🕷️ Crawler Inspector

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Direct Parameter Lookup

Raw Queries and Responses

1. Shard Calculation

Query:

Response:

Calculated Shard: 62 (from laksa151)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa062.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://www.britannica.com/technology/nuclear-reactor/Fukushima\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.britannica.com/technology/nuclear-reactor/Fukushima\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fukushima","crawl_time":1775230889,"first_indexed_time":1561817850,"http_code":200,"src_unparsed":"com,britannica!www,\/technology\/nuclear-reactor\/Fukushima s443","src_root_hash":"5455945239613777662","history_drop_reason":null,"meta_title":"Nuclear reactor - Fukushima, Meltdown, Radiation | Britannica","meta_descriptions":["Nuclear reactor - Fukushima, Meltdown, Radiation: A failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of nuclear power generation (after Chernobyl)—a partial meltdown in 2011 at the Fukushima Daiichi (“Number One”) plant in Japan. That facility, located on Japan’s Pacific coast in northeastern Fukushima prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful earthquake"],"attrs_boilerpipe_text":"damage at Fukushima Daiichi power plant\nTwo of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation.\nA failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of\nnuclear power\ngeneration (after Chernobyl)—a partial\nmeltdown\nin 2011 at the\nFukushima Daiichi (“Number One”) plant\nin\nJapan\n. That facility, located on Japan’s Pacific coast in northeastern\nFukushima\nprefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful\nearthquake\nshook all units at the plant, initiating an automatic\nshutdown\n, or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a\ntsunami\nstruck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup\nbatteries\nincorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of\nhydrogen\ngas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and\nboric acid\ninto them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment\nvessels\nwere sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated.\nThe Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events.\nIn contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the\nnatural disaster\nthat took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry.\nEmergency response\nFukushima accident\nA man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami.\nEach regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the\nUnited States\n, the\nNRC\nhas an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant:\nNotification of unusual events. Potential\ndegradation\nin the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring.\nAlert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected.\nSite area emergency. Actual or likely major failures of plant functions needed for protection of the public, with\nradioactivity\nlevels potentially above acceptable\nthresholds\nat the boundary of the power plant.\nGeneral emergency. Actual or\nimminent\nsubstantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant.\nOn a worldwide scale, the\nIAEA\nhas developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and\nradiation\nsources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific\ncriteria\nthat have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the\nenvironment\n. The seven levels and some of the important criteria are as follows:\nAnomaly\n. Minor problems with safety components, with significant defense-in-depth remaining.\nIncident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits.\nSerious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation.\nAccident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation.\nAccident with wider\nconsequences\n. Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation.\nSerious accident. Significant release of radioactive material likely to require implementation of planned countermeasures.\nMajor accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.\nUnder the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents.\nThe\nnuclear fuel cycle\nnuclear fuel cycle\nA “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies.\nNo discussion of nuclear power is complete without a brief exposition of the\nnuclear fuel\ncycle. The whole point of a reactor is, after all, to initiate and control the process of\nfission\non a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include\nuranium\nmining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and\nirradiation\nin the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and\nwaste disposal\n.\nThe nuclear fuel cycle also is an\nintegral\nstep in the production of\nplutonium\nfor\nnuclear weapon\ns, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy.\nAI-generated answers\nfrom Britannica articles. AI makes mistakes, so verify using Britannica articles.","attrs_markdown":"[![Encyclopedia Britannica](https:\/\/cdn.britannica.com\/mendel\/eb-logo\/MendelNewThistleLogo.png)](https:\/\/www.britannica.com\/)\n\n[![Encyclopedia Britannica](https:\/\/cdn.britannica.com\/mendel\/eb-logo\/MendelNewThistleLogo.png)](https:\/\/www.britannica.com\/)\n\n[SUBSCRIBE](https:\/\/premium.britannica.com\/premium-membership\/?utm_source=premium&utm_medium=global-nav&utm_campaign=blue-evergreen)\n\n[SUBSCRIBE](https:\/\/premium.britannica.com\/premium-membership\/?utm_source=premium&utm_medium=global-nav-mobile&utm_campaign=blue-evergreen)\n\nLogin\n\nhttps:\/\/premium.britannica.com\/premium-membership\/?utm\\_source=premium\\&utm\\_medium=nav-login-box\\&utm\\_campaign=evergreen\n\n[SUBSCRIBE](https:\/\/premium.britannica.com\/premium-membership\/?utm_source=premium&utm_medium=hamburger-menu&utm_campaign=blue)\n\n[Ask the Chatbot](https:\/\/www.britannica.com\/chatbot)\n\n[Games & Quizzes](https:\/\/www.britannica.com\/quiz\/browse) [History & Society](https:\/\/www.britannica.com\/History-Society) [Science & Tech](https:\/\/www.britannica.com\/Science-Tech) [Biographies](https:\/\/www.britannica.com\/Biographies) [Animals & Nature](https:\/\/www.britannica.com\/Animals-Nature) [Geography & Travel](https:\/\/www.britannica.com\/Geography-Travel) [Arts & Culture](https:\/\/www.britannica.com\/Arts-Culture) [ProCon](https:\/\/www.britannica.com\/procon) [Money](https:\/\/www.britannica.com\/money) [Videos](https:\/\/www.britannica.com\/videos)\n\n[nuclear reactor](https:\/\/www.britannica.com\/technology\/nuclear-reactor)\n\n- [Introduction](https:\/\/www.britannica.com\/technology\/nuclear-reactor)\n- [Principles of operation](https:\/\/www.britannica.com\/technology\/nuclear-reactor#ref45759)\n  - [Chain reaction and criticality](https:\/\/www.britannica.com\/technology\/nuclear-reactor#ref45760)\n  \n  - [Reactor control](https:\/\/www.britannica.com\/technology\/nuclear-reactor#ref45761)\n  \n  - [Fissile and fertile materials](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fissile-and-fertile-materials)\n  \n  - [Heat removal](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fissile-and-fertile-materials#ref45763)\n  \n  - [Shielding](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fissile-and-fertile-materials#ref45764)\n  \n  - [Critical concentration and size](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fissile-and-fertile-materials#ref45765)\n  \n  - [Thermal, intermediate, and fast reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Thermal-intermediate-and-fast-reactors)\n- [Reactor design and components](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Thermal-intermediate-and-fast-reactors#ref45767)\n  - [Core](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Thermal-intermediate-and-fast-reactors#ref45768)\n    - [Fuel types](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fuel-types)\n    - [Coolants and moderators](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fuel-types#ref45770)\n  \n  - [Reflectors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fuel-types#ref45771)\n  \n  - [Reactor control elements](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fuel-types#ref45772)\n  \n  - [Structural components](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fuel-types#ref45773)\n  \n  - [Coolant system](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Coolant-system)\n  \n  - [Containment system](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Coolant-system#ref45775)\n    - [Containment design principles](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Coolant-system#ref307270)\n    - [Containment systems and major nuclear accidents](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Coolant-system#ref307271)\n- [Types of reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors)\n  - [Power reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref45777)\n    - [Light-water reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref45778)\n      - [PWRs and BWRs](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref307272)\n      - [Advantages and disadvantages](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref307273)\n      - [Fueling and refueling LWRs](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref307274)\n      - [Global status of LWRs](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref307275)\n    - [High-temperature gas-cooled reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref45779)\n    - [Liquid-metal reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Liquid-metal-reactors)\n    - [CANDU reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Liquid-metal-reactors#ref45781)\n    - [Advanced gas-cooled reactor](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Liquid-metal-reactors#ref45782)\n    - [Other power reactor types](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Liquid-metal-reactors#ref45783)\n  \n  - [Research reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors)\n    - [Water-cooled, plate-fuel reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45785)\n    - [TRIGA reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45786)\n    - [Other research reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45787)\n  \n  - [Ship-propulsion reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45788)\n  \n  - [Production reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45789)\n  \n  - [Specialized reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Research-reactors#ref45790)\n- [Reactor safety](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety)\n  - [Preventive measures](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety#ref302439)\n    - [Design and operating standards](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety#ref307276)\n    - [The Windscale accident of 1957](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety#ref307277)\n    - [The Reactor Safety Study of 1972–75](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety#ref307278)\n    - [Three Mile Island and Chernobyl](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Three-Mile-Island-and-Chernobyl)\n  \n  - [Mitigating measures](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Three-Mile-Island-and-Chernobyl#ref302440)\n    - [Systems and structures](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Three-Mile-Island-and-Chernobyl#ref307280)\n    - [Fukushima](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fukushima)\n  \n  - [Emergency response](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fukushima#ref307282)\n- [The nuclear fuel cycle](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Fukushima#ref302441)\n  - [Uranium mining and processing](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing)\n  \n  - [Enrichment](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref302443)\n  \n  - [Fabrication](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref302444)\n  \n  - [Fuel management](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref302445)\n  \n  - [Unloading and cooling](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref302446)\n  \n  - [Reprocessing](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref302447)\n    - [Reprocessing methods](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref307283)\n    - [Reprocessing policies](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Uranium-mining-and-processing#ref307284)\n  \n  - [Waste disposal](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Waste-disposal)\n    - [Waste conditioning](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Waste-disposal#ref307285)\n    - [Geologic disposal](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Waste-disposal#ref302450)\n    - [Risks of nuclear waste disposal](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Waste-disposal#ref302451)\n- [History of reactor development](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/History-of-reactor-development)\n  - [The first atomic piles](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/History-of-reactor-development#ref307286)\n  \n  - [From production reactors to commercial power reactors](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/From-production-reactors-to-commercial-power-reactors)\n  \n  - [Growth of nuclear programs](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Growth-of-nuclear-programs)\n\n[References & Edit History](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/additional-info)  [Related Topics](https:\/\/www.britannica.com\/facts\/nuclear-reactor)\n\n[Images & Videos](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/images-videos)\n\n[![Temelín Nuclear Power Plant, South Bohemia, Czech Republic](https:\/\/cdn.britannica.com\/01\/95901-004-E2ECE82B\/Czech-Republic-operation-Temelin-Nuclear-Power-Plant-2003.jpg)](https:\/\/cdn.britannica.com\/01\/95901-050-49DFA760\/Czech-Republic-operation-Temelin-Nuclear-Power-Plant-2003.jpg)  [![Fission](https:\/\/cdn.britannica.com\/79\/22479-004-672B4B99\/Sequence-events-fission-neutron-uranium-nucleus.jpg)](https:\/\/cdn.britannica.com\/79\/22479-050-FD2BBACB\/Sequence-events-fission-neutron-uranium-nucleus.jpg)  [![chain reaction in a nuclear reactor at a critical state](https:\/\/cdn.britannica.com\/10\/162210-004-FAAAD4E4\/reactor-Chain-reaction-neutrons-nuclei-state-fission.jpg)](https:\/\/cdn.britannica.com\/10\/162210-050-698010AF\/reactor-Chain-reaction-neutrons-nuclei-state-fission.jpg)  [![nuclear power plant at Angra dos Reis, Brazil](https:\/\/cdn.britannica.com\/27\/108127-004-55BFE14C\/Angra-nuclear-power-plant-reactors-Rio-de.jpg)](https:\/\/cdn.britannica.com\/27\/108127-050-508CDB0D\/Angra-nuclear-power-plant-reactors-Rio-de.jpg)  [![Cherenkov radiation](https:\/\/cdn.britannica.com\/34\/163334-004-85D3FD57\/Cherenkov-radiation-core-Reed-Research-Reactor-Oregon.jpg)](https:\/\/cdn.britannica.com\/34\/163334-050-62755157\/Cherenkov-radiation-core-Reed-Research-Reactor-Oregon.jpg)  [![nuclear reactor: basic components](https:\/\/cdn.britannica.com\/89\/62989-004-CC63E739\/components-reactor.jpg)](https:\/\/cdn.britannica.com\/89\/62989-050-6737CF1F\/components-reactor.jpg)  [![reactor core](https:\/\/cdn.britannica.com\/97\/102097-004-FE989361\/core-nuclear-reactor.jpg)](https:\/\/cdn.britannica.com\/97\/102097-050-1D7987E3\/core-nuclear-reactor.jpg)  [![nuclear fuel assembly for a pressurized-water reactor.](https:\/\/cdn.britannica.com\/81\/162181-004-C9F17D1F\/rods-fuel-control-grid-spacers-assembly-reactor.jpg)](https:\/\/cdn.britannica.com\/81\/162181-050-975991EB\/rods-fuel-control-grid-spacers-assembly-reactor.jpg)  [![basic power cycles in nuclear power plants](https:\/\/cdn.britannica.com\/13\/313-004-C93A8968\/power-cycles-cycle-nuclear-plants-reactor-loop.jpg)](https:\/\/cdn.britannica.com\/13\/313-050-8ABCA946\/power-cycles-cycle-nuclear-plants-reactor-loop.jpg)  [![pressurized-water reactor](https:\/\/cdn.britannica.com\/96\/102096-004-4018C7BB\/Section-reactor-outlets-inlets-core-water-coolant.jpg)](https:\/\/cdn.britannica.com\/96\/102096-050-58C9DE3D\/Section-reactor-outlets-inlets-core-water-coolant.jpg)\n\nAt a Glance\n\n[![default image](https:\/\/cdn.britannica.com\/mendel-resources\/3-178\/images\/shared\/new-thistle.svg?v=3.178.5)](https:\/\/www.britannica.com\/summary\/nuclear-reactor)\n\n[nuclear reactor summary](https:\/\/www.britannica.com\/summary\/nuclear-reactor)\n\nQuizzes\n\n[![Men of the Royal Norfolk Regiment at Aldershot now undergoing a course of revolver shooting wear gas masks while at practice in order to got used to wearing the masks under all conditions. Two Tommies sighting the target in their gas masks. (World War I)](https:\/\/cdn.britannica.com\/54\/200654-131-1B599C36\/Men-Royal-Norfolk-Regiment-gas-masks-revolver-1915.jpg?w=200&h=200&c=crop)](https:\/\/www.britannica.com\/quiz\/do-you-know-which-african-american-inventor-created-which-product)\n\n[Do You Know Which African American Inventor Created Which Product?](https:\/\/www.britannica.com\/quiz\/do-you-know-which-african-american-inventor-created-which-product)\n\n![Britannica AI Icon](https:\/\/cdn.britannica.com\/mendel-resources\/3-178\/images\/chatbot\/star-ai.svg?v=3.178.5)\n\nContents\n\nAsk Anything\n\n[Technology](https:\/\/www.britannica.com\/browse\/Technology)    [Industry](https:\/\/www.britannica.com\/browse\/Industry)\n\nCITE\n\nShare\n\nFeedback\n\nExternal Websites\n\n## Fukushima\nin  [nuclear reactor](https:\/\/www.britannica.com\/technology\/nuclear-reactor) in\n\n# [Reactor safety](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Reactor-safety)\n\nHomework Help\n\nWritten by\n\n[Wade Marcum Assistant professor of nuclear engineering, Oregon State University, U.S.](https:\/\/www.britannica.com\/contributor\/Wade-Marcum\/9281651)\n\nWade Marcum[All](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/additional-info#contributors)\n\nFact-checked by\n\n[Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....](https:\/\/www.britannica.com\/editor\/The-Editors-of-Encyclopaedia-Britannica\/4419)\n\nBritannica Editors\n\nLast updated\n\nMar. 19, 2026\n\n•[History](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/additional-info#history)\n\n![Britannica AI Icon](https:\/\/cdn.britannica.com\/mendel-resources\/3-178\/images\/chatbot\/star-ai.svg?v=3.178.5) Britannica AI\n\nAsk Anything\n\nTable of Contents\n\nTable of Contents\n\nAsk Anything\n\n[![damage at Fukushima Daiichi power plant](https:\/\/cdn.britannica.com\/93\/148893-050-26E0E6CE\/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg?w=300)](https:\/\/cdn.britannica.com\/93\/148893-050-26E0E6CE\/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg)\n\n[damage at Fukushima Daiichi power plant](https:\/\/cdn.britannica.com\/93\/148893-050-26E0E6CE\/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg)Two of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation.\n\n(more)\n\nA failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of [nuclear power](https:\/\/www.britannica.com\/technology\/nuclear-power) generation (after Chernobyl)—a partial [meltdown](https:\/\/www.britannica.com\/technology\/meltdown) in 2011 at the [Fukushima Daiichi (“Number One”) plant](https:\/\/www.britannica.com\/event\/Fukushima-accident) in [Japan](https:\/\/www.britannica.com\/place\/Japan). That facility, located on Japan’s Pacific coast in northeastern [Fukushima](https:\/\/www.britannica.com\/place\/Fukushima-prefecture-Japan) prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful [earthquake](https:\/\/www.britannica.com\/science\/earthquake-geology) shook all units at the plant, initiating an automatic [shutdown](https:\/\/www.britannica.com\/dictionary\/shutdown), or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a [tsunami](https:\/\/www.britannica.com\/science\/tsunami) struck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup [batteries](https:\/\/www.britannica.com\/dictionary\/batteries) incorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of [hydrogen](https:\/\/www.britannica.com\/science\/hydrogen) gas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and [boric acid](https:\/\/www.britannica.com\/science\/boric-acid) into them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment [vessels](https:\/\/www.britannica.com\/dictionary\/vessels) were sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated.\n\nThe Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events.\n\nIn contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the [natural disaster](https:\/\/www.britannica.com\/science\/natural-disaster) that took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry.\n\n## Emergency response\n[![Fukushima accident](https:\/\/cdn.britannica.com\/60\/148660-050-47FA35DE\/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg?w=300)](https:\/\/cdn.britannica.com\/60\/148660-050-47FA35DE\/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg)\n\n[Fukushima accident](https:\/\/cdn.britannica.com\/60\/148660-050-47FA35DE\/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg)A man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami.\n\n(more)\n\nEach regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the [United States](https:\/\/www.britannica.com\/place\/United-States), the [NRC](https:\/\/www.britannica.com\/topic\/Nuclear-Regulatory-Commission) has an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant:\n\n1. Notification of unusual events. Potential [degradation](https:\/\/www.merriam-webster.com\/dictionary\/degradation) in the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring.\n2. Alert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected.\n3. Site area emergency. Actual or likely major failures of plant functions needed for protection of the public, with [radioactivity](https:\/\/www.britannica.com\/science\/radioactivity) levels potentially above acceptable [thresholds](https:\/\/www.merriam-webster.com\/dictionary\/thresholds) at the boundary of the power plant.\n4. General emergency. Actual or [imminent](https:\/\/www.merriam-webster.com\/dictionary\/imminent) substantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant.\n\nOn a worldwide scale, the [IAEA](https:\/\/www.britannica.com\/topic\/International-Atomic-Energy-Agency) has developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and [radiation](https:\/\/www.britannica.com\/science\/radiation) sources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific [criteria](https:\/\/www.merriam-webster.com\/dictionary\/criteria) that have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the [environment](https:\/\/www.merriam-webster.com\/dictionary\/environment). The seven levels and some of the important criteria are as follows:\n\n1. [Anomaly](https:\/\/www.merriam-webster.com\/dictionary\/Anomaly). Minor problems with safety components, with significant defense-in-depth remaining.\n2. Incident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits.\n3. Serious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation.\n4. Accident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation.\n5. Accident with wider [consequences](https:\/\/www.britannica.com\/dictionary\/consequences). Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation.\n6. Serious accident. Significant release of radioactive material likely to require implementation of planned countermeasures.\n7. Major accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.\n\n[![exclusion zone: Chernobyl disaster; Fukushima accident](https:\/\/cdn.britannica.com\/00\/196800-050-E30A2B4A\/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg)](https:\/\/cdn.britannica.com\/00\/196800-050-E30A2B4A\/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg)\n\n[exclusion zone: Chernobyl disaster; Fukushima accident](https:\/\/cdn.britannica.com\/00\/196800-050-E30A2B4A\/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg)Map of the exclusion zones following the nuclear accidents at Chernobyl, Soviet Union (now in Ukraine), and Fukushima, Japan.\n\n(more)\n\nUnder the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents.\n\n## The [nuclear fuel cycle](https:\/\/www.britannica.com\/technology\/nuclear-fuel-cycle)\n[![nuclear fuel cycle](https:\/\/cdn.britannica.com\/78\/162178-050-43849118\/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)](https:\/\/cdn.britannica.com\/78\/162178-050-43849118\/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)\n\n[nuclear fuel cycle](https:\/\/cdn.britannica.com\/78\/162178-050-43849118\/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)A “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies.\n\n(more)\n\nNo discussion of nuclear power is complete without a brief exposition of the [nuclear fuel](https:\/\/www.britannica.com\/technology\/nuclear-fuel) cycle. The whole point of a reactor is, after all, to initiate and control the process of [fission](https:\/\/www.britannica.com\/science\/nuclear-fission) on a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include [uranium](https:\/\/www.britannica.com\/science\/uranium) mining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and [irradiation](https:\/\/www.britannica.com\/science\/radiation-therapy) in the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and [waste disposal](https:\/\/www.britannica.com\/technology\/waste-disposal-system).\n\nThe nuclear fuel cycle also is an [integral](https:\/\/www.merriam-webster.com\/dictionary\/integral) step in the production of [plutonium](https:\/\/www.britannica.com\/science\/plutonium) for [nuclear weapon](https:\/\/www.britannica.com\/technology\/nuclear-weapon)s, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy.\n\n![Britannica AI Icon](https:\/\/cdn.britannica.com\/mendel-resources\/3-178\/images\/chatbot\/star-ai.svg?v=3.178.5)Britannica AI\n\n*chevron\\_right*\n\nNuclear reactor - Fukushima, Meltdown, Radiation\n\n*close*\n\n[AI-generated answers](https:\/\/www.britannica.com\/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles.\n\n# [Uranium](https:\/\/www.britannica.com\/science\/uranium) mining and processing\n[Uranium](https:\/\/www.britannica.com\/technology\/uranium-processing) is extracted from ores whose uranium content is often less than 0.1 percent (one part per thousand). Most ore deposits occur at or near the surface; whether they are mined through open-pit or underground techniques depends on the depth of the deposit and its slope. The mined ore is crushed and the uranium chemically extracted from it at the mouth of the mine. The residue remains naturally radioactive, as it contains long-lived radioactive daughter nuclei of uranium and has to be carefully managed to minimize the release of radioactive contaminants into the [environment](https:\/\/www.merriam-webster.com\/dictionary\/environment). The uranium concentrate, which is known as yellow cake, consists of uranium [compounds](https:\/\/www.merriam-webster.com\/dictionary\/compounds) (typically 75 to 95 percent). It is shipped to a chemical plant for further purification and chemical conversion.\n\n## [Enrichment](https:\/\/www.britannica.com\/technology\/enrichment-nuclear-fuel-processing)\n[![uranium-enrichment processes](https:\/\/cdn.britannica.com\/05\/158605-050-5318BAB2\/uranium-enrichment-processes.jpg)](https:\/\/cdn.britannica.com\/05\/158605-050-5318BAB2\/uranium-enrichment-processes.jpg)\n\n[uranium-enrichment processes](https:\/\/cdn.britannica.com\/05\/158605-050-5318BAB2\/uranium-enrichment-processes.jpg)Three uranium-enrichment processes.\n\n(more)\n\nSeveral enrichment techniques have been developed, though only two of these methods are used on a large scale; these are [gaseous diffusion](https:\/\/www.britannica.com\/science\/gaseous-diffusion) and gas centrifuging. In gaseous [diffusion](https:\/\/www.merriam-webster.com\/dictionary\/diffusion), natural [uranium](https:\/\/www.britannica.com\/science\/uranium) in the form of [uranium hexafluoride](https:\/\/www.britannica.com\/science\/uranium-hexafluoride) gas (UF6), a product of chemical conversion, is encouraged (through a mechanical process) to seep through a porous barrier. The molecules of 235UF6 penetrate the barrier slightly faster than those of 238UF6. Since the percentage of 235U increases by only a very small amount after traversal of the barrier, the process must be repeated over and over in thousands of stages to obtain the necessary enrichment for commercial [nuclear power](https:\/\/www.britannica.com\/technology\/nuclear-power) use.\n\nIn gas centrifuging, the UF6 gas is fed into a high-speed centrifuge. The centrifuge is balanced very well at the top bottom and spins at an extremely high rate. Because of the relative centripetal forces that each atom experiences, the lighter species of this mixture of gaseous molecules, including 235U, tend to concentrate near the centre of the spinning centrifuge, while the heavier ones [accumulate](https:\/\/www.britannica.com\/dictionary\/accumulate) along the wall. These mixtures are then siphoned off. The degree of enrichment per stage in a centrifuge is greater than that obtained in a gaseous [diffusion chamber](https:\/\/www.britannica.com\/science\/diffusion-chamber), and the process uses less energy than gaseous diffusion does, but centrifuges are more expensive pieces of equipment.\n\nAn experimental enrichment method with much commercial potential is laser separation. This process is based on the principle that [isotope](https:\/\/www.britannica.com\/science\/isotope)s of different [molecular weight](https:\/\/www.britannica.com\/science\/molecular-weight) absorb [light](https:\/\/www.britannica.com\/science\/light) of different frequencies. Once a specific isotope has absorbed [radiation](https:\/\/www.britannica.com\/science\/radiation) and has reached an excited state, its properties may become quite different from the other isotopes; it is then separated on the basis of this difference. In one method known generically as MLIS ([molecular laser isotope separation](https:\/\/www.britannica.com\/science\/molecular-laser-isotope-separation))—or commercially as SILEX (separation of isotopes by laser excitation)—gaseous UF6 is exposed to high-powered [lasers](https:\/\/www.britannica.com\/technology\/laser) tuned to the correct frequencies to cause the molecules containing 235U (but not 238U) to lose [electron](https:\/\/www.britannica.com\/science\/electron)s. In this (ionized) form, the 235U-containing molecules are separated from the stream on the basis of their different [electric charge](https:\/\/www.britannica.com\/science\/electric-charge). Proponents of laser separation claim that the method consumes less energy and wastes less starting material than, for example, gaseous diffusion.\n\n## Fabrication\nThis step involves the conversion of the suitably enriched product material to the chemical form desired for reactor fuel. The only fuel fabricated on a large scale is for light-water reactors (LWRs).\n\nThe chemical form prepared for the LWR is [uranium dioxide](https:\/\/www.britannica.com\/science\/uranium-dioxide). Produced in the form of a [ceramic](https:\/\/www.britannica.com\/technology\/ceramic-composition-and-properties) powder, this [compound](https:\/\/www.merriam-webster.com\/dictionary\/compound) is ground to a very fine flourlike consistency and inserted into a die, where it is pressed into a pellet shape—in the case of some LWR fuels, approximately 6 mm in diameter and 10 mm in length (that is, about 0.25 × 0.4 inch). Next the pellet is sintered in a [furnace](https:\/\/www.britannica.com\/technology\/furnace) at 1,500–1,800 °C (approximately 2,700–3,300 °F). This sintering, similar to the firing of other ceramic ware, produces a dense ceramic pellet. The pellets are loaded into prefabricated [zirconium](https:\/\/www.britannica.com\/science\/zirconium) alloy cladding tubes, which are then filled with an [inert gas](https:\/\/www.britannica.com\/science\/noble-gas) and welded shut. Once the zirconium alloy tubes have been sealed, they go through significant testing to verify that there are no leaks. These tubes, called rods or pins, are then bundled together with proper spacing ensured by top and bottom manifolds through which the ends of the pins pass as well as spacer grids distributed along the middle portion of the pins. Together with other necessary hardware, the bundle [constitutes](https:\/\/www.merriam-webster.com\/dictionary\/constitutes) a fuel assembly.\n\n## Fuel management\nFuel is loaded into a reactor in a very specific and well-controlled pattern so as to obtain the most energy production before the material becomes unusable. Fresh fuel is more reactive than old fuel. Typically, a reactor is fueled in cycles, each cycle lasting one to two years, and a fuel batch is kept in the reactor for three or four cycles. At the end of each cycle, the oldest fuel is removed—normally this consists of about one-third the fuel content in the core—and fresh fuel loaded. The partially burned fuel that remains, however, is [shuffled](https:\/\/www.britannica.com\/dictionary\/shuffled) before the fresh fuel is installed. The objective of this procedure is to achieve a fuel assembly arrangement of maximum reactivity while keeping the power distribution among the different fuel assemblies as even as possible and within technical specifications.\n\nFuel burnup—that is, energy production—is limited by two factors. After significant burnup has occurred, the physical properties of the fuel become degraded, and it is not prudent to continue to keep it in the reactor. Also, after some burnup, the old fuel no longer contributes useful reactivity to the reactor. The fuel design, including its initial enrichment, is such that these two limits are made to coincide approximately.\n\n## Unloading and cooling\n[![spent nuclear fuel rods](https:\/\/cdn.britannica.com\/51\/238851-050-3EBCB91F\/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg?w=300)](https:\/\/cdn.britannica.com\/51\/238851-050-3EBCB91F\/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg)\n\n[spent nuclear fuel rods](https:\/\/cdn.britannica.com\/51\/238851-050-3EBCB91F\/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg)Spent nuclear fuel rods stored underwater at the nuclear power plant in La Hague, near Cherbourg, northwestern France.\n\n(more)\n\nSpent reactor fuel is extremely radioactive, and its [radioactivity](https:\/\/www.britannica.com\/science\/radioactivity) also makes it a source of heat (*see above* [Fueling and refueling LWRs](https:\/\/www.britannica.com\/technology\/nuclear-reactor\/Types-of-reactors#ref307274)). When the spent fuel is removed from the reactor, it must continue to be both shielded and cooled. This is accomplished by placing the spent fuel in a water-storage pool, or spent-fuel cooling pool, located next to the reactor. The water in the pool contains a large amount of [dissolved](https:\/\/www.britannica.com\/dictionary\/dissolved) [boric acid](https:\/\/www.britannica.com\/science\/boric-acid), which is a strong absorber of [neutron](https:\/\/www.britannica.com\/science\/neutron)s; this ensures that the fuel assemblies in the pool do not go critical. (Pool water is also a common source of emergency cooling water for the reactor.)\n\nPools vary in size; older spent-fuel cooling pools are able to accommodate only about 10 years’ worth of spent fuel. As the pools fill up, more spent fuel storage is needed. Additional storage space can be gained by loading spent fuel into the pool more densely than originally planned, by building a new pool, or by removing the oldest fuel assemblies from the existing pool and storing them in air-cooled concrete and steel silos—called spent-fuel storage casks—located aboveground. This last method becomes [feasible](https:\/\/www.merriam-webster.com\/dictionary\/feasible) after fuel has been stored in cooling pools for two or three years, because radioactivity and the rate of heat generation decrease rapidly over this period. Dense storage in existing pools and casks tends to be less expensive and more economical for utilities than building new pools.\n\n## [Reprocessing](https:\/\/www.britannica.com\/science\/recycling)\nBoth the converted [plutonium](https:\/\/www.britannica.com\/science\/plutonium) and residual [uranium-235](https:\/\/www.britannica.com\/science\/uranium-235) in spent fuel can be recycled by chemically reprocessing the fuel and extracting the specific elements of interest. Reprocessing not only provides a means to recycle nuclear fuel, but it also can reduce the volume and radioactivity of the waste material that must ultimately be eliminated by some method of permanent disposal.\n\nOne motivation for reprocessing is ultimately to provide a “closed-loop” fuel cycle within the nuclear industry. Closed-loop refers to recycling with 100-percent [efficiency](https:\/\/www.merriam-webster.com\/dictionary\/efficiency) of all materials that are fabricated for use as nuclear fuel (including the most commonly used fuel, uranium dioxide pellets). Though the goal of 100-percent efficiency has yet to be attained by any country’s nuclear industry, a closed-loop fuel cycle is not an unrealistic ambition, based on current progress in reprocessing [technology](https:\/\/www.britannica.com\/technology\/technology). Many benefits would result from fuel recycling, including lower cost for fuel (once the recycling [infrastructure](https:\/\/www.merriam-webster.com\/dictionary\/infrastructure) was in place) and reduced quantities of spent fuel to be stored on reactor sites around the world.\n\n## Reprocessing methods\nThe most common method for reprocessing, known as the PUREX (for plutonium-uranium extraction) process, begins with dissolving the spent fuel in [nitric acid](https:\/\/www.britannica.com\/science\/nitric-acid) and contacting the acid solution with oil in which [tributyl phosphate](https:\/\/www.britannica.com\/science\/tributyl-phosphate) (TBP) has been dissolved. TBP is a [complexing](https:\/\/www.britannica.com\/science\/complex-in-chemistry) agent for uranium and plutonium, forming compounds with them that bring them into the oil solution. A physical separation of the (immiscible) oil and acid serves to remove the desired products from the nitric acid solution (which still contains all the fission products). The uranium and plutonium are then washed out of the TBP back into a water solution and separated from each other by various means to the degree desired. Thus, reprocessing produces three product streams: (1) a purified uranium product, (2) a plutonium product that may be either pure or mixed with uranium, and (3) a waste stream of fission products dissolved in nitric acid.\n\n## Reprocessing policies\nDuring the period of ambitious nuclear power plant construction in the [United States](https:\/\/www.britannica.com\/place\/United-States) in the 1950s and ’60s, it was generally assumed that after two to five years, spent fuel would be delivered to a reprocessing plant. Some commercial reprocessing plants were built or planned, but by the mid-1970s the cost of reprocessing had escalated to a point where its economics became questionable. Also, in 1977 Pres. [Jimmy Carter](https:\/\/www.britannica.com\/biography\/Jimmy-Carter), in order to take a public, symbolic stand against [nuclear proliferation](https:\/\/www.britannica.com\/topic\/nuclear-proliferation), declared that the federal government would permanently defer all permits for the commercial reprocessing and recycling of plutonium. Carter’s directive was [rescinded](https:\/\/www.merriam-webster.com\/dictionary\/rescinded) by his successor, [Ronald Reagan](https:\/\/www.britannica.com\/biography\/Ronald-Reagan), and it has not been reinstated by any subsequent president. Even so, reprocessing is still not done commercially in the United States, partly because of the huge costs of building a reprocessing plant in a period when the supply of uranium ore has been sufficient to satisfy demand relatively cheaply.\n\nPolicy and institutional arrangements have been different in [France](https:\/\/www.britannica.com\/place\/France) and the [United Kingdom](https:\/\/www.britannica.com\/place\/United-Kingdom), where commercial plants reprocess spent fuel not only from nuclear plants in the host countries but also from plants in other countries. The reprocessed plutonium can be used not only as fuel for proposed future liquid-metal reactors (LMRs) but also to help fuel existing LWRs. In the latter application, the plutonium is utilized in mixed oxide (MOX) form—a combination of uranium and plutonium dioxides having 3 to 6 percent plutonium.\n\nA number of other countries, including [Russia](https:\/\/www.britannica.com\/place\/Russia), [India](https:\/\/www.britannica.com\/place\/India), [Japan](https:\/\/www.britannica.com\/place\/Japan), and [China](https:\/\/www.britannica.com\/place\/China), reprocess their spent fuel or plan to do so. The [proactive](https:\/\/www.merriam-webster.com\/dictionary\/proactive) reprocessing efforts of these countries have reduced the waste scheduled for long-term disposal to amounts well below those that are accumulating in countries that do not reprocess.\n\nLoad Next Page\n\nFeedback\n\nThank you for your feedback\n\nOur editors will review what you’ve submitted and determine whether to revise the article.\n\n*print* Print\n\nPlease select which sections you would like to print:\n\n*verified*Cite\n\nWhile every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.\n\nSelect Citation Style\n\nMarcum, Wade, Spinrad, Bernard I.. \"nuclear reactor\". *Encyclopedia Britannica*, 19 Mar. 2026, https:\/\/www.britannica.com\/technology\/nuclear-reactor. Accessed 3 April 2026.\n\nCopy Citation\n\nShare\n\nShare to social media\n\n[Facebook](https:\/\/www.facebook.com\/BRITANNICA\/) [X](https:\/\/x.com\/britannica)\n\nURL\n\n<https:\/\/www.britannica.com\/technology\/nuclear-reactor>\n\nExternal Websites\n\n- [United States Nuclear Regulatory Commission - Nuclear Reactor](https:\/\/www.nrc.gov\/reactors.html)\n- [Chemistry LibreTexts - Nuclear Reactors](https:\/\/chem.libretexts.org\/Courses\/Los_Angeles_Trade_Technical_College\/Chem_51\/19%3A_Nuclear_Chemistry\/19.08%3A_Nuclear_Reactors)\n- [University of Chicago News - The first nuclear reactor, explained](https:\/\/news.uchicago.edu\/explainer\/first-nuclear-reactor-explained)\n- [The Royal Society Publishing - Proceedings A - Challenges to deployment of twenty-first century nuclear reactor systems](https:\/\/royalsocietypublishing.org\/rspa\/article\/473\/2198\/20160815\/57308\/Challenges-to-deployment-of-twenty-first-century)\n- [World Nuclear Association - Nuclear Power Reactors](https:\/\/world-nuclear.org\/information-library\/nuclear-fuel-cycle\/nuclear-power-reactors\/nuclear-power-reactors)\n- [Atomic Heritage Foundation - Theï¿½National Museum of Nuclear Science & History - Nuclear Reactors](https:\/\/ahf.nuclearmuseum.org\/ahf\/history\/nuclear-reactors\/)\n- [Energy Education - Nuclear reactor](https:\/\/energyeducation.ca\/encyclopedia\/Nuclear_reactor)\n- [University Network of Excellence in Nuclear Engineering - Introduction to Nuclear Reactors (PDF)](https:\/\/www.unene.ca\/essentialcandu\/pdf\/1%20-%20Intro%20to%20Nuclear%20Reactors.pdf)\n- [The Institution of Engineering and Technology - Nuclear Reactor Types (PDF)](https:\/\/www.theiet.org\/media\/8809\/nuclear-reactor-types.pdf)\n- [National Center for Biotechnology Information - PubMed Central - A review of the application of artificial intelligence to nuclear reactors: Where we are and what's next](https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC9988575\/)\n- [Simon Fraser University - Nuclear Reactor Basic Principles](https:\/\/www.sfu.ca\/phys\/346\/121\/lecture_notes\/lecture26_nuclear_reactors.pdf)","attrs_readable_markdown":"[damage at Fukushima Daiichi power plant](https:\/\/cdn.britannica.com\/93\/148893-050-26E0E6CE\/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg)Two of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation.\n\nA failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of [nuclear power](https:\/\/www.britannica.com\/technology\/nuclear-power) generation (after Chernobyl)—a partial [meltdown](https:\/\/www.britannica.com\/technology\/meltdown) in 2011 at the [Fukushima Daiichi (“Number One”) plant](https:\/\/www.britannica.com\/event\/Fukushima-accident) in [Japan](https:\/\/www.britannica.com\/place\/Japan). That facility, located on Japan’s Pacific coast in northeastern [Fukushima](https:\/\/www.britannica.com\/place\/Fukushima-prefecture-Japan) prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful [earthquake](https:\/\/www.britannica.com\/science\/earthquake-geology) shook all units at the plant, initiating an automatic [shutdown](https:\/\/www.britannica.com\/dictionary\/shutdown), or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a [tsunami](https:\/\/www.britannica.com\/science\/tsunami) struck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup [batteries](https:\/\/www.britannica.com\/dictionary\/batteries) incorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of [hydrogen](https:\/\/www.britannica.com\/science\/hydrogen) gas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and [boric acid](https:\/\/www.britannica.com\/science\/boric-acid) into them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment [vessels](https:\/\/www.britannica.com\/dictionary\/vessels) were sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated.\n\nThe Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events.\n\nIn contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the [natural disaster](https:\/\/www.britannica.com\/science\/natural-disaster) that took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry.\n\n## Emergency response\n[Fukushima accident](https:\/\/cdn.britannica.com\/60\/148660-050-47FA35DE\/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg)A man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami.\n\nEach regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the [United States](https:\/\/www.britannica.com\/place\/United-States), the [NRC](https:\/\/www.britannica.com\/topic\/Nuclear-Regulatory-Commission) has an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant:\n\n1. Notification of unusual events. Potential [degradation](https:\/\/www.merriam-webster.com\/dictionary\/degradation) in the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring.\n2. Alert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected.\n3. Site area emergency. Actual or likely major failures of plant functions needed for protection of the public, with [radioactivity](https:\/\/www.britannica.com\/science\/radioactivity) levels potentially above acceptable [thresholds](https:\/\/www.merriam-webster.com\/dictionary\/thresholds) at the boundary of the power plant.\n4. General emergency. Actual or [imminent](https:\/\/www.merriam-webster.com\/dictionary\/imminent) substantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant.\n\nOn a worldwide scale, the [IAEA](https:\/\/www.britannica.com\/topic\/International-Atomic-Energy-Agency) has developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and [radiation](https:\/\/www.britannica.com\/science\/radiation) sources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific [criteria](https:\/\/www.merriam-webster.com\/dictionary\/criteria) that have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the [environment](https:\/\/www.merriam-webster.com\/dictionary\/environment). The seven levels and some of the important criteria are as follows:\n\n1. [Anomaly](https:\/\/www.merriam-webster.com\/dictionary\/Anomaly). Minor problems with safety components, with significant defense-in-depth remaining.\n2. Incident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits.\n3. Serious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation.\n4. Accident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation.\n5. Accident with wider [consequences](https:\/\/www.britannica.com\/dictionary\/consequences). Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation.\n6. Serious accident. Significant release of radioactive material likely to require implementation of planned countermeasures.\n7. Major accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.\n\nUnder the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents.\n\n## The [nuclear fuel cycle](https:\/\/www.britannica.com\/technology\/nuclear-fuel-cycle)\n[nuclear fuel cycle](https:\/\/cdn.britannica.com\/78\/162178-050-43849118\/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)A “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies.\n\nNo discussion of nuclear power is complete without a brief exposition of the [nuclear fuel](https:\/\/www.britannica.com\/technology\/nuclear-fuel) cycle. The whole point of a reactor is, after all, to initiate and control the process of [fission](https:\/\/www.britannica.com\/science\/nuclear-fission) on a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include [uranium](https:\/\/www.britannica.com\/science\/uranium) mining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and [irradiation](https:\/\/www.britannica.com\/science\/radiation-therapy) in the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and [waste disposal](https:\/\/www.britannica.com\/technology\/waste-disposal-system).\n\nThe nuclear fuel cycle also is an [integral](https:\/\/www.merriam-webster.com\/dictionary\/integral) step in the production of [plutonium](https:\/\/www.britannica.com\/science\/plutonium) for [nuclear weapon](https:\/\/www.britannica.com\/technology\/nuclear-weapon)s, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy.\n\n[AI-generated answers](https:\/\/www.britannica.com\/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles.","meta_canonical":null}

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Meta Description	Nuclear reactor - Fukushima, Meltdown, Radiation: A failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of nuclear power generation (after Chernobyl)—a partial meltdown in 2011 at the Fukushima Daiichi (“Number One”) plant in Japan. That facility, located on Japan’s Pacific coast in northeastern Fukushima prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful earthquake
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Boilerpipe Text	damage at Fukushima Daiichi power plant Two of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation. A failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of nuclear power generation (after Chernobyl)—a partial meltdown in 2011 at the Fukushima Daiichi (“Number One”) plant in Japan . That facility, located on Japan’s Pacific coast in northeastern Fukushima prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful earthquake shook all units at the plant, initiating an automatic shutdown , or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a tsunami struck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup batteries incorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of hydrogen gas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and boric acid into them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment vessels were sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated. The Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events. In contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the natural disaster that took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry. Emergency response Fukushima accident A man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami. Each regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the United States , the NRC has an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant: Notification of unusual events. Potential degradation in the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring. Alert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected. Site area emergency. Actual or likely major failures of plant functions needed for protection of the public, with radioactivity levels potentially above acceptable thresholds at the boundary of the power plant. General emergency. Actual or imminent substantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant. On a worldwide scale, the IAEA has developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and radiation sources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific criteria that have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the environment . The seven levels and some of the important criteria are as follows: Anomaly . Minor problems with safety components, with significant defense-in-depth remaining. Incident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits. Serious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation. Accident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation. Accident with wider consequences . Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation. Serious accident. Significant release of radioactive material likely to require implementation of planned countermeasures. Major accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures. Under the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents. The nuclear fuel cycle nuclear fuel cycle A “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies. No discussion of nuclear power is complete without a brief exposition of the nuclear fuel cycle. The whole point of a reactor is, after all, to initiate and control the process of fission on a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include uranium mining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and irradiation in the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and waste disposal . The nuclear fuel cycle also is an integral step in the production of plutonium for nuclear weapon s, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy. AI-generated answers from Britannica articles. AI makes mistakes, so verify using Britannica articles.
Markdown	[![Encyclopedia Britannica](https://cdn.britannica.com/mendel/eb-logo/MendelNewThistleLogo.png)](https://www.britannica.com/) [![Encyclopedia Britannica](https://cdn.britannica.com/mendel/eb-logo/MendelNewThistleLogo.png)](https://www.britannica.com/) [SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=global-nav&utm_campaign=blue-evergreen) [SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=global-nav-mobile&utm_campaign=blue-evergreen) Login https://premium.britannica.com/premium-membership/?utm\_source=premium\&utm\_medium=nav-login-box\&utm\_campaign=evergreen [SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=hamburger-menu&utm_campaign=blue) [Ask the Chatbot](https://www.britannica.com/chatbot) [Games & Quizzes](https://www.britannica.com/quiz/browse) [History & Society](https://www.britannica.com/History-Society) [Science & Tech](https://www.britannica.com/Science-Tech) [Biographies](https://www.britannica.com/Biographies) [Animals & Nature](https://www.britannica.com/Animals-Nature) [Geography & Travel](https://www.britannica.com/Geography-Travel) [Arts & Culture](https://www.britannica.com/Arts-Culture) [ProCon](https://www.britannica.com/procon) [Money](https://www.britannica.com/money) [Videos](https://www.britannica.com/videos) [nuclear reactor](https://www.britannica.com/technology/nuclear-reactor) - [Introduction](https://www.britannica.com/technology/nuclear-reactor) - [Principles of operation](https://www.britannica.com/technology/nuclear-reactor#ref45759) - [Chain reaction and criticality](https://www.britannica.com/technology/nuclear-reactor#ref45760) - [Reactor control](https://www.britannica.com/technology/nuclear-reactor#ref45761) - [Fissile and fertile materials](https://www.britannica.com/technology/nuclear-reactor/Fissile-and-fertile-materials) - [Heat removal](https://www.britannica.com/technology/nuclear-reactor/Fissile-and-fertile-materials#ref45763) - [Shielding](https://www.britannica.com/technology/nuclear-reactor/Fissile-and-fertile-materials#ref45764) - [Critical concentration and size](https://www.britannica.com/technology/nuclear-reactor/Fissile-and-fertile-materials#ref45765) - [Thermal, intermediate, and fast reactors](https://www.britannica.com/technology/nuclear-reactor/Thermal-intermediate-and-fast-reactors) - [Reactor design and components](https://www.britannica.com/technology/nuclear-reactor/Thermal-intermediate-and-fast-reactors#ref45767) - [Core](https://www.britannica.com/technology/nuclear-reactor/Thermal-intermediate-and-fast-reactors#ref45768) - [Fuel types](https://www.britannica.com/technology/nuclear-reactor/Fuel-types) - [Coolants and moderators](https://www.britannica.com/technology/nuclear-reactor/Fuel-types#ref45770) - [Reflectors](https://www.britannica.com/technology/nuclear-reactor/Fuel-types#ref45771) - [Reactor control elements](https://www.britannica.com/technology/nuclear-reactor/Fuel-types#ref45772) - [Structural components](https://www.britannica.com/technology/nuclear-reactor/Fuel-types#ref45773) - [Coolant system](https://www.britannica.com/technology/nuclear-reactor/Coolant-system) - [Containment system](https://www.britannica.com/technology/nuclear-reactor/Coolant-system#ref45775) - [Containment design principles](https://www.britannica.com/technology/nuclear-reactor/Coolant-system#ref307270) - [Containment systems and major nuclear accidents](https://www.britannica.com/technology/nuclear-reactor/Coolant-system#ref307271) - [Types of reactors](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors) - [Power reactors](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref45777) - [Light-water reactors](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref45778) - [PWRs and BWRs](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref307272) - [Advantages and disadvantages](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref307273) - [Fueling and refueling LWRs](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref307274) - [Global status of LWRs](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref307275) - [High-temperature gas-cooled reactors](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref45779) - [Liquid-metal reactors](https://www.britannica.com/technology/nuclear-reactor/Liquid-metal-reactors) - [CANDU reactors](https://www.britannica.com/technology/nuclear-reactor/Liquid-metal-reactors#ref45781) - [Advanced gas-cooled reactor](https://www.britannica.com/technology/nuclear-reactor/Liquid-metal-reactors#ref45782) - [Other power reactor types](https://www.britannica.com/technology/nuclear-reactor/Liquid-metal-reactors#ref45783) - [Research reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors) - [Water-cooled, plate-fuel reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45785) - [TRIGA reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45786) - [Other research reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45787) - [Ship-propulsion reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45788) - [Production reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45789) - [Specialized reactors](https://www.britannica.com/technology/nuclear-reactor/Research-reactors#ref45790) - [Reactor safety](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety) - [Preventive measures](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety#ref302439) - [Design and operating standards](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety#ref307276) - [The Windscale accident of 1957](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety#ref307277) - [The Reactor Safety Study of 1972–75](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety#ref307278) - [Three Mile Island and Chernobyl](https://www.britannica.com/technology/nuclear-reactor/Three-Mile-Island-and-Chernobyl) - [Mitigating measures](https://www.britannica.com/technology/nuclear-reactor/Three-Mile-Island-and-Chernobyl#ref302440) - [Systems and structures](https://www.britannica.com/technology/nuclear-reactor/Three-Mile-Island-and-Chernobyl#ref307280) - [Fukushima](https://www.britannica.com/technology/nuclear-reactor/Fukushima) - [Emergency response](https://www.britannica.com/technology/nuclear-reactor/Fukushima#ref307282) - [The nuclear fuel cycle](https://www.britannica.com/technology/nuclear-reactor/Fukushima#ref302441) - [Uranium mining and processing](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing) - [Enrichment](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref302443) - [Fabrication](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref302444) - [Fuel management](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref302445) - [Unloading and cooling](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref302446) - [Reprocessing](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref302447) - [Reprocessing methods](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref307283) - [Reprocessing policies](https://www.britannica.com/technology/nuclear-reactor/Uranium-mining-and-processing#ref307284) - [Waste disposal](https://www.britannica.com/technology/nuclear-reactor/Waste-disposal) - [Waste conditioning](https://www.britannica.com/technology/nuclear-reactor/Waste-disposal#ref307285) - [Geologic disposal](https://www.britannica.com/technology/nuclear-reactor/Waste-disposal#ref302450) - [Risks of nuclear waste disposal](https://www.britannica.com/technology/nuclear-reactor/Waste-disposal#ref302451) - [History of reactor development](https://www.britannica.com/technology/nuclear-reactor/History-of-reactor-development) - [The first atomic piles](https://www.britannica.com/technology/nuclear-reactor/History-of-reactor-development#ref307286) - [From production reactors to commercial power reactors](https://www.britannica.com/technology/nuclear-reactor/From-production-reactors-to-commercial-power-reactors) - [Growth of nuclear programs](https://www.britannica.com/technology/nuclear-reactor/Growth-of-nuclear-programs) [References & Edit History](https://www.britannica.com/technology/nuclear-reactor/additional-info) [Related Topics](https://www.britannica.com/facts/nuclear-reactor) [Images & Videos](https://www.britannica.com/technology/nuclear-reactor/images-videos) [![Temelín Nuclear Power Plant, South Bohemia, Czech Republic](https://cdn.britannica.com/01/95901-004-E2ECE82B/Czech-Republic-operation-Temelin-Nuclear-Power-Plant-2003.jpg)](https://cdn.britannica.com/01/95901-050-49DFA760/Czech-Republic-operation-Temelin-Nuclear-Power-Plant-2003.jpg) [![Fission](https://cdn.britannica.com/79/22479-004-672B4B99/Sequence-events-fission-neutron-uranium-nucleus.jpg)](https://cdn.britannica.com/79/22479-050-FD2BBACB/Sequence-events-fission-neutron-uranium-nucleus.jpg) [![chain reaction in a nuclear reactor at a critical state](https://cdn.britannica.com/10/162210-004-FAAAD4E4/reactor-Chain-reaction-neutrons-nuclei-state-fission.jpg)](https://cdn.britannica.com/10/162210-050-698010AF/reactor-Chain-reaction-neutrons-nuclei-state-fission.jpg) [![nuclear power plant at Angra dos Reis, Brazil](https://cdn.britannica.com/27/108127-004-55BFE14C/Angra-nuclear-power-plant-reactors-Rio-de.jpg)](https://cdn.britannica.com/27/108127-050-508CDB0D/Angra-nuclear-power-plant-reactors-Rio-de.jpg) [![Cherenkov radiation](https://cdn.britannica.com/34/163334-004-85D3FD57/Cherenkov-radiation-core-Reed-Research-Reactor-Oregon.jpg)](https://cdn.britannica.com/34/163334-050-62755157/Cherenkov-radiation-core-Reed-Research-Reactor-Oregon.jpg) [![nuclear reactor: basic components](https://cdn.britannica.com/89/62989-004-CC63E739/components-reactor.jpg)](https://cdn.britannica.com/89/62989-050-6737CF1F/components-reactor.jpg) [![reactor core](https://cdn.britannica.com/97/102097-004-FE989361/core-nuclear-reactor.jpg)](https://cdn.britannica.com/97/102097-050-1D7987E3/core-nuclear-reactor.jpg) [![nuclear fuel assembly for a pressurized-water reactor.](https://cdn.britannica.com/81/162181-004-C9F17D1F/rods-fuel-control-grid-spacers-assembly-reactor.jpg)](https://cdn.britannica.com/81/162181-050-975991EB/rods-fuel-control-grid-spacers-assembly-reactor.jpg) [![basic power cycles in nuclear power plants](https://cdn.britannica.com/13/313-004-C93A8968/power-cycles-cycle-nuclear-plants-reactor-loop.jpg)](https://cdn.britannica.com/13/313-050-8ABCA946/power-cycles-cycle-nuclear-plants-reactor-loop.jpg) [![pressurized-water reactor](https://cdn.britannica.com/96/102096-004-4018C7BB/Section-reactor-outlets-inlets-core-water-coolant.jpg)](https://cdn.britannica.com/96/102096-050-58C9DE3D/Section-reactor-outlets-inlets-core-water-coolant.jpg) At a Glance [![default image](https://cdn.britannica.com/mendel-resources/3-178/images/shared/new-thistle.svg?v=3.178.5)](https://www.britannica.com/summary/nuclear-reactor) [nuclear reactor summary](https://www.britannica.com/summary/nuclear-reactor) Quizzes [![Men of the Royal Norfolk Regiment at Aldershot now undergoing a course of revolver shooting wear gas masks while at practice in order to got used to wearing the masks under all conditions. Two Tommies sighting the target in their gas masks. (World War I)](https://cdn.britannica.com/54/200654-131-1B599C36/Men-Royal-Norfolk-Regiment-gas-masks-revolver-1915.jpg?w=200&h=200&c=crop)](https://www.britannica.com/quiz/do-you-know-which-african-american-inventor-created-which-product) [Do You Know Which African American Inventor Created Which Product?](https://www.britannica.com/quiz/do-you-know-which-african-american-inventor-created-which-product) ![Britannica AI Icon](https://cdn.britannica.com/mendel-resources/3-178/images/chatbot/star-ai.svg?v=3.178.5) Contents Ask Anything [Technology](https://www.britannica.com/browse/Technology) [Industry](https://www.britannica.com/browse/Industry) CITE Share Feedback External Websites ## Fukushima in [nuclear reactor](https://www.britannica.com/technology/nuclear-reactor) in # [Reactor safety](https://www.britannica.com/technology/nuclear-reactor/Reactor-safety) Homework Help Written by [Wade Marcum Assistant professor of nuclear engineering, Oregon State University, U.S.](https://www.britannica.com/contributor/Wade-Marcum/9281651) Wade Marcum[All](https://www.britannica.com/technology/nuclear-reactor/additional-info#contributors) Fact-checked by [Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....](https://www.britannica.com/editor/The-Editors-of-Encyclopaedia-Britannica/4419) Britannica Editors Last updated Mar. 19, 2026 •[History](https://www.britannica.com/technology/nuclear-reactor/additional-info#history) ![Britannica AI Icon](https://cdn.britannica.com/mendel-resources/3-178/images/chatbot/star-ai.svg?v=3.178.5) Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything [![damage at Fukushima Daiichi power plant](https://cdn.britannica.com/93/148893-050-26E0E6CE/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg?w=300)](https://cdn.britannica.com/93/148893-050-26E0E6CE/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg) [damage at Fukushima Daiichi power plant](https://cdn.britannica.com/93/148893-050-26E0E6CE/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg)Two of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation. (more) A failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of [nuclear power](https://www.britannica.com/technology/nuclear-power) generation (after Chernobyl)—a partial [meltdown](https://www.britannica.com/technology/meltdown) in 2011 at the [Fukushima Daiichi (“Number One”) plant](https://www.britannica.com/event/Fukushima-accident) in [Japan](https://www.britannica.com/place/Japan). That facility, located on Japan’s Pacific coast in northeastern [Fukushima](https://www.britannica.com/place/Fukushima-prefecture-Japan) prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful [earthquake](https://www.britannica.com/science/earthquake-geology) shook all units at the plant, initiating an automatic [shutdown](https://www.britannica.com/dictionary/shutdown), or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a [tsunami](https://www.britannica.com/science/tsunami) struck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup [batteries](https://www.britannica.com/dictionary/batteries) incorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of [hydrogen](https://www.britannica.com/science/hydrogen) gas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and [boric acid](https://www.britannica.com/science/boric-acid) into them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment [vessels](https://www.britannica.com/dictionary/vessels) were sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated. The Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events. In contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the [natural disaster](https://www.britannica.com/science/natural-disaster) that took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry. ## Emergency response [![Fukushima accident](https://cdn.britannica.com/60/148660-050-47FA35DE/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg?w=300)](https://cdn.britannica.com/60/148660-050-47FA35DE/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg) [Fukushima accident](https://cdn.britannica.com/60/148660-050-47FA35DE/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg)A man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami. (more) Each regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the [United States](https://www.britannica.com/place/United-States), the [NRC](https://www.britannica.com/topic/Nuclear-Regulatory-Commission) has an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant: 1. Notification of unusual events. Potential [degradation](https://www.merriam-webster.com/dictionary/degradation) in the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring. 2. Alert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected. 3. Site area emergency. Actual or likely major failures of plant functions needed for protection of the public, with [radioactivity](https://www.britannica.com/science/radioactivity) levels potentially above acceptable [thresholds](https://www.merriam-webster.com/dictionary/thresholds) at the boundary of the power plant. 4. General emergency. Actual or [imminent](https://www.merriam-webster.com/dictionary/imminent) substantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant. On a worldwide scale, the [IAEA](https://www.britannica.com/topic/International-Atomic-Energy-Agency) has developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and [radiation](https://www.britannica.com/science/radiation) sources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific [criteria](https://www.merriam-webster.com/dictionary/criteria) that have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the [environment](https://www.merriam-webster.com/dictionary/environment). The seven levels and some of the important criteria are as follows: 1. [Anomaly](https://www.merriam-webster.com/dictionary/Anomaly). Minor problems with safety components, with significant defense-in-depth remaining. 2. Incident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits. 3. Serious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation. 4. Accident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation. 5. Accident with wider [consequences](https://www.britannica.com/dictionary/consequences). Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation. 6. Serious accident. Significant release of radioactive material likely to require implementation of planned countermeasures. 7. Major accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures. [![exclusion zone: Chernobyl disaster; Fukushima accident](https://cdn.britannica.com/00/196800-050-E30A2B4A/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg)](https://cdn.britannica.com/00/196800-050-E30A2B4A/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg) [exclusion zone: Chernobyl disaster; Fukushima accident](https://cdn.britannica.com/00/196800-050-E30A2B4A/Map-exclusion-zones-accidents-Chernobyl-Fukushima-Japan.jpg)Map of the exclusion zones following the nuclear accidents at Chernobyl, Soviet Union (now in Ukraine), and Fukushima, Japan. (more) Under the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents. ## The [nuclear fuel cycle](https://www.britannica.com/technology/nuclear-fuel-cycle) [![nuclear fuel cycle](https://cdn.britannica.com/78/162178-050-43849118/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)](https://cdn.britannica.com/78/162178-050-43849118/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg) [nuclear fuel cycle](https://cdn.britannica.com/78/162178-050-43849118/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)A “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies. (more) No discussion of nuclear power is complete without a brief exposition of the [nuclear fuel](https://www.britannica.com/technology/nuclear-fuel) cycle. The whole point of a reactor is, after all, to initiate and control the process of [fission](https://www.britannica.com/science/nuclear-fission) on a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include [uranium](https://www.britannica.com/science/uranium) mining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and [irradiation](https://www.britannica.com/science/radiation-therapy) in the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and [waste disposal](https://www.britannica.com/technology/waste-disposal-system). The nuclear fuel cycle also is an [integral](https://www.merriam-webster.com/dictionary/integral) step in the production of [plutonium](https://www.britannica.com/science/plutonium) for [nuclear weapon](https://www.britannica.com/technology/nuclear-weapon)s, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy. ![Britannica AI Icon](https://cdn.britannica.com/mendel-resources/3-178/images/chatbot/star-ai.svg?v=3.178.5)Britannica AI chevron\_right Nuclear reactor - Fukushima, Meltdown, Radiation close [AI-generated answers](https://www.britannica.com/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles. # [Uranium](https://www.britannica.com/science/uranium) mining and processing [Uranium](https://www.britannica.com/technology/uranium-processing) is extracted from ores whose uranium content is often less than 0.1 percent (one part per thousand). Most ore deposits occur at or near the surface; whether they are mined through open-pit or underground techniques depends on the depth of the deposit and its slope. The mined ore is crushed and the uranium chemically extracted from it at the mouth of the mine. The residue remains naturally radioactive, as it contains long-lived radioactive daughter nuclei of uranium and has to be carefully managed to minimize the release of radioactive contaminants into the [environment](https://www.merriam-webster.com/dictionary/environment). The uranium concentrate, which is known as yellow cake, consists of uranium [compounds](https://www.merriam-webster.com/dictionary/compounds) (typically 75 to 95 percent). It is shipped to a chemical plant for further purification and chemical conversion. ## [Enrichment](https://www.britannica.com/technology/enrichment-nuclear-fuel-processing) [![uranium-enrichment processes](https://cdn.britannica.com/05/158605-050-5318BAB2/uranium-enrichment-processes.jpg)](https://cdn.britannica.com/05/158605-050-5318BAB2/uranium-enrichment-processes.jpg) [uranium-enrichment processes](https://cdn.britannica.com/05/158605-050-5318BAB2/uranium-enrichment-processes.jpg)Three uranium-enrichment processes. (more) Several enrichment techniques have been developed, though only two of these methods are used on a large scale; these are [gaseous diffusion](https://www.britannica.com/science/gaseous-diffusion) and gas centrifuging. In gaseous [diffusion](https://www.merriam-webster.com/dictionary/diffusion), natural [uranium](https://www.britannica.com/science/uranium) in the form of [uranium hexafluoride](https://www.britannica.com/science/uranium-hexafluoride) gas (UF6), a product of chemical conversion, is encouraged (through a mechanical process) to seep through a porous barrier. The molecules of 235UF6 penetrate the barrier slightly faster than those of 238UF6. Since the percentage of 235U increases by only a very small amount after traversal of the barrier, the process must be repeated over and over in thousands of stages to obtain the necessary enrichment for commercial [nuclear power](https://www.britannica.com/technology/nuclear-power) use. In gas centrifuging, the UF6 gas is fed into a high-speed centrifuge. The centrifuge is balanced very well at the top bottom and spins at an extremely high rate. Because of the relative centripetal forces that each atom experiences, the lighter species of this mixture of gaseous molecules, including 235U, tend to concentrate near the centre of the spinning centrifuge, while the heavier ones [accumulate](https://www.britannica.com/dictionary/accumulate) along the wall. These mixtures are then siphoned off. The degree of enrichment per stage in a centrifuge is greater than that obtained in a gaseous [diffusion chamber](https://www.britannica.com/science/diffusion-chamber), and the process uses less energy than gaseous diffusion does, but centrifuges are more expensive pieces of equipment. An experimental enrichment method with much commercial potential is laser separation. This process is based on the principle that [isotope](https://www.britannica.com/science/isotope)s of different [molecular weight](https://www.britannica.com/science/molecular-weight) absorb [light](https://www.britannica.com/science/light) of different frequencies. Once a specific isotope has absorbed [radiation](https://www.britannica.com/science/radiation) and has reached an excited state, its properties may become quite different from the other isotopes; it is then separated on the basis of this difference. In one method known generically as MLIS ([molecular laser isotope separation](https://www.britannica.com/science/molecular-laser-isotope-separation))—or commercially as SILEX (separation of isotopes by laser excitation)—gaseous UF6 is exposed to high-powered [lasers](https://www.britannica.com/technology/laser) tuned to the correct frequencies to cause the molecules containing 235U (but not 238U) to lose [electron](https://www.britannica.com/science/electron)s. In this (ionized) form, the 235U-containing molecules are separated from the stream on the basis of their different [electric charge](https://www.britannica.com/science/electric-charge). Proponents of laser separation claim that the method consumes less energy and wastes less starting material than, for example, gaseous diffusion. ## Fabrication This step involves the conversion of the suitably enriched product material to the chemical form desired for reactor fuel. The only fuel fabricated on a large scale is for light-water reactors (LWRs). The chemical form prepared for the LWR is [uranium dioxide](https://www.britannica.com/science/uranium-dioxide). Produced in the form of a [ceramic](https://www.britannica.com/technology/ceramic-composition-and-properties) powder, this [compound](https://www.merriam-webster.com/dictionary/compound) is ground to a very fine flourlike consistency and inserted into a die, where it is pressed into a pellet shape—in the case of some LWR fuels, approximately 6 mm in diameter and 10 mm in length (that is, about 0.25 × 0.4 inch). Next the pellet is sintered in a [furnace](https://www.britannica.com/technology/furnace) at 1,500–1,800 °C (approximately 2,700–3,300 °F). This sintering, similar to the firing of other ceramic ware, produces a dense ceramic pellet. The pellets are loaded into prefabricated [zirconium](https://www.britannica.com/science/zirconium) alloy cladding tubes, which are then filled with an [inert gas](https://www.britannica.com/science/noble-gas) and welded shut. Once the zirconium alloy tubes have been sealed, they go through significant testing to verify that there are no leaks. These tubes, called rods or pins, are then bundled together with proper spacing ensured by top and bottom manifolds through which the ends of the pins pass as well as spacer grids distributed along the middle portion of the pins. Together with other necessary hardware, the bundle [constitutes](https://www.merriam-webster.com/dictionary/constitutes) a fuel assembly. ## Fuel management Fuel is loaded into a reactor in a very specific and well-controlled pattern so as to obtain the most energy production before the material becomes unusable. Fresh fuel is more reactive than old fuel. Typically, a reactor is fueled in cycles, each cycle lasting one to two years, and a fuel batch is kept in the reactor for three or four cycles. At the end of each cycle, the oldest fuel is removed—normally this consists of about one-third the fuel content in the core—and fresh fuel loaded. The partially burned fuel that remains, however, is [shuffled](https://www.britannica.com/dictionary/shuffled) before the fresh fuel is installed. The objective of this procedure is to achieve a fuel assembly arrangement of maximum reactivity while keeping the power distribution among the different fuel assemblies as even as possible and within technical specifications. Fuel burnup—that is, energy production—is limited by two factors. After significant burnup has occurred, the physical properties of the fuel become degraded, and it is not prudent to continue to keep it in the reactor. Also, after some burnup, the old fuel no longer contributes useful reactivity to the reactor. The fuel design, including its initial enrichment, is such that these two limits are made to coincide approximately. ## Unloading and cooling [![spent nuclear fuel rods](https://cdn.britannica.com/51/238851-050-3EBCB91F/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg?w=300)](https://cdn.britannica.com/51/238851-050-3EBCB91F/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg) [spent nuclear fuel rods](https://cdn.britannica.com/51/238851-050-3EBCB91F/Spent-nuclear-fuel-rods-stored-underwater-pool-Areva-Nuclear-Plant-France.jpg)Spent nuclear fuel rods stored underwater at the nuclear power plant in La Hague, near Cherbourg, northwestern France. (more) Spent reactor fuel is extremely radioactive, and its [radioactivity](https://www.britannica.com/science/radioactivity) also makes it a source of heat (see above [Fueling and refueling LWRs](https://www.britannica.com/technology/nuclear-reactor/Types-of-reactors#ref307274)). When the spent fuel is removed from the reactor, it must continue to be both shielded and cooled. This is accomplished by placing the spent fuel in a water-storage pool, or spent-fuel cooling pool, located next to the reactor. The water in the pool contains a large amount of [dissolved](https://www.britannica.com/dictionary/dissolved) [boric acid](https://www.britannica.com/science/boric-acid), which is a strong absorber of [neutron](https://www.britannica.com/science/neutron)s; this ensures that the fuel assemblies in the pool do not go critical. (Pool water is also a common source of emergency cooling water for the reactor.) Pools vary in size; older spent-fuel cooling pools are able to accommodate only about 10 years’ worth of spent fuel. As the pools fill up, more spent fuel storage is needed. Additional storage space can be gained by loading spent fuel into the pool more densely than originally planned, by building a new pool, or by removing the oldest fuel assemblies from the existing pool and storing them in air-cooled concrete and steel silos—called spent-fuel storage casks—located aboveground. This last method becomes [feasible](https://www.merriam-webster.com/dictionary/feasible) after fuel has been stored in cooling pools for two or three years, because radioactivity and the rate of heat generation decrease rapidly over this period. Dense storage in existing pools and casks tends to be less expensive and more economical for utilities than building new pools. ## [Reprocessing](https://www.britannica.com/science/recycling) Both the converted [plutonium](https://www.britannica.com/science/plutonium) and residual [uranium-235](https://www.britannica.com/science/uranium-235) in spent fuel can be recycled by chemically reprocessing the fuel and extracting the specific elements of interest. Reprocessing not only provides a means to recycle nuclear fuel, but it also can reduce the volume and radioactivity of the waste material that must ultimately be eliminated by some method of permanent disposal. One motivation for reprocessing is ultimately to provide a “closed-loop” fuel cycle within the nuclear industry. Closed-loop refers to recycling with 100-percent [efficiency](https://www.merriam-webster.com/dictionary/efficiency) of all materials that are fabricated for use as nuclear fuel (including the most commonly used fuel, uranium dioxide pellets). Though the goal of 100-percent efficiency has yet to be attained by any country’s nuclear industry, a closed-loop fuel cycle is not an unrealistic ambition, based on current progress in reprocessing [technology](https://www.britannica.com/technology/technology). Many benefits would result from fuel recycling, including lower cost for fuel (once the recycling [infrastructure](https://www.merriam-webster.com/dictionary/infrastructure) was in place) and reduced quantities of spent fuel to be stored on reactor sites around the world. ## Reprocessing methods The most common method for reprocessing, known as the PUREX (for plutonium-uranium extraction) process, begins with dissolving the spent fuel in [nitric acid](https://www.britannica.com/science/nitric-acid) and contacting the acid solution with oil in which [tributyl phosphate](https://www.britannica.com/science/tributyl-phosphate) (TBP) has been dissolved. TBP is a [complexing](https://www.britannica.com/science/complex-in-chemistry) agent for uranium and plutonium, forming compounds with them that bring them into the oil solution. A physical separation of the (immiscible) oil and acid serves to remove the desired products from the nitric acid solution (which still contains all the fission products). The uranium and plutonium are then washed out of the TBP back into a water solution and separated from each other by various means to the degree desired. Thus, reprocessing produces three product streams: (1) a purified uranium product, (2) a plutonium product that may be either pure or mixed with uranium, and (3) a waste stream of fission products dissolved in nitric acid. ## Reprocessing policies During the period of ambitious nuclear power plant construction in the [United States](https://www.britannica.com/place/United-States) in the 1950s and ’60s, it was generally assumed that after two to five years, spent fuel would be delivered to a reprocessing plant. Some commercial reprocessing plants were built or planned, but by the mid-1970s the cost of reprocessing had escalated to a point where its economics became questionable. Also, in 1977 Pres. [Jimmy Carter](https://www.britannica.com/biography/Jimmy-Carter), in order to take a public, symbolic stand against [nuclear proliferation](https://www.britannica.com/topic/nuclear-proliferation), declared that the federal government would permanently defer all permits for the commercial reprocessing and recycling of plutonium. Carter’s directive was [rescinded](https://www.merriam-webster.com/dictionary/rescinded) by his successor, [Ronald Reagan](https://www.britannica.com/biography/Ronald-Reagan), and it has not been reinstated by any subsequent president. Even so, reprocessing is still not done commercially in the United States, partly because of the huge costs of building a reprocessing plant in a period when the supply of uranium ore has been sufficient to satisfy demand relatively cheaply. Policy and institutional arrangements have been different in [France](https://www.britannica.com/place/France) and the [United Kingdom](https://www.britannica.com/place/United-Kingdom), where commercial plants reprocess spent fuel not only from nuclear plants in the host countries but also from plants in other countries. The reprocessed plutonium can be used not only as fuel for proposed future liquid-metal reactors (LMRs) but also to help fuel existing LWRs. In the latter application, the plutonium is utilized in mixed oxide (MOX) form—a combination of uranium and plutonium dioxides having 3 to 6 percent plutonium. A number of other countries, including [Russia](https://www.britannica.com/place/Russia), [India](https://www.britannica.com/place/India), [Japan](https://www.britannica.com/place/Japan), and [China](https://www.britannica.com/place/China), reprocess their spent fuel or plan to do so. The [proactive](https://www.merriam-webster.com/dictionary/proactive) reprocessing efforts of these countries have reduced the waste scheduled for long-term disposal to amounts well below those that are accumulating in countries that do not reprocess. Load Next Page Feedback Thank you for your feedback Our editors will review what you’ve submitted and determine whether to revise the article. print Print Please select which sections you would like to print: verifiedCite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style Marcum, Wade, Spinrad, Bernard I.. "nuclear reactor". Encyclopedia Britannica, 19 Mar. 2026, https://www.britannica.com/technology/nuclear-reactor. Accessed 3 April 2026. Copy Citation Share Share to social media [Facebook](https://www.facebook.com/BRITANNICA/) [X](https://x.com/britannica) URL <https://www.britannica.com/technology/nuclear-reactor> External Websites - [United States Nuclear Regulatory Commission - Nuclear Reactor](https://www.nrc.gov/reactors.html) - [Chemistry LibreTexts - Nuclear Reactors](https://chem.libretexts.org/Courses/Los_Angeles_Trade_Technical_College/Chem_51/19%3A_Nuclear_Chemistry/19.08%3A_Nuclear_Reactors) - [University of Chicago News - The first nuclear reactor, explained](https://news.uchicago.edu/explainer/first-nuclear-reactor-explained) - [The Royal Society Publishing - Proceedings A - Challenges to deployment of twenty-first century nuclear reactor systems](https://royalsocietypublishing.org/rspa/article/473/2198/20160815/57308/Challenges-to-deployment-of-twenty-first-century) - [World Nuclear Association - Nuclear Power Reactors](https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors) - [Atomic Heritage Foundation - Theï¿½National Museum of Nuclear Science & History - Nuclear Reactors](https://ahf.nuclearmuseum.org/ahf/history/nuclear-reactors/) - [Energy Education - Nuclear reactor](https://energyeducation.ca/encyclopedia/Nuclear_reactor) - [University Network of Excellence in Nuclear Engineering - Introduction to Nuclear Reactors (PDF)](https://www.unene.ca/essentialcandu/pdf/1%20-%20Intro%20to%20Nuclear%20Reactors.pdf) - [The Institution of Engineering and Technology - Nuclear Reactor Types (PDF)](https://www.theiet.org/media/8809/nuclear-reactor-types.pdf) - [National Center for Biotechnology Information - PubMed Central - A review of the application of artificial intelligence to nuclear reactors: Where we are and what's next](https://pmc.ncbi.nlm.nih.gov/articles/PMC9988575/) - [Simon Fraser University - Nuclear Reactor Basic Principles](https://www.sfu.ca/phys/346/121/lecture_notes/lecture26_nuclear_reactors.pdf)
Readable Markdown	[damage at Fukushima Daiichi power plant](https://cdn.britannica.com/93/148893-050-26E0E6CE/Two-earthquake-containment-buildings-nuclear-power-plant-March-11-2011.jpg)Two of the damaged containment buildings at the Fukushima Daiichi nuclear power plant, northeastern Fukushima prefecture, Japan, several days after the March 11, 2011, earthquake and tsunami that crippled the installation. A failure of the main power line and a loss of backup power were at the heart of the second worst nuclear accident in the history of [nuclear power](https://www.britannica.com/technology/nuclear-power) generation (after Chernobyl)—a partial [meltdown](https://www.britannica.com/technology/meltdown) in 2011 at the [Fukushima Daiichi (“Number One”) plant](https://www.britannica.com/event/Fukushima-accident) in [Japan](https://www.britannica.com/place/Japan). That facility, located on Japan’s Pacific coast in northeastern [Fukushima](https://www.britannica.com/place/Fukushima-prefecture-Japan) prefecture, was made up of six boiling-water reactors (BWRs) constructed between 1971 and 1979, three of which were operational and one of which was under maintenance, its fuel having been stored out of the core in the reactor’s spent fuel storage pool. A powerful [earthquake](https://www.britannica.com/science/earthquake-geology) shook all units at the plant, initiating an automatic [shutdown](https://www.britannica.com/dictionary/shutdown), or scram. Immediately after the earthquake, all safety systems in each unit were operable, though a few were slightly damaged. However, less than one hour after the earthquake, a [tsunami](https://www.britannica.com/science/tsunami) struck the shoreline where the reactor units were built. The tsunami reached heights much greater than the reactors were designed to withstand, and ultimately it cut off the main power supply to the facility and damaged the backup generators by flooding their housing structures. Although the reactors withstood both an earthquake and a tsunami beyond their design requirement, the prolonged power outage drained backup [batteries](https://www.britannica.com/dictionary/batteries) incorporated into the emergency core-cooling system, which led to a loss of capability to remove decay heat. Despite the best efforts of the reactor operators and emergency responders, rising temperatures within each reactor’s core eventually caused a partial meltdown of the fuel rods, a fire in the storage reactor, explosions in the outer containment buildings (caused by a buildup of [hydrogen](https://www.britannica.com/science/hydrogen) gas), the release of radioactive steam into the air, and the leakage of radioactive water into the ocean. As workers struggled to cool and stabilize the three cores by pumping seawater and [boric acid](https://www.britannica.com/science/boric-acid) into them, government officials established a 30-km (18-mile) evacuation zone around the plant. Approximately one month after the initiating event, the reactor cores were stabilized, cracks in the foundations of the containment [vessels](https://www.britannica.com/dictionary/vessels) were sealed, and irradiated cooling water began to be pumped to a storage building until it could be properly treated. The Fukushima accident made it all too clear that another type of risk can arise from external events: earthquakes and tsunamis may not be two separate events but rather be two successive events in which an earthquake will cause structural damage to a reactor and will also initiate a tsunami. The risk associated with an earthquake of plausible magnitude is minimized by building plants away from faults and by making use of earthquake-resistant mechanical design and construction features. Furthermore, the addition of dikes and water barriers reduces the risk of damage by a tsunami. Added construction features such as water barriers must be able to withstand both an earthquake and a tsunami, as these are likely to be coupled events. In contrast to the Three Mile Island and Chernobyl accidents, which were largely blamed on staffing issues, the “weak link” in the Fukushima accident seemed upon immediate observation to be the physical plant itself rather than human error. However, because the plants were not designed to handle the [natural disaster](https://www.britannica.com/science/natural-disaster) that took place, fault can be found with the design process, in a sense pointing out human error once again as the most failure-prone component in the nuclear industry. ## Emergency response [Fukushima accident](https://cdn.britannica.com/60/148660-050-47FA35DE/safety-workers-evacuee-radiation-exposure-civilians-quarantine-March-11-2011.jpg)A man is checked for radiation exposure after having been evacuated from the quarantine area around a nuclear power station in Fukushima prefecture, Japan, that was damaged in the March 11, 2011, earthquake and tsunami. Each regulating body that oversees the operations of a country’s nuclear power has its own methods for identifying and responding to emergency conditions. In the [United States](https://www.britannica.com/place/United-States), the [NRC](https://www.britannica.com/topic/Nuclear-Regulatory-Commission) has an emergency classification system that identifies four levels of severity in conditions at a nuclear power plant: 1. Notification of unusual events. Potential [degradation](https://www.merriam-webster.com/dictionary/degradation) in the level of safety of the plant, but no release of radioactive material requiring off-site response or monitoring. 2. Alert. Actual or potential substantial degradation in the level of safety of the plant, with a release of radioactive material from the plant expected. 3. Site area emergency. Actual or likely major failures of plant functions needed for protection of the public, with [radioactivity](https://www.britannica.com/science/radioactivity) levels potentially above acceptable [thresholds](https://www.merriam-webster.com/dictionary/thresholds) at the boundary of the power plant. 4. General emergency. Actual or [imminent](https://www.merriam-webster.com/dictionary/imminent) substantial core damage or melting of reactor fuel with the potential for loss of containment integrity; radioactive material is released and may be above acceptable thresholds beyond the boundary of the power plant. On a worldwide scale, the [IAEA](https://www.britannica.com/topic/International-Atomic-Energy-Agency) has developed the International Nuclear and Radiological Event Scale (INES), to be applied to any event occurring in the agency’s signatory states that is associated with nuclear facilities and with the transport or storage of nuclear materials and [radiation](https://www.britannica.com/science/radiation) sources. The INES offers a common event scale for all parties that interact with nuclear power or radiological sources in any part of the world. The scale includes seven independent event levels; the lower three are referred to as “incidents” and the upper four as “accidents.” A declaration of a specific level is determined by identifying specific [criteria](https://www.merriam-webster.com/dictionary/criteria) that have an impact on defense-in-depth of the nuclear power plant, radiological barriers and controls, and people and the [environment](https://www.merriam-webster.com/dictionary/environment). The seven levels and some of the important criteria are as follows: 1. [Anomaly](https://www.merriam-webster.com/dictionary/Anomaly). Minor problems with safety components, with significant defense-in-depth remaining. 2. Incident. Significant contamination within the facility into an area not expected by design, with exposure of a worker in excess of the statutory annual limits. 3. Serious incident. Severe contamination in an area not expected by design, with a nonlethal health effect such as a burn on a worker from radiation. 4. Accident with local consequences. Fuel melt or damage to fuel resulting in more than 0.1 percent release of core inventory; release of significant quantities of radioactive material within an installation, with a high probability of significant public exposure and at least one death from radiation. 5. Accident with wider [consequences](https://www.britannica.com/dictionary/consequences). Severe damage to reactor core; release of large quantities of radioactive material within an installation, with a high probability of significant public exposure and several deaths from radiation. 6. Serious accident. Significant release of radioactive material likely to require implementation of planned countermeasures. 7. Major accident. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures. Under the INES, Three Mile Island is classified as Level 5, an accident with wider consequences, whereas both Fukushima and Chernobyl are Level 7, major accidents. ## The [nuclear fuel cycle](https://www.britannica.com/technology/nuclear-fuel-cycle) [nuclear fuel cycle](https://cdn.britannica.com/78/162178-050-43849118/loop-nuclear-fuel-cycle-plutonium-use-reprocessing.jpg)A “closed loop” nuclear fuel cycle, showing the reprocessing of uranium-235 and plutonium from spent fuel for use in new fuel assemblies. No discussion of nuclear power is complete without a brief exposition of the [nuclear fuel](https://www.britannica.com/technology/nuclear-fuel) cycle. The whole point of a reactor is, after all, to initiate and control the process of [fission](https://www.britannica.com/science/nuclear-fission) on a very large scale in nuclear fuel, and the low cost of fueling is the chief reason for the economic competitiveness of nuclear power. The principal steps of the fuel cycle include [uranium](https://www.britannica.com/science/uranium) mining and extraction from its ore (processing), uranium enrichment, fuel fabrication, loading and [irradiation](https://www.britannica.com/science/radiation-therapy) in the reactor (fuel management), unloading and cooling, reprocessing, waste packaging, and [waste disposal](https://www.britannica.com/technology/waste-disposal-system). The nuclear fuel cycle also is an [integral](https://www.merriam-webster.com/dictionary/integral) step in the production of [plutonium](https://www.britannica.com/science/plutonium) for [nuclear weapon](https://www.britannica.com/technology/nuclear-weapon)s, and the technologies of enrichment and reprocessing in particular have been key factors in the proliferation of these weapons around the world. For this reason and also for a host of other political, environmental, and economic reasons, the various steps in the nuclear fuel cycle are closely regulated and frequently observed under terms of international treaties. Conflicts between some countries’ nuclear ambitions and various international conventions have sometimes generated great controversy. [AI-generated answers](https://www.britannica.com/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles.
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