The disaster that struck Japan's Fukushima Daiichi nuclear power station on March 11, 2011, caused the most extensive release of radioactivity since the Chernobyl accident in 1986 and was far worse than the 1979 Three Mile Island accident in the United States. Unlike at Chernobyl and Three Mile Island, the destruction at Fukushima was initiated by natural disasters—a huge earthquake and tsunami—rather than equipment failure and human error. The tsunami knocked out backup power systems that were needed to cool the reactors at the plant, causing several of them to undergo fuel melting, hydrogen explosions, and radioactive releases. Studies of the Fukushima disaster have identified design changes, response actions, and other safety improvements that could have reduced or eliminated the amount of radioactivity released from the plant. As a result, Fukushima has prompted a reexamination of nuclear plant safety requirements around the world, including the United States. Radioactive contamination from the Fukushima plant forced the evacuation of communities up to 25 miles away, affecting up to 100,000 residents, many of whom remain indefinitely barred from their homes. The evacuations are believed to have prevented radiation exposure among the population from exceeding Japanese regulatory limits in most cases. Near-term deaths and illnesses resulting from radiation are believed to be unlikely, although cancer and other long-term health effects remain possible. Workers at the plant site were exposed to far higher radiation levels, with at least two suffering radiation burns on their legs after wading in contaminated water. Two other workers drowned during the tsunami. Recovery from the disaster has focused on restoring cooling systems at the three most severely damaged reactors at the six-unit plant and halting radioactive emissions into the air and water. That work has been hampered by continuing high levels of radiation in the plant and severe structural damage. The Japanese government announced December 16, 2011, that the damaged Fukushima reactors had achieved "cold shutdown," a milestone in which the reactor cooling water is below boiling temperature at atmospheric pressure. At cold shutdown, the diminished threat of further radioactive releases may allow some residents to begin returning to the least contaminated evacuation zones. Japan's environment minister announced December 19, 2011, that about $15 billion had been allocated for decontaminating the area around the Fukushima Daiichi plant, an unprecedented undertaking. Complete decommissioning and dismantlement of the plant is expected to take 40 years, and the total cost of the disaster was recently estimated by a Japanese government committee to exceed $75 billion. The Institute of Nuclear Power Operations (INPO), a safety organization established by the U.S. nuclear power industry after the Three Mile Island accident, issued a detailed description of the Fukushima accident in November 2011. The INPO report provides timelines of the response actions taken at each unit of the Fukushima Daiichi plant and the sequences of events that led to major reactor core damage and radioactive releases. It is intended "to provide an accurate, consolidated source of information" about the event. However, the report notes, "Because of the extensive damage at the site, some details of the event remain unknown or have not been confirmed." The purpose of this CRS report is to highlight the aspects of the Fukushima disaster that could bear on U.S. nuclear plant safety and nuclear energy policy in general. It provides a brief explanation of the Fukushima events, including new details provided by the INPO report, a general discussion of the consequences of the disaster, and a description of U.S. assistance provided to Japan. Other CRS reports provide additional information and analysis: CRS Report R41805, Nuclear Power Plant Design and Seismic Safety Considerations , by [author name scrubbed] and [author name scrubbed]. CRS Report R41728, The Japanese Nuclear Incident: Technical Aspects , by Jonathan Medalia (for information on nuclear plant radioactive releases and effects). CRS Report RL33558, Nuclear Energy Policy , by [author name scrubbed] (for information on U.S. nuclear safety responses to Fukushima). CRS Report R41751, Effects of Radiation from Fukushima Daiichi on the U.S. Marine Environment , by [author name scrubbed] and [author name scrubbed]. The earthquake on March 11, 2011, off the east coast of Honshu, Japan's largest island, reportedly caused an automatic shutdown of 11 of Japan's 55 operating nuclear power plants. Most of the shutdowns proceeded without incident. However, the plants closest to the epicenter, Fukushima and Onagawa (see Figure 1 ), were damaged by the earthquake and resulting tsunami. The Fukushima Daiichi plant subsequently suffered hydrogen explosions and severe nuclear fuel damage, releasing significant amounts of radioactive material into the environment. Tokyo Electric Power Company (TEPCO) operates the Fukushima nuclear power complex in the Futaba district of Fukushima prefecture in Northern Japan, consisting of six nuclear units at the Fukushima Daiichi station and four nuclear units at the Fukushima Daini station. All the units at the Fukushima complex are boiling water reactors (BWRs), with reactors 1 to 5 at the Fukushima Daiichi site being the General Electric Mark I design (see Figure 2 ). The Fukushima Daiichi reactors entered commercial operation in the years from 1971 (reactor 1) to 1979 (reactor 6). Nuclear reactors produce power through the fission (splitting) of the nuclei of heavy isotopes, such as uranium-235 and plutonium-239, resulting from the absorption of neutrons. Each fission event generates additional neutrons that induce more fission events, creating a continuous nuclear chain reaction. The heavy nuclei split into lighter isotopes called fission products, many of which are highly radioactive, such as iodine-129, iodine-131, strontium-90, and cesium-137. To shut down the nuclear chain reaction, neutron-absorbing control rods are inserted into the reactor core. However, even though the fission process has stopped, the fission products and other radioactive isotopes in the reactor core continue to generate significant heat through radioactive decay. Until the decay heat sufficiently diminishes, a source of electricity is needed to operate pumps and circulate water in the reactor. Under normal conditions, it would take a few days for a reactor core to cool down to a "cold shutdown" state. Reactors 1, 2, and 3 at Fukushima Daiichi were operating and automatically shut down when the quake struck, while reactors 4, 5, and 6 were already shut down for routine inspections. All four of the Fukushima Daini reactors were operating at the time of the earthquake and taken down after the quake. Although horizontal ground acceleration from the magnitude 9.0 earthquake exceeded the maximum designed level at two of the operating Fukushima Daiichi units (2 and 3), all the operating units maintained normal cooling immediately after the emergency shutdown. However, the earthquake triggered a tsunami that struck the Fukushima Daiichi station about 40 minutes later, devastating much of the area and overtopping the plant's six-meter-high seawall. TEPCO estimated the tsunami's height at Fukushima Daiichi to be 14 meters (46 feet). The station was cut off from Japan's national electricity grid, leaving the plant dependent on backup diesel generators for alternating current (AC) power. The tsunami flooded most of the generators and electrical switchgear rooms, knocking out the backup AC power for cooling the nuclear reactors in units 1-5. An air-cooled diesel generator at Unit 6 survived the tsunami and provided backup power to cool unit 5 and 6, which therefore did not suffer fuel damage. Although backup batteries at the Fukushima Daiichi plant were designed to provide direct current (DC) power for eight hours, most of the DC power was lost when the tsunami flooded the plant's DC power distribution systems. As a result, the shared control room for units 1 and 2 went completely dark, and the control room for units 3 and 4 had only emergency lighting available. TEPCO immediately began to experience problems cooling the Fukushima Daiichi reactors. With alternating current no longer available to power the primary and secondary cooling systems, and batteries for backup cooling and control systems out of service, TEPCO began trying to cool the reactor cores with seawater. Neutron-absorbing boron was added to the seawater to prevent restart of the nuclear chain reaction. Despite those efforts, cooling water levels in the reactor cores remained low for many days, causing the reactors to overheat. TEPCO estimated on December 2, 2011, that all of the nuclear fuel in Unit 1 had melted and that much of it had leaked from the reactor pressure vessel into the primary containment. Melting was considered likely for 57% of the fuel in Unit 2 and 63% of Unit 3. Because heat removal systems failed, pressure built up to high levels in the primary containments of units 1 through 3. The containments were vented to the atmosphere to relieve the pressure, allowing the escape of radioactivity leaking from the reactor cores. A chemical reaction between the fuel's zirconium cladding and high-temperature steam is believed to have generated large amounts of hydrogen in the containments of units 1 through 3. The hydrogen leaked from the containments or the venting systems into the reactor buildings and caused large explosions in units 1, 3, and 4. Japanese safety regulators estimated that each unit had produced 800-1,000 kilograms of hydrogen. The hydrogen explosions caused tremendous damage and hindered TEPCO's efforts to restore plant cooling capability. The loss of cooling also affected the plant's spent fuel pools (shown in Figure 2 ), which hold fuel rods that have been removed from the reactors after their ability to sustain a nuclear chain reaction has diminished. Although much of the radioactivity in the spent fuel has been decaying for many years, the large volumes of spent fuel in the pools represent a significant total heat load.