	Introduction and Overview

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. 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 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). All four of the Fukushima Daini reactors were operating at the time of the earthquake and taken down after the quake. 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. The hydrogen leaked from the containments or the venting systems into the reactor buildings and caused large explosions in units 1, 3, and 4. 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. Diesel-generated backup power had been available at units 5 and 6 since March 19. Top priorities were restoring core cooling to units 1-3 and to the spent fuel pools in units 1-4 and eliminating discharges of highly contaminated water into the ocean. Unit 3

Unit 3 was generating electricity and shut down automatically during the earthquake and then lost AC power during the tsunami. Unit 4

Unit 4 was out of service for maintenance when the earthquake struck. The station reportedly retained offsite power to maintain its ability to circulate cooling water in the reactor. U.S. Assistance

The United States and other countries, as well as the International Atomic Energy Agency, are providing assistance to Japan to deal with the nuclear crisis. The United States provided advice and equipment, including transport of pumps, boron, fresh water, remote cameras, use of Global Hawk surveillance drones, evacuation support, medical support, and decontamination and radiation monitoring equipment.
The huge earthquake and tsunami that struck Japan's Fukushima Daiichi nuclear power station on March 11, 2011, knocked out backup power systems that were needed to cool the reactors at the plant, causing three of them to undergo fuel melting, hydrogen explosions, and radioactive releases. Radioactive contamination from the Fukushima plant forced the evacuation of communities up to 25 miles away and affected up to 100,000 residents, although it did not cause any immediate deaths.
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, with reactors 1 to 5 at the Fukushima Daiichi site being the General Electric Mark I design, which is also used in the United States. The Fukushima Daiichi reactors entered commercial operation in the years from 1971 (reactor 1) to 1979 (reactor 6). The Fukushima Daini reactors shut down automatically after the earthquake and were able to maintain sufficient cooling.
When the earthquake struck, Fukushima Daiichi units 1, 2, and 3 were generating electricity and shut down automatically. The earthquake caused offsite power supplies to be lost, and backup diesel generators started up as designed to supply backup power. However, the subsequent tsunami flooded the electrical switchgear for the diesel generators, causing most AC power in units 1 to 4 to be lost. Because Unit 4 was undergoing a maintenance shutdown, all of its nuclear fuel had been removed and placed in the unit's spent fuel storage pool. One generator continued operating to cool units 5 and 6.
The loss of all AC power in units 1 to 3 prevented valves and pumps from operating that were needed to remove heat and pressure that was being generated by the radioactive decay of the nuclear fuel in the reactor cores. As the fuel rods in the reactor cores overheated, they reacted with steam to produce large amounts of hydrogen, which escaped into the unit 1, 3, and 4 reactor buildings and exploded (the hydrogen that exploded in Unit 4 is believed to have come from Unit 3). The explosions interfered with efforts by plant workers to restore cooling and helped spread radioactivity. Cooling was also lost in the reactors' spent fuel pools, although recent analysis has found that no significant overheating took place.
Radioactive material released into the atmosphere produced extremely high radiation dose rates near the plant and left large areas of land uninhabitable, especially to the northwest of the plant. Contaminated water from the plant was discharged into the sea, creating international controversy.
The United States and other countries, as well as the International Atomic Energy Agency, are providing assistance to Japan to deal with the nuclear disater. U.S. assistance has included transport of pumps, boron, fresh water, remote cameras, use of Global Hawk surveillance drones, evacuation support, medical support, and decontamination and radiation monitoring equipment.
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 in the United States.