"The earthquake on March 11, 2011, off the east coast of Honshu, Japan's largest island, reportedly caused an automatic shutdown of eleven of Japan's fifty-five 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 probable 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 Daiichi station and four nuclear units at the Daini station. All the units at the Fukushima complex are boiling water reactors, with reactors 1 to 5 at the 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). At the time of the earthquake, reactors 1, 2, and 3 at Daiichi were operating and shut down after the quake, while reactors 4, 5, and 6 were already shut down for routine inspections. All four of the Daini reactors were operating at the time of the earthquake and taken down after the quake. Nuclear reactors produce power by fissioning (splitting) the nuclei of heavy isotopes, such as uranium-235 and plutonium-239, through 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."
CRS Report for Congress, R41694
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