At 2:46:23 PM JST on 11 March 2011, the Great Tohoku Earthquake occurred off the coast of Tohoku peninsula, Japan, with a magnitude of 9.0, making it the fourth strongest earthquake worldwide since 1900. The quake produced a destructive tsunami striking the Pacific coast of northern Honshu, Japan, attaining heights of at least 12 meters. Major aftershocks included one of magnitude 7.9 occurring 29 minutes after the main shock, and five more over the next 24 hours with magnitudes from 6.5 to 7.7. Total casualties from the earthquake and tsunami (as of 18 July 2012) were 15,867 killed, 2,906 missing, and 6,109 injured; this includes earthquake and tsunami casualties within Fukushima prefecture numbering 1,606 killed, 212 missing, and 182 injured.
The Fukushima Daiichi nuclear power plant is located on the Pacific coast of northeast Honshu, Japan, and included six boiling water power reactors. Unit 1 was completed in 1970 with a net capacity of 439 MW(e); units 2-5 were completed in 1973, 1974, 1978, and 1977, respectively, each with a net capacity of 760 MW(e); and unit 6 was completed in 1979 with a net capacity of 1067 MW(e). In 2011 units 1, 2, and 4-6 used uranium fuel while unit 3 used MOX (mixed uranium and plutonium oxide) fuel. The plant is located 4 km from Futaba town (population 1,200), 5 km from Okuma (population 11,000), 9 km from Tomioka (population 16,000), 24 km from Minamisoma (population 71,000) and 60 km from Fukushima city (population 292,000).
On 11 March 2011, units 1, 2, and 3 of Fukushima Daiichi were operating, unit 4 was defueled (with fuel rods moved to the spent fuel pond), and units 5 and 6 were in cold shutdown. The main Tokoku quake, at 2:46 PM with epicenter 150 km from Fukushima Daiichi, produced ground motion at the reactors with east-west accelerations of 0.44-0.52 gees (comparable to the design requirement to withstand acceleration of 0.45 gees). This resulted in loss of primary power maintaining coolant to the reactors. Units 1-3 were automatically shut down at 2:48 PM, with control rods successfully inserted, and emergency diesel power generators began providing coolant to the reactors. The tsunami struck Fukushima Daiichi 55 minutes later with a height of 8-14 meters, well over the 5.7 meter height the plant was designed to withstand. Two workers in turbine building of unit 4 were killed by the tsunami. The backup diesel generators failed as as result of flooding from the tsunami. Backup coolant systems (RCIC or HPCI) continued providing cooling to units 1-3, but these systems failed by the following day as battery power was depleted.
As cooling to units 1-3 failed, temperatures and pressure in the cores increased; heat at this point was from decay of fission products, not from nuclear reactions which had ended when the reactors were shut down. Controlled venting of unit 1 began on 12 March, but at 3:36 PM JST on 12 March a hydrogen explosion destroyed the roof of the outer building at unit 1, causing non-radiation injuries to 4 workers. Workers began injecting seawater into unit 1 later that day. On 13 March controlled venting began for unit 3, followed by injection of seawater, but at 11:15 AM JST on 14 March a hydrogen explosion destroyed the roof of the outer building of unit 3, causing non-radiation injuries to 11 workers. Later on 14 March the plant operator reported that fuel rods in the cores of units 1-3 were not fully covered by water, and that fuel damage was suspected at all three units; seawater injection into unit 2 began along with controlled venting the next day. At 6:14 AM JST on 15 March a hydrogen explosion occurred at unit 2, apparently damaging the containment vessel. Spent fuel ponds in units 1-3 were all exposed to atmosphere at this point, with these ponds near the top of the reactor buildings.
At about 9:40 AM JST on 15 March a fire started at the spent fuel pond in unit 4; in contrast to the other units on site, all of unit 4's fuel rods had been tranferred to the spent fuel pond during its current shutdown. The fire was extinguished within a few hours, but recurred briefly on the following day. Water in the spent fuel pond at unit 3 likely was boiling by 16 March, when steam began coming from the building. Workers began efforts to spray seawater into the spent fuel ponds at unit 3 on 17 March and units 1-2 and 4 on 18 March. Efforts also concentrated on restoring off-site power to the reactors, with this partially accomplished for units 1, 2, 5, and 6 on 21 March and units 3 and 4 on 22 March. Core cooling to the unit 1 reactor was increased 24 March after its temperature rose above its design limit.
On 24 April three workers were moving a power line in the turbine room of unit 3 when water overflowed into their boots and set off their personal radiation monitors; they proceeded to complete the work anyway and were subsequently determined to have resulting exposures of 17-18 rems. Two of the workers were temporarily hospitalized with possible beta radiation burns to their legs, with local skin doses estimated at 200-300 rems; all were released 28 April with no treatment required. As of 31 March the plant operator reported that 21 workers had received doses of at least 10 rem, but none exceeding 25 rem (the regulatory limit for emergency workers).
Japanese authorities began evacuating civilians from within 3 km of the plant on 11 March, the first day. This evacuation radius was extended to 10 km and then to 20 km on 12 March; this evacuation was completed on 16 March and included about 70,000 people. Residents to a distance of 30 km were asked on 15 March to voluntarily evacuate or to stay indoors. Radiation levels in local food and in Fukushima tap water reportly exceeded regulatory limits on 19 March, and Japanese authorities placed various food restrictions over the following days (these radiation levels are far below levels for acute radiation injury, and the restrictions appear adequate with regard to chronic radiation risk). Drinking water restrictions were generally lifted by 2 April. On 25 March authorities reported that 66 children from near Fukushima were examined for possible thyroid exposure, but no significant exposure was identified. Similar results were reported for 1,083 children screened 26-30 March. Through 31 March authorities had screened 114,488 evacuees for radioactivity; detectable radioactivity was found on 102 people but limited to their clothing in all cases.
Radiation releases into discharge water on site were reported on 22 March, and plutonium contamination of soil on site was reported 28 March. On 2 April the plant operator determined that contaminated water was leaking from the unit 2 turbine building into the ocean. Several efforts to stop this leak were implemented without success. On 4 April Japanese authorities gave the operator permission to discharge low-level contaminated water into the ocean to empty tanks for storage of high-level contaminated water from unit 2.
Japanese authorities estimated that about 10 megacuries of radioactive iodine and cesium had been released through 12 April, with most of this release in the first week or so.
Consequences: 17 injuries--15 from mechanical or thermal effects of explosions, 2 from localized radiation exposure.
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