Fukushima Disaster

Fukushima I nuclear accidents

The Fukushima I nuclear accidents (福島第一原子力発 電所事故, Fukushima Dai-ichi  are series of ongoing equipment failures and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the 2011 Tōhoku earthquake and tsunami at 14:46 JST on 11 March 2011. The plant comprises six separate boiling water reactors maintained by the Tokyo Electric Power Company (TEPCO). Reactors 4, 5 and 6 had been shut down prior to the earthquake for planned maintenance.^[3] The remaining reactors were shut down automatically after the earthquake, but the subsequent 14 metres (46 ft) tsunami^[4] flooded the plant, knocking out emergency generators needed to run pumps which cool and control the reactors. The flooding and earthquake damage prevented assistance being brought from elsewhere.

Evidence arose of partial core meltdown in reactors 1, 2, and 3; hydrogen explosions destroyed the upper cladding of the buildings housing reactors 1, 3, and 4; an explosion damaged the containment inside reactor 2; and multiple fires broke out at reactor 4. In addition, spent fuel rods stored in spent fuel pools of units 1–4 began to overheat as water levels in the pools dropped. Fears of radiation leaks led to a 20 kilometres (12 mi) radius evacuation around the plant. Workers at the plant suffered radiation exposure and were temporarily evacuated at various times. On 18 March, Japanese officials designated the magnitude of the danger at reactors 1, 2 and 3 at level 5 on the 7 point International Nuclear Event Scale (INES).^[5] Power was restored to parts of the plant from 20 March, but machinery damaged by floods, fires and explosions remained inoperable.^[6]

Measurements taken by the Japanese science ministry and education ministry in areas of northern Japan 30-50 km from the plant showed radioactive caesium levels high enough to cause concern.^[7] Food grown in the area was banned from sale. World wide measurements of iodine-131 and caesium-137 suggested release levels had reached 2/3 of those from the Chernobyl accident, although at Chernobyl this was accompanied by releases of many other radioactive materials from the reactor fire.^[8] Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants.^[9][10] Plutonium contamination has been detected in the soil at two sites in the plant.^[11]

The IAEA announced on 27 March that workers hospitalized as a precaution on 25 March had been exposed to between 2 and 6 Sv of radiation at their ankles when standing in water in unit 3.^[12][13][14] The international reaction to the accidents was also concerned. The Japanese government and TEPCO have been criticized for poor communication with the public.^[15][16] On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be closed once the crisis was over.^[17] On 30 March 2011, the president of Tepco, Masataka Shimizu, was hospitalised with symptoms of dizziness and high blood pressure.^[18]

Fukushima I Nuclear Power Plant

The Fukushima I Nuclear Power Plant is located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan, about 210 kilometers (130 miles) north of Tokyo.^[102] It consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).

Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971.^[103] It was designed for a peak ground acceleration of 0.18 g (1.74 m/s²) and a response spectrum based on the 1952 Kern County earthquake.^[104] Units 2 and 3 are both 784 MWe type BWR-4 reactors, Unit 2 commenced operating in July 1974 and Unit 3 in March 1976. The design basis for Units 3 and 6 were 0.45 g (4.41 m/s²) and 0.46 g (4.48 m/s²) respectively.^[105] All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s²) for 30 seconds, but no damage to the critical parts of the reactor was discovered.^[104]

Units 1–5 have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure.^[104] From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.^[106]

At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:^[107]

Radiation levels and radioactive contamination

Radioactive material was released from containment on several occasions after the tsunami struck, the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Using Japanese Nuclear Safety Commission numbers, Asahi Shimbun reported that by 24 March the accident might have emitted 30,000 to 110,000 TBq of iodine-131.^[292] On the INES scale, the accident would rate 6 rather than the official level 5, according to the newspaper. The radiation dose rate at one location between reactor units 3 and 4 was measured at 400 mSv/h at 10:22 JST, 13 March, causing experts to urge rapid rotation of emergency crews as a method of limiting exposure to radiation.^[293] 1 Sv/h were reported (but not confirmed by the IAEA)^[2] close to the leaking reactor units on 16 March, prompting a temporary evacuation of plant workers, with radiation levels subsequently dropping back to 800–600 millisieverts.^[43] On 29 March, at times near unit 2, radiation monitoring was hampered by a belief that some radiation levels may be higher than 1000 mSv/hr, but that "1,000 millisieverts is the upper limit of their measuring devices."^[211] The maximum permissible dose for Japanese nuclear workers was increased to 250 mSv/year, for emergency situations after the accidents.^[294][295]

The Japanese Ministry of Health, Labour and Welfare announced that levels of radioactivity exceeding legal limits had been detected in milk produced in the Fukushima area and in certain vegetables in Ibaraki. Measurements made by Japan in a number of locations have shown the presence of radionuclides on the ground.^[296] On 23 March, Tokyo drinking water exceeded the safe level for infants, prompting the government to distribute bottled water to families with infants.^[297] World wide measurements of wind-born radioactive iodine and caesium vented from reactors suggest that levels have reached around 2/3 those during the Chernobyl disaster, though in that case other radioactive material was also released.^[8]