At 14:46 local time on 11 March, a magnitude 9 earthquake struck off Japan’s north-east coast.
The 11 operating nuclear power reactors in the region all “tripped” as designed (the nuclear fission process was stopped).
However, the fuel in a nuclear reactor continues to produce considerable amounts of heat even when fission has stopped, and the key task – the main plot in this drama – is to keep water circulating over the fuel to remove that decay heat. This is to prevent damage to the fuel rods, and to the containment around the reactor – the thick steel pressure vessel and the surrounding concrete structure designed to keep fissile material isolated from the outside world in all circumstances.
Eleven Reactors in total affected
Reactors 1-3 at Fukushima Daiichi (Reactors 4-6 were not operating)- Four at Fukushima Daini
- Three at Onagawa
- One at Tokai
Mains electric power to the pumps providing this cooling water was lost in the earthquake, so back-up diesel generators kicked in, again as designed, and alllooked good. But an hour later the tsunami hit, taking out the diesel generators and the oil storage tanks. Fukushima Daiichi was designed to withstand a six-metre tsunami – 15 metres was just too much.
All the reactors except Daiichi 1-3 were brought into “cold shutdown”, with water circulating as required, some after minor problems. But at the three oldest Fukushima plants, connected to the grid between 1970 and 1974, the loss of power to the pumps led to water in the pressure vessel boiling and the fuel heating up hugely.
“estimates suggesting that water levels are now above half-way up the fuel in the core of Reactors 1-3, enough to introduce an element of stability.”
Guidelines for Japan’s nuclear plants fail to account for worst-case earthquake and tsunami scenarios and need to be revised, said the head of a government committee drafting new seismic-safety standards, raising the prospect of further delays in the government’s push to get idled reactors around Japan restarted.
“All plants, not just new ones, must be able to withstand a 9.0 magnitude earthquake and a 15-meter tsunami,” said Kojiro Irikura, a professor emeritus of seismology at Kyoto University who chairs the standard-writing body for Japan’s Nuclear Safety Commission, in an interview. He said revising the standards, which are used in so-called stress tests of reactors, “will take years, not months”
The worst in the history of Japan — severely damaged the Tokyo Electric Power Co. (TEPCO)-operated Fukushima No. 1 nuclear complex, located along the coast of the towns of Futaba and Okuma in Fukushima Prefecture. As a result of the disaster, all external power sources were lost, causing the supply of cooling water to the plant’s No. 1, 2 and 3 reactors to stop. Hydrogen was generated as a result of a chemical reaction between fuel rods and water, leading to hydrogen explosions which badly damaged reactor buildings. The government, which initially estimated the accident level at 4 on the International Nuclear Event Scale (INES), later raised the level to 7 — the highest rank. This matched the level of the Chernobyl catastrophe, which at that stage was the worst nuclear accident in history.
TIMELINE OF EVENTS
8.15 p.m.: The Japanese government declares an emergency at Fukushima Daiichi power plant.
10.30 p.m.: Authorities reveal the cooling system at the plant is not working, and admit they are “bracing for the worst.”
Saturday, March 12
2.06 a.m.: Radiation levels in the No.1 reactor at Fukushima are reported to be rising.
3.24 a.m.: Japanese trade minister Banri Kaieda warns that a small radiation leak could occur at the plant.
6.45 a.m.: TEPCO says radioactive substances may have leaked at Fukushima.
Japan’s Nuclear and Industrial Safety Agency says radiation near the plant’s main gate is more than eight times the normal level.
4.19 p.m.: Japan’s Nuclear and Industrial Agency reveals a small amount of radioactive cesium has escaped from the power plant, possibly caused by a fuel rod melting.
EXPLOSION: 6.22 p.m.: A hydrogen explosion at Fukushima’s reactor No.3 blows the roof off the containment structure around the No.1 reactor and injures four people.
8.18 p.m.: Residents living within 20 kilometers of the plant are told to evacuate the area. Some 200,000 people leave.
REFERENCES:
1. http://www.guardian.co.uk/world/2011/sep/09/fukushima-japan-nuclear-disaster-aftermath
2. http://articles.cnn.com/2011-03-15/world/japan.nuclear.disaster.timeline_1_power-plant-reactor-containment-structure?_s=PM:WORLD
4. http://www.shimbun.denki.or.jp/en/news/20110614_01.html
5. http://mdn.mainichi.jp/mdnnews/news/20110910p2a00m0na008000c.html