On March 11, 2011, a 15 metre tsunami after a major earthquake disabled the power and colling of three Fukushima Daiichi reactors, causing a nuclear accident. All three cores largely melted in the first three days of the accident. The accident was rated a 7 on the INES scale, due to high radioactive releases over days 4 to 6.
Eleven reactors at four nuclear power plants in the region were operating at the time of the eathquake and all shut done automatically. The operating units which shut down were Tokyo Electric Power Company’s (Tepco) Fukushima Daiichi 1, 2, 3, and Fukushima Daini 1, 2, 3, 4, Tohoku’s Onagawa 1, 2, 3, and Japco’s Tokai, total 9377 MWe net. Fukushima Daiichi units 4, 5 & 6 were not operating at the time, but were affected. The main problem initially centred on Fukushima Daiichi units 1-3. The tsunami was essentially the issue for the reactors, not the earthquake.
Power from the grid or backup gnerators was available to run the Residual Heat Removal system cooling pump. The three reactors at Fukushima Daiichi lost power an hour after the earthquake, when the entire site was flooded by the tsunami. This disables 12 of 13 backup generators on site and also the heat exchangers for dumping reactor watse heat and decay heat to the sea. The three units lost the ability to maintian proper reactor cooling and water circulation functions. Many weeks of hard work by hundreds fo Tepco employees centered on restoring heat removal from the reactors and coping with overheated spent fuel ponds. Some of the Tepco staff had lost homes, and even families in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. Three Tepco employees at the Daiichi and Daini plants were killed directly by the eathquake and tsunami, but none were killed from the nuclear accident.
The Fukushima Daiichi reactors are GE boiling water reactors. Below is a diagram of the reactor.
When the power failed at the site (about an hour after shutdown of the fission reactions), the reactor cores were still producting about 1.5% of their norminal thermal power from fission product decay. Without heat removal by circulation to an outside heat exchanger, this produced a lot of steam in the reactor pressure vessels housing the cores, and was released into the dry primary containment through saftey valves. Later this was accompanied by hydrogen, produced by the interaction of the fuel’s hot zirconium cladding with steam after the water level dropped.
The disaster at Fukushima has raised much internation concern about the future of nuclear energy, but Japan is working to improve their future by building a wall of ice to stem the Fukushima leak. the project is expected to be completed by March 2015, costing $320 million and using a substantial amount of power (enough power each day to run 3300 Japanese households). The country’s government decided a wall of ice is the best solution to stem the flow of radioactive water leaking from Fukushima Daiichi’s four stricken nuclear reactors. The wall will stop 400 tons of groundwater being containimated everyday. It is currently being stored in huge tanks.
three years after the accident, engineers working on site
Resources:
http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Fukushima-Accident/
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