Sunday, 13 March 2011
The earthquake and tsunami that battered northern Japan on Friday set in motion one of the worst nuclear accidents in over two decades.
The International Atomic Energy Agency rates the severity of radiological events, with a scale starting at one, an “anomaly,” and rising to seven, a “major” accident. Six and seven designate full meltdown, where the nuclear fuel or core of a reactor overheats and melts. The scale of the ensuing uncontrolled release of radiation that follows differentiates the two. Partial meltdowns, in which the fuel is damaged, are rated a four or a five.
The accident at Chernobyl in the former Soviet Union in 1986 — which killed 56 people directly and thousands of others through cancer and other diseases — was the only nuclear accident so far to have been designated a seven. Just one other accident has surpassed five on the scale: an explosion of dried radioactive waste at the Mayak Nuclear Power Plant near the Soviet city of Kyshtym in 1957. The blast produced a radioactive cloud that spread for hundreds of miles over what is now Russia, forcing the evacuation of 10,000 people and causing the deaths of at least 200.
The Mayak blast was rated a six on the atomic agency’s scale.
The full extent of the damage at the Fukushima Daiichi Nuclear Power Station in Japan is yet to be determined. On Saturday, before emergency measures were announced at a second reactor at that plant, Japanese nuclear safety experts rated the accident a four, putting it just behind the Three Mile Island accident in 1979 near Harrisburg, Pa. That accident, the worst in United States history, was designated a five.
That accident was caused by what began as a seemingly minor plumbing glitch. A valve that opened to reduce pressure in the reactor failed to close, letting cooling water escape and leading the core to overheat. That set in motion a series of missteps by the machines and plant operators monitoring the reactor, a crisis that almost led to a full meltdown.
Officials monitoring the Daiichi (or No. 1) plant in Fukushima have said they detected a radioactive byproduct, cesium, that could indicate that some of the nuclear fuel in Reactor No. 1 was damaged and a partial meltdown had occurred. Officials at the plant filled the reactor with seawater to prevent a full meltdown. But early Sunday, they were struggling to inject water into another reactor. The government issued evacuation orders for about 200,000 people in the surrounding area.
Even as the accident continued to unfold, it was already considered worse than the most severe nuclear accident in Japanese history. In 1999, at a plant just outside Tokyo, a team of operators put a batch of highly enriched uranium in a precipitation tank that was not designed to handle it, setting off a critical reaction. Two operators died from radiation poisoning, and dozens of workers and people living nearby were hospitalized.
Posted by Reg Illingworth at 07:50