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I think I may have identified the potential class action lawsuit that the US Government might be nervous about. There was a news story several months ago which announced that the death toll of 1656 for people currently living in evacuation camps in the ƒukushima prefecture now exceeds the death toll of 1607 that occurred in the original tsunami event in 2011. The 1656 evacuee deaths are apparently due to stress and other illnesses related to the disaster. I assume there have been no radiation-induced deaths amongst the evacuees, otherwise the BBC and the Guardian would have been rather enthusiastically reporting such deaths and I'm sure I would have heard about it.
The problem with the super-conservative radiation limits for evacuation is that they don't appear to recognise the potential mental health problems that might be associated with evacuating more people than is necessary. A radioactive contamination hazard will be different from other hazards requiring evacuation in that it is likely to require a much lengthier evacuation, which might even be permanent, and it also has a unique feature where a powerful lobby group (Big Green), assisted by the mainstream news media, has a vested interest in making the evacuees think they are 'doomed' or have been subject to the most terrible of injustices, which won't help anybody with mental health issues.
There has also been a news story in the last day or two that the ƒukushima nuclear power plant operator, TEPCO, has been successfully sued by the family of a Japanese woman who committed suicide and has been ordered to pay nearly $500K in compensation.
I would deduce from this recent story that TEPCO must be responsible for setting the evacuation criteria in Japan rather than the Japanese government, and I would guess TEPCO have adopted something like USEPA criteria for the evacuation.
I'm not sure what we do about evacuation criteria for nuclear accidents and other radioactive contamination incidents in the UK. The only nuclear accident we have had in the UK was the 1957 Windscale fire. No evacuation was carried out for that incident even though it did involve a reasonably big release of radioactive material, probably because Big Green didn't exist back in those days.