Saturday, 9 November 2013

So now 20 mSv/year is OK!

It's just been announced that the Nuclear Regulation Authority is proposing that up to 20 mSv/year of additional air-borne radiation poses no risk to health. This will form part of government guidelines for repatriation of evacuees to be announced at the end of the year. Well, well, well! This has been such a touchy subject. After the accident people were evacuated from areas over 20 mSv/year. The ICRP said at the time that the range in such an emergency situation was between 1 and 20 mSv. But there was an outcry when the government first announced a level of 20 mSv - it seemed too close to evacuation levels. What was safe? Everyone was confused. The government's knee-jerk reaction was to lower the 'safe' level to 1 mSv - that became the target for decontamination, and people here came to believe that exposure over 1 mSv/year was not safe. There were even some ugly scenes when experts arguing for less strict standards were heckled by residents. 

The meter in my local park today showing 0.281 μSv/hour. Falling very gradually
but still not down to the magic figure of 0.23 μSv/hour
Here in Koriyama, 1 mSv/year is the target for decontamination work currently underway. By a strange logic this is worked out to 0.23 μSv (microsieverts)/hour. But as I have said many times, the equation is based on being outdoors for 8 hours of the day and spending the other 16 hours in a house constructed of wood. This is unrealistic for most people. When I carried a dosimeter for a month in June my estimated accumulated additional exposure came to only 0.3 mSv/year - way below 1 mSv. There are still places in Koriyama registering 0.5 or 0.6 μSv/hour but for most people this would still come to under 1 mSv/year. So I'm convinced Koriyama is safe.

But what of the evacuated areas? Pretty soon those in the 'hard to return areas' of 50 mSv/year are going to be told they can't go back. In the surrounding areas (the 'restricted residence areas' of 20 - 50 mSv, and the 'preparation for the lifting of the ban areas' up to 20 mSv/year) it's become evident that levels can't be reduced to 1 mSv/year. Many evacuees are understandably reluctant to return as they don't think levels of say 5 mSv/year are safe - 1 mSv has become the yardstick in the popular perception. Today's news is a move in the right direction but people will need convincing. The governor of Fukushima has long campaigned for the government to provide scientific evidence for the 1 mSv level. This time round the government's going to have to do a better job explaining the new standards to the public.
Measuring radiation is such a tricky and emotive subject.
More about this next time.

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