The government will raise the maximum ambient radiation level target for the cleanup operation around the stricken Fukushima No. 1 nuclear plant from an hourly 0.23 microsieverts to 0.3-0.6 microsieverts.
The Environment Ministry made the decision after analysis showing that people living in contaminated zones would still have an annual dose of less than the 1 millisievert maximum even at the new, higher hourly target. While relaxing the 0.23 microsievert hourly maximum (which does not include normal background radiation) will make the cleanup operation more efficient, residents of the municipalities affected may find the sudden shift upwards of the “safe” dose worrying.
The ministry will present the revised number to municipalities concerned at a meeting sometime soon. At the same time, the government will present a plan to distribute dosimeters to residents to track their individual doses. Individual exposure varies depending on the person’s location and their daily activities, and the ministry plans to use this detailed data to adjust the intensity of the cleanup operations to best suit local conditions and focus on areas of the most need.
In short, the government is looking to shift away from the blanket cleanup operation of the past three years to prioritizing reducing the radiation doses of residents.
Environment Ministry guidelines issued for the cleanup areas in 2011 advised residents to stay indoors 16 hours a day, adding that the dose absorbed inside a wooden structure was 40 percent of that when in the open air. The ministry also stated that it had calculated the maximum hourly ambient radiation level at 0.23 microsieverts for residents to stay under the prescribed 1 millisievert per year dose.
Based on the ministry advisory, the cities of Fukushima, Koriyama and other local bodies drew up plans to clear away contaminated soil to get the radiation level below the 0.23 microsievert per hour mark. Authorities discovered, however, that the lower the radioactive contamination in a particular spot, the less effective was the cleanup operation in getting rid of it. There have been many cases where radiation levels have remained stubbornly above 0.23 microsieverts per hour even after decontamination, sparking resident demands that cleanups be repeated.
The cities of Date and Soma, meanwhile, distributed dosimeters to their residents in affected areas, and compared ambient radiation levels with the actual doses absorbed by their citizens. Environment Ministry analysis of the data showed that residents living in areas with an ambient radiation level of 0.3-0.6 microsieverts per hour for the most part stayed under the maximum annual dose of 1 millisievert.
With these results in hand, the ministry declared that it was “necessary to respond to the actual situation with respect to resident’s radiation dose levels” and embarked on the policy shift.
July 31, 2014(Mainichi Japan)