Japanese authorities are to start detailed studies which could lead to a re-assessment of the evacuation measures in Fukushima prefecture.
The Ministry of Economy, Trade and Industry (METI) today summarised the overall level of safety at the Fukushima Daiichi nuclear power plant. It then revealed what it called a conservative analysis of potential effects on people living 20 kilometres from the plant after various loss of coolant scenarios.
The calculated effects were small enough for METI to say it would evaluate the possibilities in more detail using the latest data, and consider the results in the context of a revision of the evacuation measures.
At present nobody is allowed to live within 20 kilometres of the Fukushima Daiichi plant, while people in the zone between 20 and 30 kilometres away have been told to be prepared to evacuate. In addition, some other areas such as Itate village have been evacuated due to radiation dose rates over 20 millisieverts per year. This prolonged displacement of about 100,000 people has been by far the primary negative effect of the nuclear accident on health and wellbeing.
In its plant status run-down, METI noted that cooling was in place for the melted 'comparatively stable' reactor cores, while back-ups for pumps, pipes, power supply, and water supply were in place. In addition, the risk of a hydrogen explosion was very low due to the injection of nitrogen to the cores. Analysis of the damaged buildings has shown that they are not in danger of collapse in earthquake or tsunami scenarios.
At the same time, radiation releases from the plant have all but stopped. Recent air sampling at the site border has shown no detection of iodine-131, caesium-134 or caesium-137 and METI noted the maximum dose rates from newly released radioactivity of 1.7 millisieverts per year at the site border. This decreases substantially with distance.
Against this backdrop, METI has considered two sets of scenarios for loss of cooling and analysed the effect this could have on people living 20 kilometres away from the plant. With the exact status of the Fukushima Daiichi cores unknown, the first set of scenarios assumes the more likely status of near total melt; the second set assumes a 'substantial' amount of fuel still present above the core support plate in unit 3.
METI considered the effect of loss of coolant incidents across the site lasting five, ten and 15 hours. For the first set, it found that the additional internal and external radiation dose to people living 20 kilometres away would be 0.17 millisieverts - less than the 10 millisievert maximum permitted for a nuclear emergency.
For comparison, the global average radiation dose per year from all sources is 2.4 millisieverts per year.
Most at risk during a nuclear accident are children with fast-growing thyroid glands that would absorb and concentrate iodine-131. The total dose to an infant thyroid gland under METI's first set of scenarios would be 0.038 millisieverts, which the ministry said was also small.
For the second set of scenarios, where more fuel is assumed in place within unit 3, the situation was more serious but still considered small by METI. Total internal and external radiation dose to people living 20 kilometres away was no more than 2.3 millisieverts, with children's thyroids receiving less than 1.0 millisieverts.
In that incident scenario, METI said that an adult living 20 kilometres away from the plant could receive 17 millisieverts over the course of a year, but noted the possibility to make countermeasures during the time.
The ministry is to go on to conduct more detailed analyses, based on the latest data from the plant. It will consider a range of possible events and the degrees to which they may affect people living in the area now covered by evacuation precautions.
Aside from the risk of further incidents at the plant and their possible effects, a separate question for METI will be the acceptable dose rate to members of the public from radioactive materials already present on the ground. However, chief secretary of the cabinet Yukiyo Edano has previously said that the evacuation orders are primarily in place due to the possibility of sudden deterioration at the plant, rather than specifically to measured dose rates.
Researched and written
by World Nuclear News