On April 8, 2011, 28 days after the nuclear accident at the Fukushima nuclear power plant (NPP), IRSN published on its website the first worldwide map of doses likely to be received by the Japanese population as a result of external irradiation occurring the 1st year following the accident. This map was derived from dose rate data collected by the US DoE/NNSA based on airborne measurements and published on their website on April 7, 2011. The IRSN map revealed significant external doses in a northwest zone from the NPP, about 20 km in width and 50 to 70 km in depth. Other dose maps were then produced and published by DoE/NNSA on April 18, 2011 and more recently by the Japan „Ministry of Education, Culture, Sports, Science and Technology“ (MEXT), on the 44th day after accident. These dose maps were consistent with the first dose assessment carried out by IRSN and show dose values of the same order of magnitude (difference less than a factor of 2.5). On the 56th day after the accident, MEXT published the first maps of caesium depositions. They revealed high values comparable with the most contaminated areas of Chernobyl, even beyond the initial 20 km-radius evacuation zone around the Fukushima plant. A new dose assessment was carried out by IRSN on the 66th day after the accident to estimate projected doses due to external exposure from radioactive deposits, for exposure durations of 3 months, 1 year and 4 years before evacuation.
The estimated projected doses reach particularly significant values, some of them even above 200 mSv, which are no longer in the range of „low doses“ according to UNSCEAR definition. Moreover these dose levels do not take into account neither the doses received from other pathways such as immersion within the plume and inhalation of particles in the plume during the accident nor the doses already received or to be received from ingestion of contaminated foodstuffs. The total effective doses to be received (external + internal) could be much higher according to the type of deposit (dry or wet), diet and source of food.
The number of Japanese people living in the most contaminated areas outside the initial 20 kmradius evacuation zone around the Fukushima plant (874 km2 with caesium 134+137 deposits higher than 600,000 Bq/m2) was estimated to 70,000 people including 9,500 children of 0-14 years in age. This significant number reaches about 26% of that of Chernobyl (270,000 people) for a surface area only 8.5% of that of Chernobyl (10,300 km2).
IRSN have also studied: the impact of the selection of a dose reference level, within the range of 20 to 100 mSv recommended by ICRP in emergency situations, on the number of people to be evacuated; averted doses for these populations resulting from an evacuation according to 3 different scenarios: evacuation 3 months, 1 year or 4 years after the accident. The level of projected external doses in upcoming years – up to 4 Sv lifetime dose in the most contaminated areas (30 million Bq/m2 of caesium-137 + 134) – requires the implementation of protective actions such as evacuation of population. Report DRPH/2011-010 5/28
Assessment on the 66th day of projected external doses from the nuclear accident in Fukushima According to the ICRP recommendations in emergency situations, the selection of the highest protective reference level, i.e. 20 mSv, would avert external doses above this level for 15,000 to 20,000 people.
If the Japanese authorities decide to take an even more protective reference level, for example 10 mSv for the 1st year, the averted external doses for the affected populations (70,000 people) would be much higher if the evacuation is quickly prescribed. An evacuation one year after the accident would result in a 59% decrease of the projected external dose for this population; evacuation three months after the accident would result in an 82% decrease. This policy for preventing the risk of developing long-term leukaemia and radiation-induced cancer has been clearly understood by the Japanese authorities as shown in the map of population evacuation beyond the initial zone of exclusion of 20 km brought to the IRSN knowledge on May 16, i.e. the 66th day after the accident. The prescribed evacuation area seems to meet the 20 mSv reference level – the most protective dose value within the range recommended by ICRP in an emergency situation. This decision made by the Japanese authorities proves retrospectively the relevance of the IRSN’s radiological assessment map – the first to have been published worldwide, 28 days after the accident. IRSN-Report DRPH/2011-10