Development of an External Radiation Dose Estimation Model for Children Returning to Their Homes in Areas Affected by the Fukushima Nuclear Accident

Abstract

On 1 April 2017, around 6 y after the Fukushima Daiichi nuclear power station accident, evacuation orders for large affected areas were lifted, and areas to which people could return were expanded. In the current study, a dose estimation model based on a probabilistic approach has been developed to estimate the external radiation doses children would receive after returning to these areas. The target groups are children from infants to high school students, and the target areas are nine municipalities including evacuation areas as of 5 September 2015. The estimation period is for 4 y starting 1 April 2017. Validation of the model in an area for which individual personal dosimetry measurements were available showed that it is valid for infants, kindergarteners, 3rd and 4th grade elementary school students, 5th and 6th grade elementary school students, and junior high school students. Considering the statement of the International Commission on Radiological Protection, the estimated radiation doses for these five age groups were taken to be the 95th percentiles of the predicted distributions as an index of conservative judgement. As a result of our estimations, the 95th percentile doses to all age groups were less than 20 mSv y−1 in all periods and in all areas. The 95th percentile doses in some areas were less than 1 mSv y−1, which is the long-term dosimetric target set by the Japanese government. It should be noted that our results are preliminary, being based on several assumptions and limitations regarding environmental contamination conditions and the behavioral patterns of children. To estimate the children’s doses precisely, further considerations for these assumptions and limitations will be needed.

Low dose of external exposure among returnees to former evacuation areas: a cross-sectional all-municipality joint study following the 2011 Fukushima Daiichi nuclear power plant incident

“…Recent research efforts have estimated dose levels among individuals returning to former no-go zones through a variety of modelling methods [17,18]. However, few dose assessments have been conducted based on actual measurements.…”

Individual Doses to the Public after the Fukushima Nuclear Accident

“…Then, probabilistic dose assessments were made for these population groups by considering the spatial variability of ambient dose rate and interpopulation differences resulting from behavior patterns. Mori et al [20] applied the probabilistic approach to estimate the external doses that children would receive after returning to evacuated areas. Application of this approach is described in another paper of this journal in more detail [8].…”

Age Dependence of Individual External Doses in an Early Stage After the Fukushima Nuclear Accident

“…These doses can differ from person to person even in the same area; they vary by such characteristics as age, behavior (or evacuation) pattern after the accident, food intake and so on. In the case of the external dose integrated for a period, the difference can be caused by: (1) a location factor depending on ambient doses for sites (indoors/outdoors) where residents spent time after the accident; (2) an occupancy factor depending on daily time budget for the sites; and (3) a body size factor depending on extent of absorption and scattering of gamma rays in bodies of different sizes (2)(3)(4) . The location factor is a similar concept to "shielding factor" or "dose reduction factor" (5) .…”