Estimation of Dose Rate for the Large Japanese Field Mouse (Apodemus speciosus) Distributed in the “Difficult-to-Return Zone” in Fukushima Prefecture

Following the Fukushima Daiichi Nuclear Power Plant (FNPP) accident, dose rate for Muridae species in forests of Iitate Village, Fukushima, was estimated as 3.9 mGy/day over the first 30 days. According to the derived consideration reference levels (DCRLs) determined by the International Commission on Radiological Protection (ICRP), this dose level could be affecting reproduction of these rodents. However, information on dose rate for forest rodents after 2012 is limited. Therefore, the dose rate of forest rodents was calculated for large Japanese field mice (Apodemus speciosus) captured in the “difficult-to-return zone” in the Fukushima Prefecture from 2012 to 2016. External dose rate was calculated based on the ambient dose equivalent rate of gamma-radiation at the ground level of the trapping site. Internal dose rate was simulated using the EGS5 program based on cesium (Cs)-137 concentrations in the captured mice. Combining the external and internal doses, the total daily dose rate for the mice within the zone was estimated to be 0.201–0.547 mGy/day. In addition, the ratio of external dose rate to total dose rate was estimated to be 61.2–95.4%. Thus, it is concluded that the present radiation exposure of the field mice distributed in the trapping site did not affect their reproduction. However, it must be noticed that total dose rate exceeding 0.1 mGy/day, which offers very low probability of the occurrence of certain effects according to the DCRLs determined by ICRP, is still present in most of the zone (September 2018, Nuclear Regulation Authority, Japan). Thus, various indexes should be applied to evaluate the exposure effects on the field mice in this zone.