Structure shielding of school and gymnasium buildings against fallout gamma radiation from the Fukushima Daiichi Nuclear Power Plant accident

The radiation dose reduction factor represents a building's shielding capability against radiation in scenarios of potential nuclear accidents. Notably, the radiation dose reduction for school buildings and gymnasiums have not yet been determined, even though the building types are planned to be used as evaluation shelters in Japan. This study evaluated dose reduction factors in relation to the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). In this study, we separately measured the indoor and outdoor ambient dose equivalent rates and the dose rates from natural radionuclides at two school sites in an evacuation-order-lifted area in Fukushima Prefecture, Japan, to evaluate the dose reduction factors of the school buildings and gymnasiums. The factors depended not only on the structure and materials of the buildings, but also on the geographical characteristics of the building locations, which could govern the geometries of irradiation. We noted a non-negligible overestimation of the dose reduction factors by the conventional method, due to the presence of natural radiation in areas with low dose rates (<1 mu Sv/h). Note that the dose reduction factor is used as an uncertainty source for external dose evaluation related to the FDNPP accident. This study can serve as a foundation for future investigations on improving the resilience of buildings confronted by artificial radiation due to scenarios of nuclear plant accidents.