Fluctuation factors of ambient dose rates in the vicinity of Hamaoka nuclear power plant

It is important to clearly distinguish between variations in natural ambient dose rates and those by small accidents at nuclear facilities from the viewpoint of radiation control. Therefore, we must have exact data of ambient dose rates during daily life. Ambient dose rates are influenced by meteorological conditions as well as geographical and geological features. We investigated the factors which cause fluctuations in ambient dose rates on fine and rainy days around Hamaoka nuclear power plant in Shizuoka Prefecture. Fluctuations in ambient dose rates depended on the concentration of radon daughters on fine days, whereas on rainy days they depended on gamma-rays from the uranium-series. Contributions of gamma-rays from the thorium-series and potassium-40 to fluctuations in ambient dose rates were not observed. (author)