Development of discrimination method for the effects from nuclear facilities on environmental dose rate measurement

The final commissioning test (the Active Test) was carried out at the Rokkasho Reprocessing Plant from March 31, 2006. We recognized the influence of the Active Test on the environmental γ-dose rate when the plant was in operation. Alternatively, the environmental γ-dose rate can also be changed by natural phenomena (e.g. precipitation, snow cover, etc). In this research, we developed a method to discriminate the effects from nuclear facilities on measurement of environmental γ-dose rate by estimation of contribution from natural radiation. We judged that SCA (Bi), a counting rate of γ-rays (1.76 MeV, 2.20 MeV) emitted from 214Bi natural radioactivity, was a good index for determining increase of dose rate by precipitation, because there was a positive correlation between SCA(Bi) and environmental γ-dose rate. Additionally, we considered that SCA(Tl), a counting rate of γ-rays (2.62 MeV) emitted from 208Tl natural radioactivity, was a good index for measuring depression of dose rate by snow cover on the ground because SCA(Tl) was almost unchanged by precipitation. We obtained the following equation (1) by multiple linear regression analysis using SCA(Bi) and SCA(Tl) as parameters, and derived 'Estimated natural dose rate'. We determined that the difference between the observed data and Estimated natural dose rate' indicated the magnitude of the influence from nuclear facilities or other artificial sources of radiation (e.g. medical radiation). Estimated natural dose rate=a SCA(Bi)+b SCA(Tl)+c (1) where a, b and c are constants. We obtained the equation using data obtained at Obuchi monitoring station from April 2005 to March 2006 and calculated the difference between the observed data and the 'Estimated natural dose rate'. The mean value and standard deviation of the calculated results was 0.00±0.010 nGy/h (99% confidence interval) and 0.36 nGy/h, respectively. We could discriminate the effects of the Active Test on the environmental γ-dose rate observed from April 2006 at Obuchi monitoring station by utilizing this method. (author)