Dose Assessment of Natural Radiation and Distribution of Natural –emitting
Radionuclides in the Environment
Yoshihito OHTSUKA, Takashi IYOGI, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Natural background radiation doses for residents and the ecosystem in Rokkasho, Japan are importantfor evaluating the significance of the dose from radionuclides discharged from the first commercial spent
nuclear fuel reprocessing plant located there. The aims of this study were to establish methods for measuring
the environmental -ray radiation dose to people in their actual living environment and the natural radiationdose to biota in the aquatic environment of Lake Obuchi neighboring the plant. Since the background levelof natural -emitting radionuclides is also useful as a reference for such radionuclides as Pu assumed to bedischarged from the plant, this study also had another purpose of obtaining the concentration level of natural
-emitting radionuclides in various environmental samples.
Environmental -ray dose rates to people living in Aomori City were measured for 1 week in eachseason in FY 2013 by using personal electrical dosimeters that were able to log data. Dose rates in various
working and living environments were also measured for a year with glass dosimeters. The mean personalradiation dose for 10 volunteers was 38 nGy h-1 and higher than the value of 32 nGy h-1 estimated using
measurement results in various environments obtained with glass dosimeters and the mean spending time ineach environment from the literature. The values of this study were measured by personal dosimeters which
had been calibrated by 226Ra absorbed dose in air. Self-dose of the detectors and the cosmic ray dose werecorrected by using data from control detectors in a box shielded with 5-cm-thick lead.
To estimate external radiation dose to biota in Lake Obuchi, radiation dose in the lake water and its out
flow, Obuchi River, were measured with glass dosimeters placed in watertight boxes and submerged in thewater. Concentrations of radionuclides in Pacific herring (Clupea pallasii) were also measured to estimate
internal dose to the fish. The radiation dose rate at the surface of the lake water was found to be 27 nGy h-1,and the dose rate exponentially decreased with depth until a point 1 m above the bottom sediment. Dose ratein the Obuchi River was measured as 30 nGy h-1 which was slightly higher than the value in the lake surface
due to gamma-rays from bottom sediment. The radionuclide in the highest concentration in the Pacificherring was 40K. The fish internal dose rate was evaluated to be 75 nGy h-1 using dose conversion
coefficients of pelagic fish calculated by FASSET with ~50% contribution by 210Po to the dose rate. A voxel
phantom of the Pacific herring was constructed by using a three-dimensional scanner for future dose
calculations.
Natural -emitting radionuclides were determined for samples of various environmental media in
Rokkasho: soil, sediment, lake water, precipitation, and aerosol samples. Rice paddy soil samples from fivesites in Rokkasho were analyzed, and it was found that 210Po was present in the highest concentration among
natural -emitting radionuclides. Mean concentration of 210Po in the 0 – 60 cm depth of these soil samples
was about 650 times larger than the assumed 238, 239, 240Pu concentrations discharged from the plant.
Polonium-210 also was found in the highest concentration of alpha-emitters in sediment samples in Lake

Obuchi, and atmospheric deposition and aerosol samples collected in IES, while 238U originating fromseawater was the highest in lake water samples from Lake Obuchi.