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 level ofnatural -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 in various working and living environments in Hirosaki City weremeasured for a year in FY 2015 with glass dosimeters. Environmental -ray dose rates to people living in thecity were also measured for 1 week in each season by using personal electronic dosimeters that were able tolog data. Using results obtained, we examined whether measured dose rates in various environments canreproduce personal dose rates. The mean personal dose rate for 10 volunteers was found as 0.18 mSv y-1 andslightly lower than the value of 0.22±0.03 mSv y-1 estimated using measurement results in various
environments obtained with glass dosimeters and the mean spending time in each environment from theliterature. This showed that the personal dose rate measured by the personal dosimeter could be substantially
reproduced by the combination of the measured dose rates in various environments and the mean spendingtime.
To estimate radiation dose to biota in the Obuchi River, Pacific oyster (Crassostrea gigas) and blue mussel(Mytilus galloprovincialis) were collected and analyzed for radionuclides to obtain internal radiation dose rates.
Voxel phantoms of the bivalves were constructed on the basis of their X-ray computed tomographic (CT)images for accurately estimating their radiation dose rates. Internal and external dose rates of chum salmon(Oncorhynchus keta), starry flounder (Platichthys stellatus), oyster and blue mussel were estimated by theMonte Carlo code (EGS5) using each voxel phantom developed by us, getting the dose rates of 1.3E+2 nGyh-1, 1.1E+2 nGy h-1, 1.4E+2, and 1.7E+2 nGy h-1, respectively. Polonium-210 burden in their bodies
contributed 46 – 80% of total dose rates.
Natural -emitting radionuclides were determined for samples of various environmental media in
Rokkasho: soil, lake water, precipitation and aerosol samples. Soil samples from sites in five forests in
Rokkasho were analyzed, and we found that 210Po in the surface layer was in the highest concentration among
natural -emitting radionuclides in most of the sites. Mean concentration of 210Po in litter and soil samples in
0 to ~5 cm depth was found to be 9,800 and 4,300 times larger than the assumed 238, 239, 240Pu concentrations
discharged from the plant, respectively. In surface water samples collected in the Obuchi River and LakeObuchi in and after rainfall, 238U had the highest concentration among the natural -emitting radionuclides.

The natural -emitting radionuclide concentrations in the water samples were not affected by the rainfall.
Daily -emitting radionuclide concentrations in atmospheric deposition and aerosol samples were observed
during about one week in three types of weather conditions: rain, Kosa (yellow dust) and snow. Polonium-210
was found to have the highest concentration of alpha-emitters in both deposition and aerosol samples in every

Dose rate (nGy h-1)

weather condition studied.

200150External
Internal (others)
Internal（40K）
Internal（210Po）
Cosmic ray

10050Fig. 1

Blue mussel

OysterFlat fishSalmon0

Internal and external dose rates of salmon (Oncorhynchus keta), flat fish (Platichthys stellatus), oyster(Crassostrea gigas Thunberg) and blue mussel (Mytilus galloprovincialis) estimated by the Monte Carlocode (EGS5) using each voxel phantom developed by us. Radiation dose rates from cosmic rays isevaluated from vertical distribution of dose rates measured by using glass dosimeters in Lake Obuchi
during FYs 2011 – 2013.