Natural Radiation Dose of People in Aomori Prefecture and Development of the Dose
Evaluation Method for the Forest Ecosystem in Rokkasho
Yoshihito OHTSUKA, Takashi IYOGI, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
The natural background radiation dose is important for evaluating the significance of the dose from thesmall amounts of radionuclides that are being released from the first commercial spent nuclear fuelreprocessing plant in Rokkasho, Japan. In this study, the background radiation dose of Aomori Prefecture
residents was assessed and a dose evaluation method for the forest ecosystem in Rokkasho was developed tostudy environmental protection.
Although we previously measured environmental γ-ray dose rates throughout the prefecture,measurement points around Rokkasho were scarce. In this study, the dose rates were measured in areas
surrounding Rokkasho to get their detailed distribution. The internal dose rate was also evaluated byanalyzing radionuclides in the so-called total diet and individual food item samples collected in Rokkasho
and Aomori City. The cumulative γ-ray dose at five locations in the southwestern area of ShimokitaPeninsula was measured using glass dosimeters in FY 2010. The dose rates were within the range of the dose
rates in Aomori Prefecture.
The sums of the committed effective dose due to annual ingestion of natural radionuclides in the total
diet samples and the effective dose rate from 40K cited in the literature were estimated to be 0.55 mSv forlivestock farm workers in a study done in FY 2010 and 0.58 mSv for office workers in a study done during
FYs 2006-2010. Household-numbers weighted mean dose for Aomori Prefecture was evaluated as 0.58 mSv
from the doses for fishery, agricultural, livestock and office workers in Aomori Prefecture in a study done
during FYs 2006-2010. Analytical results of edible wild plants and beverages including seasoning,
confectioneries and pickles showed that the committed effective doses from natural radionuclides ingested
annually through the products were 0.023 and 0.020 mSv for the farm and office workers, respectively.
Mice, moles, and the fox as their predator were selected as representative animals of the forest
ecosystem in this study, and their radiation doses derived from natural sources, i.e. environmental γ-rays, Rnand internal radionuclides, were measured during FYs 2006 - 2010. The study for the fox starting in FY 2009
was continued in FY 2010. Environmental γ-ray dose rates and Rn concentrations were measured in burrows
for medium-sized mammalians, presumably foxes, in the northern part of Rokkasho during June-November
2010. A female Japanese red fox (Vulpes vulpes japonica) and a female raccoon dog (Nyctereutes
procyonoides viverrinus) were caught with traps set around some of the burrows. After sacrificing them, theirbody burdens of natural radionuclides were analyzed. The environmental γ-ray dose rates ranged from 15 to61 nGy h-1 in the burrows, and the dose in the burrows decreased with distance from their entrance. Rangesof the mean 222Rn concentrations in the burrows were 4.1E+1 - 2.1E+3 Bq m-3. The radon equilibrium factor
varied from 0.18 to 0.27 around the burrows. Mean 210Pb and 210Po concentrations in the body of the raccoon
dog were two and eight times higher than that in the red fox, respectively, while mean 40K concentrations inboth animals were similar to each other.
Radiation dose rates from 222Rn and its progenies in the lung of mouse (Apodeums argenteus), shrewmole (Urotrichus talpoides) and middle-size mammalians were calculated on the assumption of theircircadian life history as 1.1E+2 nGy h-1, 1.7E+2 nGy h-1 and 1.2E+2 nGy h-1, respectively, and the dose ratesof bronchus were 1.4E+2 nGy h-1, 2.3E+2 nGy h-1 and 2.0E+2 nGy h-1, respectively.
External dose rates of the mouse, the shrew mole and Japanese red fox were evaluated by using therespective phantoms that had been constructed in FY 2009 and a Monte Carlo radiation dose calculation code(EGS4). Respective mean external dose rates were evaluated as 51 nGy h-1, 51 nGy h-1 and 34 nGy h-1 for themouse, the shrew mole and the red fox. Mean internal dose rates of the mouse, shrew mole, red fox andraccoon dog were estimated as 37 nGy h-1, 4.0E+2 nGy h-1, 50 nGy h-1 and 2.3E+2 nGy h-1, respectively.