Evaluation of Natural Radiation Dose of People in Aomori Prefecture and 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.
Natural radiation consists of cosmic rays, environmental γ-rays and radiation from Rn and internalradionuclides. The cosmic ray dose rate can be estimated from the height above sea level and the magnetic
latitude of the target area. Radon concentrations have already been measured throughout the prefecture, anddose rates from Rn were evaluated. 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 wasalso evaluated by analyzing 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 northwestern areaof Shimokita Peninsula was measured using glass dosimeters in FY 2009. The dose rates ranged from 22 to39 nGy h-1, and 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 for agricultural farm workers in a study
done during FYs 2008-2009 and 0.59 mSv for office workers in a study done during FYs 2006-2009. Over
98% of these doses were attributed to 210Po, 40K, 210Pb, 14C and 228Ra. Analytical results of vegetables, fruits,mushroom and processed foods (tofu, kon-nyaku and natto) showed that the committed effective doses fromnatural radionuclides ingested annually through the products were 0.022 and 0.019 mSv for the farm andoffice workers, respectively, corresponding to 6 and 5% of each committed effective dose.
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, Rn,
and internal radionuclides, have been planned to be measured during FYs 2006 - 2010. The study for the fox
was started from FY 2009 succeeding that for the small mammals. Environmental γ-ray dose rates and Rnconcentrations were measured in and around burrows for medium-sized mammalians, presumably foxes, inthe southern part of Rokkasho during June-November 2009. A female Japanese red fox (Vulpes vulpesjaponica) and a female raccoon dog (Nyctereutes procyonoides viverrinus) were caught with traps set
around some of the burrows. After sacrificing them, their body burdens of natural radionuclides wereanalyzed. Before dissecting the red fox, its MRI tomography images were taken for construction of a voxel
phantom. The environmental γ-ray dose rates ranged from 17 to 33 nGy h-1 in and around the burrows, andthe dose in the burrows decreased with distance from their entrance. Ranges of the mean 222Rnconcentrations in and around the burrows were 2.5E+1 - 1.4E+3 Bq m-3 and 4 - 6 Bq m-3, respectively. The

radon equilibrium factor varied from 0.29 to 0.43 around the burrows. The mean internal dose rates of thered fox and the raccoon dog were estimated as 56 nGy h-1 and 0.44 μGy h-1, respectively, with doseconversion factors using a very simple phantom. The difference between them was mainly caused by the12times higher body burden of 210Po in the raccoon dog than the red fox. Voxel phantoms which had internalorgans structure were produced for a mouse (Apodemus argenteus), a mole (Urotrichus talpoides) and a fox.
The small animal phantoms were made by size adjustment of the Digimouse, which is a mouse voxel
phantom published by the South California University. The phantom of the fox was constructed from theMRI tomography images for the red fox caught in this study.
NJapan Nuclear Fuel Limited

Spent nuclear fuel reprocessing plantArea 1

Trap area 1-a●●Trap area 1-b●●

Area 2

Trap area 2-b
Trap area 2-aInternal dose rate (nGy h-1)

500
400300

Others
228Th
226Ra
210Pb
210Po
40K

200
1000FoxCommitted effective dose (mSv)

Fig. 1 Sampling sites (●) for medium-sized mammals
in the southern part of Rokkasho

1.2Others
228Ra
210Pb
210Po
40K *

1.0
0.8
0.6
0.4
0.2
0.0
Office worker

Agricultural farm worker

Fig. 2 Mean committed effective dose fromnatural radionuclides ingested annuallythrough total diet samples of agriculturalfarm workers in FYs 2008-2009 and ofoffice workers during FYs 2006-2009 inRokkasho Village and Aomori City. Eacherror bar shows the standard deviation ofresults from 16 farm workers and 32 officeworkers, respectively. “Others” means thesum of doses for 14C, 137Cs, 226Ra, 228,232Th,and 238U.

Raccoon dog

Mice

Moles

Fig. 3 Mean internal radiation dose rates of a fox, araccoon dog, mice and moles. The rates anderror bars of mice and moles show the meanand standard deviations of results from 20
samples in beech (2006), oak (2007) andconiferous (2008) forests of Rokkasho.
“Others” means the sum of doses for 137Cs,
228
Ra, 232Th, and 234,238U.

(A)(B)

Fig. 4 Diagonal views of (A) a mouse (Apodemus argenteus) and (B) afox (Vulpes vulpes japonica) for voxel phantoms