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 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 Noheji Town wasmeasured using glass dosimeters in FY 2008. The dose rates ranged from 22 to 28 nGy h-1, and were lower
than the mean dose rate 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 theliterature were estimated to be 0.56 mSv for agricultural farm workers in FY 2008 and 0.65 mSv for office
workers during FYs 2006-2008. Over 98% of these doses were attributed to 210Po, 40K, 210Pb, 14C and 228Ra.
Analytical results of polished rice and processed cereals (breads and noodles) showed that the committed
effective doses from natural radionuclides ingested annually through the products were 0.049 and 0.042 mSvfor the farm and office workers, respectively, corresponding to 13 and 9% of each committed effective dose.
Small mammals, mice and moles, and a predator, the fox, were selected as representative animals of theforest 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 from FYs 2006 - 2010. The smallmammals in coniferous forests were caught using traps, and their natural radionuclides burdens wereanalyzed in 2008. Environmental γ-ray dose rates and Rn concentrations were also measured in the forests.
The mean internal dose rates of the mouse (Apodemus argenteus) and mole (Urotrichus talpoides) were
estimated to be 44 nGy h-1 and 0.50 μGy h-1, respectively, with dose conversion factors using a very simple
phantom. An almost 10 times higher body burden of 210Po in moles than mice led to this difference. The
environmental γ-ray dose rates ranged from 24 to 31 nGy h-1, and the range of the mean 222Rn concentration
was 3 - 10 Bq m-3 during May-December 2008. The radon equilibrium factor varied from 0.16 to 0.33, being
lower than those reported for general outdoor environments. All these results for coniferous forests werecomparable with those for beech forests and oak forests obtained in FYs 2006 and 2007, respectively.

N

Area No. 1
1-1
1-2
1-3

Area No. 2

Area No. 3
3-1
3-2
3-3

2-1
2-2
2-3
核燃料サイクル施設
Japan Nuclear Fuel Limited

再処理施設
Spent nuclear fuel reprocessingplant

4-1 4-2

4-3

Area No. 4
Coniferous forest

Committed effective dose (mSv)

1.2
Others
C-14
Ra-228
Pb-210Po-210
K-40

1.0
0.8
0.6
0.4
0.2
0.0
Office worker

Agricultural farm worker

Fig. 2 Mean committed effective dose from naturalradionuclides ingested annually through total
diet samples of agricultural farm workers in FY
2008 and of office workers during FYs of2006-2008 in Rokkasho Village and AomoriCity. Each error bar shows a standard deviationof results from 8 farm workers and 24 office
workers, respectively.

Internal dose rate (nGy h-1)

Fig. 1 Sampling sites (○) for small mammals and thedistribution of coniferous forests in Rokkasho.

800
600
400

Others
Ra-226
K-40
Po-210
Pb-2102000Coniferous Ork forest
forestMouse

BeechforestConiferous Ork forest
forestBeechforest

MoleFig. 3 Mean internal radiation dose rates of mice andmoles in beech (2006), oak (2007) and coniferous
forests (2008) of Rokkasho from naturalradionuclides in their bodies. “Others” means thesum of doses for 137Cs, 232Th, and 234,238U. Error
bars show a standard deviation of results of 20
samples.