Validation of Environmental Transfer and Dose Assessment Model for Radionuclides
Released from the Nuclear Fuel Reprocessing Plant in Rokkasho
Takashi IYOGI, Koichi ABE, Naofumi AKATA, Hideki KAKIUCHI, Shinji UEDA,Hidenao HASEGAWA, Yoshihito OHTSUKA, Kunio KONDO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
The first commercial nuclear fuel reprocessing plant in Japan, located in Rokkasho, Aomori Prefecture,
is now undergoing its final testing using actual spent nuclear fuels. An environmental transfer and doseassessment model (ETDAM) was developed for estimating areal and temporal distribution of theradionuclides around the plant and the radiation dose that result from them. To validate the model using
actual field data, we measured concentrations of radioactive nuclides (3H, 14C, and 129I, etc.) in various
environmental samples around the plant and the environmental γ-ray dose at IES. In FY 2008, we foundhigher levels in part of the measurement results than the background and the FY 2007 levels. The model wasvalidated using the data obtained in FY 2007, and validation results are reported here.
Concentrations of free water tritium (FWT) in pine needle samples collected in October 2007 atKami-iyasaka, Rokkasho were found to be higher than their background level. The concentrations of 3H inatmospheric water vapor were estimated with the model. The estimated concentrations of 3H in atmosphericwater vapor were the same level as the measured FWT concentrations in the pine needle samples. This
showed that the high FWT concentrations in the pine needle samples originated from the 3H released fromthe reprocessing plant.
Concentrations of 129I in soil samples collected at Obuchi, Rokkasho in 2007 have already been reported.
The inventory of 129I in the surface soil was higher than that in 2003, before the start of testing using actual
spent nuclear fuels. The measurement of 129I in soil samples collected at the same place in 2008 showed thatthe surface inventory of 129I further increased from that in 2007. The inventory of 129I in 2007 estimated by
the model was higher than the measured one. This may be caused by the conservative deposition velocity of129
I adopted in the model.
Concentrations of 129I in brackish lake water samples collected from Lake Obuchi on 10 July and 15
October 2007 were higher than their background level and showed a slight increase during the period
between the samplings. The concentration of 129I originating from atmospheric deposition to the lake surface
was estimated by the model assuming 129I concentration of inflow water from a river and the Pacific Ocean
as their background level. The measured 129I concentration on 10 July was used as the initial concentration in
the model, and then the atmospheric deposition and advection-diffusion of 129I were simulated until 15
October. Although the estimated concentration of 129I on 15 October was approximately 2 - 3 times higher
than the observed value, the atmospheric deposition was suggested to contribute significantly to 129I
concentration in the lake water.

25

Estimated concentration of

3Deposition(mBq m-2)

H in atmospheric water
FWT concentration in pineneedle samples at 1 m height
FWT concentration in pineneedle samples at 2 m height
FWT concentration in pineneedle samples at 3 m height

20-1

H concentration (Bq L -H2O)

315

40-50
30-40
20-30
10-20
1-10
0-1

10

5

Estimated, 15 mBq m-2; Measured, 8.6 mBq m-2

03-Oct4-OctFig. 2 Cumulative deposition of 129I from April 2006 until
2007
August 2007 estimated with ETDAM.
Fig. 1 3H concentrations in atmospheric water vapor estimatedCalculation conditions were the same as those inwith ETDAM and measured FWT concentrations inFig. 1.
pine needle samples collected at Kami-iyasaka,
Rokkasho. Calculation conditions with ETDAM: area,
50 x 50 km; grid resolution, 500 x 500 m; height, 1.4km above the mean sea level; 50 m interval.

2-Oct

0.005

0.003Estimated concentration of129

Measured concentration of129

Discharge rate of129

I

I discharge rate
(Arbitrary Unit)

0.004

II

129

0.002
0.001

129

I concentration (mBq L-1)

1-Oct

01 JulFig. 3

1 Aug

1 Sep
2007

1 Oct

1 Nov

Concentrations of 129I in surface lake water at the center of Lake Obuchi estimated with ETDAM.
Calculation conditions were the same as those in Fig. 1.