Validation of an 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 distributions of theradionuclides around the plant and the radiation dose that results from them. To validate the model using
actual field data, concentrations of radionuclides (3H, 14C, and 129I, etc.) in various environmental samples
around the plant and the environmental γ-ray dose at IES were measured. Because no nuclear fuel rods havebeen sheared or dissolved at the plant since October 2008, concentration levels of the radionuclides in mostenvironmental samples collected in FY 2009 were similar to the background ones before the plant test
operation. Although radionuclide concentrations higher than the background were detected in several
samples of soil, sediment and food, the validation results of ETDAM are reported here using the dataobtained in FY 2006-2008 when many nuclear fuel rods were sheared and dissolved.
The monthly atmospheric HTO concentration has been continuously measured in nine locations aroundthe plant by using a passive sampling method. Distributions of HTO concentration released from the spent
nuclear fuel reprocessing plant were estimated with ETDAM, and compared with the measured values.
Although the distribution patterns of the estimated HTO concentrations agreed fairly well with themeasurements, the estimated concentrations tended to be larger than the measured values.
Concentrations of 129I in brackish lake water samples collected from Lake Obuchi on 15 July and 20
October 2008 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 15 July was used as the initial concentration in
the model, and then the atmospheric deposition and advection-diffusion of 129I were simulated until 20October. The estimated concentrations of 129I on 20 October agreed well with the observed values, suggestingthat the atmospheric deposition significantly contributed to the 129I concentration in the lake water.

1x10 -2 – 5x10 -2③
5x10 -3 – 1x10 -2⑤-3

HTO (Bq m )1.5E-02

HTO concentrations（Bq m -3）

5x10 -4 – 1x 10 -3

5.0E-03
0.0E+00-5.0E-03①②③④⑨-3

HTO (Bq m )②⑧⑥⑦⑧⑨(b) January 2008.

1.5E-02⑦⑤Location number< 5x10 -4①⑥Measured
Estimated-1.0E-02

1x10 -3 – 5x10 -3④January

1.0E-02

June

Measured
Estimated

1.0E-02
5.0E-03
0.0E+00-5.0E-03-1.0E-02①②③④⑤⑥⑦⑧⑨Location number(c) June 2008.

-3(a) Distribution of HTO concentrations estimatedby ETDAM in January 2008. Calculation
conditions with ETDAM: calculation area, 50 x50 km with a grid resolution of 500 x 500 m; 1.4
km above the mean sea level with 50 m
calculation interval.
①－⑨ indicate the location number.
★ indicate the main stack of the nuclear fuelreprocessing plant.

HTO (Bq m )1.5E-02

September

Measured
Estimated

1.0E-02
5.0E-03
0.0E+00-5.0E-03-1.0E-02①②③④⑤⑥⑦Location number(d) September 2008.

Fig.1 Measured monthly atmospheric HTO concentrations around the spent nuclear fuelreprocessing plant in Rokkasho and those estimated with ETDAM.
The measured concentrations were obtained by subtracting background concentrations
estimated as mean 3H concentrations in monthly precipitation before test operation using
actual spent nuclear fuels (April 2001 – March 2005).

⑧⑨