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, 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 the radionuclides. To validate themodel 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 nonuclear fuel rods have been sheared or dissolved at the plant since October 2008, concentration levels of theradionuclides in most environmental samples collected in FY 2010 were similar to the background ones
before the plant test operation. Although radionuclide concentrations higher than the background weredetected in several samples of soil and sediment, the validation results of ETDAM, that is monthly γ-ray
dose rates from 85Kr and monthly atmospheric concentrations of 14C, 3H, and 129I at IES, are reported
here using the data obtained from April 2006 to February 2009 (FY 2006-2008) when many nuclear fuelrods were sheared and dissolved and significant amounts of radionuclides were released from theplant.

Accuracy of the simulation was evaluated by using the ratio of the number of cases in which
the estimated value was within a factor of 2 or 5 of the measured one to that of the total cases as anindex. The agreement ratio within a factor of 2 for the γ-ray dose rate from 85Kr was approximately
40% and within a factor of 5, approximately 80%. Similar ratios were obtained for the atmosphericconcentration of 14C, while the ratios for 3H and 129I were smaller and much smaller than those forthe γ-ray dose rate, respectively. The other indices for the difference between estimated andmeasured values such as the absolute ratio of mean of error to the average of the measured values(|MEnd|) and the ratio of the square root of the error squared to the average of the measured values(RMSEnd) showed similar results.
Although the estimated values of atmospheric concentration of 14C were evenly scatteredaround the measured ones, the γ-ray dose rate from 85Kr was systematically higher than themeasured one. As well, the estimated concentrations of 129I were almost 10 times higher than theobserved values. The reasons for those systematic differences are unknown, and further study isrequired to improve the accuracy of the model.

Monthly 3 H concentration（Bq m-3 ）4x10-2
Measured
Estimated

3x10-22x10-2
1x10-20January 2007

100806040200Factor
214KrFactor
5Factor2Factor
53C

Factor2Factor
5129

HI|MEnd | and RMSEnd

The agreement ratios of number of cases in which the estimated value was within a factorof 2 or 5, for monthly γ-ray dose rate from 85Kr and atmospheric concentration of 14C, 3H,and 129I at IES.
30
20100|MEnd |85Fig.3

Factor5Factor
285Fig.2

January 2009

Monthly concentrations of 3H in atmospheric water vapor estimated with ETDAM andmeasured at IES. The measured concentrations were obtained by subtracting backgroundconcentrations estimated as mean 3H concentrations in monthly precipitation obtained
before test operation using actual spent nuclear fuels (April 2001 – March 2006). ETDMA
calculation conditions: calculation area, 50 x 50 km with a grid resolution of 500 x 500 m.
Agreement ratio（％）

Fig.1

January 2008

RMSEndKr|MEnd |RMSEnd14C|MEnd |RMSEnd3H|MEnd |RMSEnd
129I

The absolute ratios of mean of error to the average of the measured value (|MEnd|) and theratios of the square root of the error squared to the average of the measured value(RMSEnd), for monthly γ-ray dose rate from 85Kr and atmospheric concentrations of 14C,3H, and 129I at IES.