Validation of the Advanced Environmental Transfer and Dose Assessment Model forRadionuclides Released from the Nuclear Fuel Reprocessing Plant in Rokkasho
Hidenao HASEGAWA, Shinya OCHIAI, Hideki KAKIUCHI, Shinji UEDA,Shun’ichi HISAMATSU
Department of Radioecology
Abstract
The first commercial nuclear fuel reprocessing plant in Japan, located in Rokkasho, Aomori Prefecturefinished its final testing using actual spent nuclear fuels and is now under safety assessment by the Nuclear
Regulation Authority. The advanced environmental transfer and dose assessment model (AdvETDAM) wasdeveloped for estimating areal and temporal distributions of the radionuclides around the plant and theradiation doses resulting from these radionuclides. To validate the model using actual field data, we measured
the concentrations of radionuclides (3H, 14C, 129I, etc.) in various environmental samples collected at points
around the plant and the environmental -ray dose rates at IES.
Because no nuclear fuel rods have been sheared or dissolved at the plant since October 2008,
concentration levels of the radionuclides in most environmental samples collected in FY 2015 were similar tothe background ones before the plant test operation, excluding several samples. Iodine-129 deposited on soil
and lake sediment surfaces around the plant has still remained at a higher level than each background level.
Since the accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011, we have studied
the distribution and transfer of several radionuclides in Fukushima Prefecture to clarify their movement in theterrestrial environment. The obtained results will be used to improve the accuracy of the model prediction inAdvETDAM. In FY 2015, we studied the following subjects: 1) the distribution of 3H in plant and soil samples
collected around the FDNPP, 2) the discharge rate of radiocesium via two small rivers in a mountainous areain Iitate Village, and 3) the re-suspension rate of radiocesium in Namie Town and Koriyama City.
Free-water tritium (FWT) concentrations in plant samples collected in Fukushima Prefecture have been
gradually decreasing from 2011 to 2015. The effective half-life of FWT in those were estimated as a few days
for a week after the accident, and as 1 to 2 months from 1 week after the accident until August 2011. Thedischarge rate of radiocesium from the river catchments of the two small rivers during 2015 was less than 1%of radiocesium deposited in the catchments, showing that most of the radiocesium has still remained on thesoil surface in the catchments. The atmospheric 137Cs concentration showed a clear seasonal fluctuation withhigh values during summer and autumn in Namie and with high values during winter and spring in Koriyama.
We defined the wind direction-weighted mean 137Cs deposition densities, which was a sum of the products ofthe frequency of wind direction for a month and the 137Cs deposition density in each sector of 16 directions in
a 5 km radius, and the wind direction-corrected resuspension factor, which was the quotient of atmospheric137
Cs concentration divided by the wind direction-weighted mean 137Cs deposition densities. The range of there-suspension factor of 137Cs (Namie, 4×10-11 to 8×10-10; Koriyama, 8×10-11 to 1×10-8) was similar to theannual mean resuspension factor, which was defined as the quotient of atmospheric 137Cs concentration
divided by local 137Cs deposition densities, observed at Chernobyl and in European countries (1×10-10 to 1×106). In addition, our re-suspension factor has been gradually decreasing with an effective half-life ofapproximately 1 year.

1×10-7: < 1.1 m: > 1.1 m5 km

1.0

No data

No data

0.5

2

10
0.15

0.0
1.5
: < 1.1 m: > 1.1 m5 kmb) Koriyama
1.00.5No data

Atmospheric 137Cs concentration (mBq m-3)a) Namie

0.00.10.05

Weighted mean 137Cs deposition density (MBq m-2)

31.5Wind direction corrected resuspension factor (m-1)

Namie
Koriyama

1×10-8y = 5.2×10-9 exp(-0.74x)

1×10-9

1×10-10
y = 8.3×10-10 exp(-0.55x)

1×10-1100

1
2
34
Time scince the accident (y)57 9 11 1 3 5 7 9 11 1 3 5 7 9 11 1 3 5 7 9 11 1 3 5 7 9 112013
2014
2011
2012
2015

Month / Year

Fig. 1 Time series of atmospheric 137Cs concentrations and137

Fig. 2Time series of wind direction-correctedCs deposition densities,

resuspension factor, which was the quotient of

which was a sum of the products of the frequency of windatmospheric 137Cs concentration divided by thedirection for a month and the 137Cs deposition density in eachwind direction weighted mean 137Cs deposition

sector of 16 directions in a 5 km radius. Blue and light blue bars

densities, during study period in Namie Town andwind direction-weighted mean

indicate

Cs concentrations in particulates <1.1 m and >1.1

137

m, respectively. (a) Namie Town and (b) Koriyama City.
Arrows indicate the most frequent wind direction in a month.

Koriyama City.