1.2 環境移行・線量評価モデルの高度化
Development of the Advanced Environmental Transfer and Dose Assessment Model forRadionuclides Released from the Nuclear Fuel Reprocessing Plant in Rokkasho
久松 俊一, 五代儀 貴, 植田 真司, 長谷川 英尚, 阿部 康一
環境動態研究部

Shun’ichi HISAMATSU, Takashi IYOGI, Shinji UEDA, Hidenao HASEGAWA, Koichi ABE
Department of Radioecology
Abstract
We have developed an environmental transfer and dose assessment model (ETDAM) for radionuclides
released from the first Japanese commercial nuclear fuel reprocessing plant located in Rokkasho. Thecomputer code system was developed on a PC to describe atmospheric dispersion, terrestrial and aquatictransfers, and dose calculations for the released radionuclides. We have used the ETDAM to estimate areal
and temporal distributions of the radionuclides around the plant and the radiation doses resulting from theradionuclides. The aquatic transfer model targeted transfer of radionuclides in Lake Obuchi, a brackish lakeneighboring the reprocessing plant. The Lake Obuchi model consists of a water current model and anecosystem model including lower trophic level organisms.
To increase accuracy and target areas of ETDAM, a meteorological model (MM5) for the atmosphericdispersion model, a catchment area model for Lake Obuchi, an ecosystem model of higher trophic level
organisms for the lake, and a coastal marine model have been developed during FYs 2006 - 2010. Thosemodels were bound to ETDAM, and integrated as the advanced environmental transfer and dose assessment
model 1.0 (AdvETDAM 1.0) in FY 2010. Meteorological elements such as wind direction and velocity
calculated by the meteorological model are introduced into the atmospheric dispersion model. The catchment
area model describes the inflow of radionuclides from the Futamata River, which is the main river flowing
into the lake. A sub-model including ellgrass (Zostera marina), fishes and benthos was developed as theecosystem model of higher trophic level organisms in the lake. The coastal marine model for Rokkasho
Coast simulates inflow of radionuclides from the Pacific Ocean into the lake through Obuchi Fishing Port at
the mouth of the lake to the ocean.
The 3H concentrations in lake water samples collected from Lake Obuchi in November 2007, whichwere higher than the background level, were simulated by using AdvETDAM 1.0 to confirm its function.
The possible sources of 3H in the lake water were surface deposition of atmospheric released 3H, and inputs
from the Futamata River and the Pacific Ocean. Estimated 3H concentrations of surface lake water at thecenter of the lake agreed fairly well with the measured values; estimated values were 1.7 - 2.8 times lower
than the measured values. The source of 3H in the lake was considered as the liquid waste release from theplant to the ocean because of an underestimated factor of 9 - 10 without input from the ocean. The
contributions of the atmospheric deposition and input from Futamata River were minor in comparison withthe input from the ocean. Although AdvEDAM 1.0 simulated fairly well the 3H concentrations in the lakewater, the estimated concentrations were systematically lower than the observed ones. The input from theFutamata River was also overestimated with the model. Further study is required for improvement ofsimulation with AdvETDAM 1.0.

1. 目的築。

六ヶ所村とその周辺の環境を対象として大型再処

これらの構築したモデルを結合して総合モデル

理施設より排出される放射性核種による中長期的な1.0 とし、平成 19 年 11 月に尾駮沼湖水中3H濃度が上線量評価を可能とするため、総合的環境移行・線量昇した事例について計算し、計算結果と観測値を比評価モデル 1.0（総合モデル 1.0）を作成する。

較した。

平成 22 年度は六ヶ所沿岸海域放射性核種移流拡散モデル（六ヶ所沿岸海域モデル）を構築した後、これまで構築・高度化した各種モデルを結合して総合モデル 1.0 とし、その機能を確認した。

3. 成果の概要
総合モデル 1.0 による計算結果の一例をFig. 2 に示す。この結果は大気拡散モデル、尾駮沼集水域モデル及び六ヶ所沿岸海域モデルの計算結果を尾駮沼高2. 方法次生態系モデルに入力して計算した結果である。

総合モデル 1.0 の概要を Fig. 1 に示す。総合モデル

Fig.2 には3H濃度の観測値及び六ヶ所沿岸海域モデ1.0 構築のため、まずは平成 17 年度までに開発したルを結合しない計算結果も示した。総合モデル 1.0

陸域移行･線量評価モデル(⑥)と尾駮沼低次栄養段による尾駮沼湖心表層水中3H濃度の計算結果は、観階生態系放射性核種移行モデル(②)（尾駮沼低次生測値に比べ 1/1.7 から 1/2.8 と低いものであった。六態系モデル）を結合して環境移行･線量評価モデル

ヶ所沿岸海域モデルを結合しない計算結果は 1/9 か(⑦)を構築し、更に気象モデル(①)を導入し、モデル

ら 1/10 と大きく過小評価しており、平成 19 年 11 月を高度化した。更に、下記の尾駮沼関連モデルを構の尾駮沼湖沼水の3H濃度の上昇は、その大部分が海築してきた。

洋排出によるものであり、大気排出による寄与は小・尾駮沼集水域から湖内への放射性核種等の移行をさいと判断された。

評価するための尾駮沼集水域放射性核種移行モデ
ル(④)（尾駮沼集水域モデル）の構築。
・尾駮沼低次生態系モデル(②)にアマモ場、底生生物、魚類等を含む高次栄養段階生態系における放上記の結果により、総合モデル 1.0 として結合された六ヶ所沿岸海域モデル、尾駮沼集水域モデル及び尾駮沼高次生態系モデルが、それぞれの機能を果たしていることが確認できた。

射性核種等の移行を評価するための尾駮沼高次栄

今後、ウェザリングや形態別挙動等、これまでの養段階生態系放射性核種移行モデル(③)（尾駮沼

調査で得られた放射性核種の移行に関する機構及び高次生態系モデル）の構築。

地域の自然環境を考慮した放射性核種の挙動の組み・尾駮沼と沿岸海域間の海洋排出放射性核種の移行を予測するための六ヶ所沿岸海域モデル(⑤)の構込みを行うとともに、鷹架沼及び鷹架沼集水域に関するモデルを構築する予定である。

Discharge tothe atmosphere

Concentration
in atmosphere

The atmosphericdiffusion model

The terrestrial
transfer model

Atmosphericdeposition

The meteorologicalmodel ①

External dose

Concentration inagricultural productsInternal dose

Concentrationin lake water

The ecosystemmodel of lower
trophic level
organisms in
Lake Obuch ②

Concentration inlivestock products

The ecosystemmodel of higher
trophic level
organisms in
Lake Obuch ③

Concentrationin plankton

Concentration inlake sediments
Concentration ineelgrass
Concentration infish and shellfish

Discharge
to the sea

The catchment areamodel of Lake Obuch ④

Concentration inriver water

The coastal marine modelof Rokkaysho ⑤

Concentration in
sea water

Concentration infish and shellfish： The terrestrial transfer and dose assessment model ⑥： The environmental transfer and dose assessment model ⑦

Fig.1

Scheme of the AdvETDAM 1.0 for transfer and dose assessment forradionuclides discharged from the nuclear fuel reprocessing plant in Rokkasho

Mouth of Futamata River (MFR)

Obuchi Fishing Port (OFP)

Center of Lake Obuchi(CLO)

Measured

Estimated

Estimated (without input from the ocean)● At CLO

At CLO------ At CLO◆ At OFPAt OFP------ At OFP▲ At MFR

At MFR-1

H concentration (Bq L )

100

101

3

0.1

0.01
1-Sep-2007

Fig.2

1-Oct-2007

1-Nov-2007

1-Dec-2007

1-Jan-2008

Concentrations of 3H in surface water samples at the center of Lake Obuchi (CLO), Obuchi
Fishing Port (OFP) and the mouth of Futamata River (MFR) estimated with AdvETDAM 1.0.
The dashed line (in red and green) shows the simulation without input of 3H from the ocean.