濃度干渉を考慮した天然バリア中の核種移行モデルの開発

Development of repository-wide radionuclide transport model considering the effects of multiple sources

畑中 耕一郎; 亘 真吾; 井尻 裕二*

not registered; Watari, Shingo; Ijiri, Yuji*

地下水シナリオを想定した地層処分システムの安全評価においては、従来より、単一のガラス固化体を基本として、人工バリア中及び天然バリア中の核種移行解析が行われ、その結果に生物圏の解析で得られた線量換算係数及び処分場に設置される予定のガラス固化体の総本数を乗じることにより全体システムの安全性が論じられている。このような性能評価手法に対し、本研究では、処分場内に埋設されたガラス固化体が多重ソースとして作用し、その結果生じると考えられる核種濃度の干渉現象に着目し、その影響を評価することができるモデル・コードを新たに構築した。また、既存コードとの比較解析を通して、新たに構築したコードの検証を行うとともに、第2次取りまとめレファレンスケースのデータを使用して濃度干渉効果の概括的評価を試みた。その結果、単一のガラス固化体を基本とした従来の評価手法から得られる核種移行率に比べ、濃度干渉効果を考慮に入れた解析から得られる核種移行率は約2オーダーのピーク値の低減効果が見られた。ここでの結果は、濃度干渉効果が最大の場合に相当し、処分場の2次元、3次元的な流れを考慮した検討が今後必要である。

Safety assessment of the geological isolation system according to the groundwater scenario has traditionally been conducted based on the single canister configuration and then the safety of total system has been evaluated based on the dose rates which were obtained by multiplying the migration rates released from the engineered barrier and/or the natural barrier by dose conversion factors and total number of canisters disposed in the repository. The dose conversion factors can be obtained from the biosphere analysis. In this study, we focused on the effect of multiple sources due to the disposal of canisters at different positions in the repository. By taking the effect of multiple sources into consideration, concentration interference in the repository region is possible to take place. Therefore, radionuclide transport model/code considering the effect of concentration interference due to the multiple sources was developed to make assessments of the effect quantitatively. The newly developed model/code was verified through the comparison analysis with the existing radionuclide transport analysis code used in the second progress report. In addition, the effect of the concentration interference was evaluated by setting a simple problem using the newly developed analysis code. This results shows that the maximum peak value of the migration rates from the repository was about two orders of magnitude lower than that based on single canister configuration. Since the analysis code was developed by assuming that all canisters disposed of along the one-dimensional groundwater flow contribute to the concentration interference in the repository region, the assumption should be verified by conducting two or three-dimensional analysis considering heterogeneous geological structure as a future work.