TRU廃棄物処分に特有な放射性核種を考慮した生物圏評価データセットの整備と線量への換算係数の算出

Biosphere assessment data set and flux-to-dose conversion factors of radionuclides specific for TRU waste geological disposal

鈴木 祐二*; 加藤 智子 ; 牧野 仁史 ; 大井 貴夫 

Suzuki, Yuji*; Kato, Tomoko; Makino, Hitoshi; Oi, Takao

高レベル放射性廃棄物の地層処分研究開発第2次取りまとめでは、レファレンスバイオスフィアの考え方を導入し線量の評価を行った。その後、放射線防護に関する法令が改訂され、新法令に基づく線量への換算係数の整備を高レベル放射性廃棄物処分の生物圏評価対象核種について実施した。一方、超ウラン核種を含む放射性廃棄物の中の地層処分相当の廃棄物(「TRU廃棄物」という)の安全評価では、レファレンスバイオスフィアとは別の生物圏評価モデルに基づく評価が行われていた。TRU廃棄物処分に関しては、高レベル放射性廃棄物の地層処分場にTRU廃棄物も併置して処分する概念が検討され、高レベル放射性廃棄物と安全評価におけるモデル,評価体系の整合性の確保が求められてきている。また、気候変動などを踏まえた将来の生物圏における環境条件が考慮されるなど、生物圏システムの変動を考慮した検討を行う重要性が指摘されている。これらの背景に基づき、高レベル放射性廃棄物の地層処分の安全評価と整合性のあるTRU廃棄物の地層処分の安全評価を行うため、TRU廃棄物処分に特有な放射性核種についても新法令に基づき設定された実効線量係数を用いたデータセットを準備し、線量への換算係数を算出した。また、気候変動による生物圏システムへの影響を生物圏モデルに取り入れた場合の影響の程度を把握するために、寒冷化した気候状態を想定して線量への換算係数を算出した。

The project to establish a technical basis for HLW disposal in Japan (H12) has used "reference biosphere" methodology. The radiation dose which was an index as a basis of the safety assessment was calculated. (In fact, the flux-to-dose conversion factors were calculated as coefficient reduced flux of radionuclide caused by geological disposal to radiation dose in biosphere.) And then, ICRP has introduced the new system for radiological protection in the publication 60 published in 1990. New regulations in Japan have been issued in 2001 and it defines new dose conversion factors of ingestion and inhalation as effective dose coefficients based on the publication 60. And so, nuclides for biosphere assessment of HLW disposal, the flux-to-dose conversion factors for biosphere assessment were calculated based on the new regulations. On the other hands, recent safety assessment on a waste of geological disposal equivalent in radioactive waste containing transuranic (TRU) nuclide was carried out the safety assessment based on the biosphere assessment model constructed by the methodology other than the reference biosphere. "Co-disposal concept" disposed of by also juxtaposing TRU waste with geological disposal site of high-level radioactive waste, was examined the geological disposal of TRU waste, and the consistency of models and assessment system in the geological disposal of HLW and TRU waste would have been required. Moreover, the environmental conditions in future various biosphere based on climatic change are considered, the importance of discussion considering the variation of the biosphere system has been indicated. The data set for appropriate models that should be considered in geological disposal safety assessment of TRU waste was also set, and flux-to-dose conversion factors were calculated for the new regulation based on the same concept and models as HLW disposal.