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地層処分システムの変遷を考慮した安全評価手法の整備,3; 移行パラメータの空間的・時間的変化を考慮した核種移行解析

Development of safety assessment methodology considering the evolution of geological disposal system, 3; Analysis of radionuclide migration with consideration of spatial and temporal change of migration parameters

島田 太郎; 田窪 一也; 武田 聖司; 田中 忠夫

Shimada, Taro; Takubo, Kazuya; Takeda, Seiji; Tanaka, Tadao

地層処分の長期安全評価では、地質,水質,水理特性などのサイト状態や人工バリア特性の空間的・時間的変化を考慮する必要がある。そこで、人工バリア及び天然バリアに関する分配係数、拡散係数、間隙率、地下水流速、移行距離などの核種移行パラメータの時間的・空間的変化を評価に反映できる核種移行解析コードを開発し、人工バリア及び天然バリアの状態に応じて変化するパラメータをリンクさせた解析を試行した。本報では、仮想的な堆積岩サイトに処分施設を設定し、施設の設置位置やサイトの隆起・侵食を考慮した地下水流動・塩分濃度の解析から求めた施設から生物圏へ至る移行経路と距離、水質,流速の条件、ならびに人工バリアの長期変遷の解析から決定した核種移行パラメータを入力データとして、各バリアからの主要な放射性核種の移行フラックスを解析し、核種移行フラックスへの影響の大きい因子を整理した結果を報告する。

In long-term safety assessment of geological disposal system, it is necessary to evaluate the impact on the nuclides migration where groundwater flow and water composition are changed with decreasing a depth of the repository by uplifts and denudation. Therefore, we developed an integrated safety assessment methodology for uplifts and denudation where nuclides migration was evaluated reflecting temporal and spatial changes of parameters of natural and engineered barriers such as distribution coefficient, porosity and groundwater velocity, which were obtained by calculation of groundwater flow, salt water distribution and long-term transition of engineered barriers. The methodology was applied to an assumed disposal site composed of sedimentary rocks with uplift and denudation. Migration parameters of engineered barrier such as coefficient of water permeability and porosity of buffer material were evaluated considering the interaction between overpack and surrounding bentonite clay under the condition of water composition. Migration parameters of the natural barrier such as distribution coefficient were determined by the combination of the geology section and water composition section on the migration pathway from the repository to the outlet of the natural barrier at the depth of 40m from the surface. The migration fluxes of some radionuclides at the outlet of natural barrier were evaluated and then the impacts on nuclides migration by uplifts and denudation were compared.

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