報告書番号

N13011

タイトル（和文）

飽和したCa型ベントナイト混合土のガス移行特性評価

タイトル（英文）

Evaluation of gas migration characteristics of compacted and saturated Ca-bentonite mixture

概要 （図表や脚注は「報告書全文」に掲載しております）

低レベル放射性廃棄物処分のうち，ピット処分施設においては，核種移行抑制の観点から，コンクリート製の埋設設備の周囲に透水性の低い，締固めたベントナイト混合土を設置することが考えられている．一方，廃棄体からはアルミの腐食などにより水素ガスが発生するが，透水性が低いため，速やかに排出されない可能性がある．そこで，Ca型ベントナイト混合土のガス移行特性ならびにガス移行が飽和透水係数に及ぼす影響を実験的に把握するとともに原位置におけるガス移行特性を評価した．その結果， 1) ガス移行試験により得られる大破過圧は初期応力条件によっても変化する．2) ガス移行試験後に測定した飽和透水係数は，試験前に測定した値と同程度である． 3) 当所が開発した力学連成気液2相流モデルによる数値解析により，ガス移行試験時の供試体の主たる挙動を再現できる．4) 施設を模擬した小寸法模型を対象にガス移行解析を行い，大破過圧に及ぼす境界条件の影響を調べた．その結果，小寸法模型の大破過圧は，境界条件の違いによる発生応力の違いにより，ガス移行試験における大破過圧より小さくなることがわかった.

概要 （英文）

In the current concept of near-surface pit disposal for low level radioactive waste, compacted bentonite mixture will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides. Hydrogen gas can be generated inside the engineered barrier mainly by the chemical interaction between aluminum and the alkaline component of cement, or water. If the gas generation rate exceeds the diffusion rate of gas molecules inside of the compacted bentonite mixture, gas will accumulate in the void space inside of the compacted bentonite mixture until its pressure becomes large enough for it to enter the compacted bentonite mixture as a discrete gaseous phase. It is expected to be not easy for gas to entering into the compacted bentonite mixture as a discrete gaseous phase because the pore of the compacted bentonite mixture is so minute. Therefore in this study, the gas migration characteristics and the effect of gas migration on the hydraulic conductivity of the compacted Ca-bentonite mixture are investigated by the gas migration tests. The effect of stress state on the migration characteristics is also investigated by the gas migration tests and by parametric study using the model of two phase flow through deformable porous media, which was originally developed by CRIEPI. Results of this study imply that : (1) Large gas breakthrough pressure, which is defined as a rapid increase of amount of discharged gas, is affected by initial stress conditions as well as Ca-bentonite content of the mixture. (2) Hydraulic conductivity measured after the large gas breakthrough is substantially the same that measured before the gas migration test. (3) Axial stress change and volume change of the specimen during the gas migration test can be reproduced by the numerical simulation using the model of two-phase flow through deformable porous media, which was originally developed by CRIEPI. (4) Gas migration of a small scale model is numerically simulated to investigate the effect of boundary conditions of the large gas breakthrough pressure. As a result, it is revealed that the large gas breakthrough pressure of the small scale model is smaller than that of the gas migration test specimen because of difference in stress caused by different boundary conditions.

報告者

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