報告書番号

N23

タイトル（和文）

ベントナイトならびに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 bentonite and 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，余裕深度処分ならびに浅地中ピット処分施設においては，核種移行抑制の観点から，廃棄体を収納した埋設設備の周囲に透水性の低い締固めたベントナイトまたはCa型ベントナイト混合土（以下，混合土と略称する）を設置することが検討されている．一方，廃棄体からは金属の腐食などにより水素ガスが発生するが，ベントナイトならびに混合土の透水性が低いため，速やかに排出されない可能性がある．そのため，ガス移行特性ならびにガス移行による影響を検討する必要がある．そこで，本研究では，ガス移行試験によりガス移行特性，試験前後の飽和透水係数の変化を把握するとともにガス移行メカニズムを明らかにした．さらに，力学非連成気液2相流モデルの適用性を検討するとともに，新たに力学連成気液2相流モデルを提案し，その適用性を示した．さらに数値シミュレーションによる検討の例示を行った．

概要 （英文）

In the current concept of subsurface disposal and near-surface pit disposal for low level radioactive waste, compacted bentonite and Ca-bentonite mixture will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides, respectively. Hydrogen gas can be generated inside the engineered barrier of subsurface disposal facilities mainly by anaerobic corrosion of metals used for containers, etc. Hydrogen gas can be also generated inside the engineered barrier of near-surface pit disposal facilities 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 and Ca-bentonite mixture, gas will accumulate in the void space inside of the compacted bentonite and Ca-bentonite mixture until breakthrough occurs. 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 and Ca-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 bentonite and Ca-bentonite mixture are investigated by the gas migration tests. The applicability of the two phase flow model without considering deformability of the specimen is investigated. The applicability of the model of two phase flow through deformable porous media, which was originally developed by CRIEPI, is also investigated. Results of this study imply that : (1) Gas migration mechanism of the compacted bentonite and Ca-bentonite mixture is revealed through gas migration test. (2) Hydraulic conductivity measured after the large gas breakthrough is substantially the same that measured before the gas migration test. (3) Stress change, pore-water pressure 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 though the stress change of the specimen cannot be reproduced by the two phase flow model without considering deformability of the specimen. (4) It is revealed through the numerical analyses that large gas breakthrough pressure, which is defined as a rapid increase of amount of discharged gas, is affected by initial stress conditions and the boundary conditions.

報告者

キーワード