報告書番号

N09014

タイトル（和文）

飽和した高密度ベントナイト原鉱のガス移行特性

タイトル（英文）

Gas migration characteristics of highly compacted bentonite ore

概要 （図表や脚注は「報告書全文」に掲載しております）

放射性廃棄物処分においては，放射性核種の移行を抑止するためなどの理由により高密度ベントナイトが用いられる．ところが，高密度ベントナイトは緻密であるため透気性が低く，地下深部の還元性環境下における金属廃棄物の腐食等により発生する可能性がある水素ガスを速やかに排出しないと思われるため，ガス圧上昇による影響の評価を行なう必要がある．そこで本報告では高密度ベントナイトとして用いられる可能性がある締固めたベントナイト原鉱を対象として室内ガス移行試験を行った．その結果，ガス圧が初期軸方向土圧を超えると収縮し，大破過に至るという基本的なガス移行メカニズムは既報告の粉末状ベントナイトと同様であること，粉末状ベントナイトと同様，大破過後の供試体の透水係数は試験前と同程度であり，大破過によるバリア性能低下は，無視し得るか小さい可能性があること，既提案の大破過圧の簡易推定法が適用可能であることなどがわかった．

概要 （英文）

In the current concept of repository for radioactive waste disposal, compacted bentonite will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides. Hydrogen gas can be generated inside the engineered barrier by anaerobic corrosion of metals used for containers, etc. If the gas generation rate exceeds the diffusion rate of dissolved gas inside of the engineered barrier, gas will accumulate in the void space inside of the engineered barrier until its pressure becomes large enough for it to enter the bentonite as a discrete gaseous phase. It is expected to be not easy for gas to entering into the bentonite as a discrete gaseous phase because the pore of compacted bentonite is so minute. Gas migration characteristics of highly compacted are already reported by CRIEPI. In this report, gas migration characteristics of bentonite ore, which is a candidate for construction material of repository for radioactive waste, is investigated.
The following conclusions are obtained through the results of the gas migration tests which are conducted in this study:
1) By increasing the gas pressure more, gas breakthrough, which defined as a sudden and sharp increase in gas flow rate out of the specimen, occurs. When the total gas pressure exceeds the initial total axial stress, the total axial stress is always equal to the total gas pressure because specimens shrink in the axial direction with causing the clearance between the end of the specimen and porous metal. Therefore gas migration mechanism of compacted bentonite ore is basically identical to that of compacted powdered bentonite.
2) Hydraulic conductivity measured after the gas breakthrough is somewhat smaller than that measured before the gas migration test. This fact means that it might be possible to neglect decline of the function of bentonite as engineered barrier caused by the gas breakthrough. These characteristics of compacted bentonite ore are identical to those of compacted powdered bentonite.
3) It is revealed that the method for calculating breakthrough gas pressure, which is already proposed based on the test results of powdered bentonite, is applicable to compacted bentonite ore. Therefore, by the method, the breakthrough gas pressure of bentonite ore can be calculated only using parameters physical meaning of which is very clear.

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