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Mudstone redox conditions at the Horonobe Underground Research Laboratory, Hokkaido, Japan; Effects of drift excavation

幌延深地層研究センターにおける泥岩の酸化還元状態; 坑道掘削の影響

望月 陽人; 石井 英一; 宮川 和也  ; 笹本 広

Mochizuki, Akihito; Ishii, Eiichi; Miyakawa, Kazuya; Sasamoto, Hiroshi

高レベル放射性廃棄物の地層処分場の建設・操業時には、坑道内の大気が掘削損傷領域(EDZ)に侵入し、坑道周辺の岩盤や地下水が酸化状態に変化することが想定される。坑道掘削が岩盤や地下水の酸化還元状態に与える影響を評価するために、幌延深地層研究センターの調査坑道周辺において酸化還元状態に関する調査を実施した。調査坑道周辺に掘削されたボーリング孔において、地下水のEhは-150mVより低く、酸化の指標となるSO$$_{4}$$$$^{2-}$$濃度はおおむね1$$mu$$mol L$$^{-1}$$未満であった。ボーリング孔に設けられた区間の50%以上をガスが占め、その組成は主にCH$$_{4}$$とCO$$_{2}$$からなり、N$$_{2}$$とO$$_{2}$$の割合はわずかであった。坑道から採取されたコア試料を分析した結果、黄鉄鉱の酸化溶解や硫酸カルシウムの沈殿は認められなかった。以上の結果から、今回調査したEDZにおいては酸化の顕著な兆候は認められず、その理由として、地下水圧の低下に伴い脱ガスしたCH$$_{4}$$やCO$$_{2}$$が岩盤中の割れ目を占め、大気の侵入が抑制された可能性が示唆された。

The mechanical and hydraulic properties of rocks around mine drifts change significantly during the construction and operation of a radioactive-waste repository, with air intrusion causing the oxidation of rock and groundwater in excavation-damaged zones (EDZ). Redox conditions in such zones associated with niches excavated in mudstone at the Horonobe Underground Research Laboratory (URL), which is believed to be generally representative of conditions that could exist in the EDZ of a repository, were studied with the aim of improving our understanding of factors that control redox conditions in such rock-groundwater systems. Groundwater Eh values around the niches have reducing values of less than -150 mV. The SO$$_{4}$$$$^{2-}$$ concentration, regarded as an oxidation indicator, is consistently as low as 1 $$mu$$mol L$$^{-1}$$. Gas occupies more than 50% of zone volumes, including CH$$_{4}$$ and CO$$_{2}$$ with traces of N$$_{2}$$ and O$$_{2}$$. Cores drilled from host rock around a URL gallery were analyzed, with no pyrite dissolution or precipitation of calcium sulfates being found. It is concluded that oxidizing conditions do not exist in the excavation-damaged zones, which is attributed to the suppression of air intrusion by the release of CH$$_{4}$$ and CO$$_{2}$$ from groundwater as pressures decreased and their accumulation in fractures. The modeling of oxygen diffusion into host rock further indicates that a reducing environment is maintained around the URL drifts.

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