報告書番号

N10037

タイトル（和文）

放射性廃棄物処分におけるベントナイト系材料のアルカリ溶液による変質挙動（その2）－アルカリ溶液の種類がベントナイト混合土の透水性へ及ぼす影響－

タイトル（英文）

Alteration behavior of bentonite barrier of radioactive waste disposal by alkaline solutions -part 2- - Effect of type of alkaline solution on permeability of compacted bentonite-sand mixture -

概要 （図表や脚注は「報告書全文」に掲載しております）

浅地中ピット処分の難透水性覆土や余裕深度処分等の空洞充てん材には，ベントナイト10～30wt%程度混合したベントナイト混合土を使用することが検討されている．ベントナイト混合土は，核種移行抑制のため低透水性である必要がある．本研究では，ベントナイト混合土の低透水性に影響を与える要因の一つであるアルカリ溶液による変質と透水性の変化について評価した．その結果，NaOHを用いた試験では，有効モンモリロナイト密度が減少し透水係数が増大した．一方，Ca(OH)2を用いた試験では，有効モンモリロナイト密度が減少していたが，透水係数は2桁程度低下した．この原因として，C-S-Hの沈殿の可能性が考えられた．以上より，ベントナイト混合土のアルカリ溶液による透水性の変化を現実的に評価するためには，モンモリロナイトの溶解のみならず，二次生成物の影響も考慮することが重要であると考えられた．（387文字）

概要 （英文）

Permeability tests were carried out using compacted bentonite-sand mixture with initial dry density of 1.55 Mg/m3 and alkaline solutions at 50C for about two years to estimate the alteration behavior and the change in the permeability. Bentonite-sand mixtures which contain bentonites of 15wt% were made using Na-bentonite or Ca-exchanged bentonite. 0.3M-NaOH solution with pH 13.3 and 5mM-Ca(OH)2 solution with pH 12.0 were used to the permeability tests of Na-bentonite-sand mixture and of Ca-exchanged bentonite-sand mixture, respectively.
In the case of the permeability test conducted using NaOH solution, montmorillonite and other associated minerals were dissolved, and consequently, the dry density and effective montmorillonite density of Na-bentonite-sand mixture were decreased. Furthermore, the mineralogical feature of montmorillonite was changed (i.e. beidellitization and an increase in the layer charge). The permeability of Na-bentonite-sand mixture was increased 5.6 times by the end of permeability test as a result of above alteration. In the case of the permeability test conducted using Ca(OH)2 solution, montmorillonite and other associated minerals were dissolved, and calcium silicate hydrate (C-S-H) was precipitated. Consequently, the dry density of Ca-exchanged bentonite-sand mixture was increased, while the effective montmorillonite density was decreased. The mineralogical feature of montmorillonite was changed (i.e. beidellitization and an increase in the layer charge). The permeability of Ca-exchange bentonite-sand mixture was decreased by more than two orders of magnitude due to fill the pore of Ca-exchange bentonite-sand mixture by the precipitation of C-S-H. From above results, the type of alkaline solution affects the mineralogical alteration behavior of the compacted bentonite-sand mixture, and consequently, affects the changing trend of permeability.
In conclusion, it is important not only to consider the dissolution of montmorillonite, but also to consider the precipitation of secondary minerals for the assessment of the change in permeability of the compacted bentonite-sand mixture.

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