報告書番号

N09015

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１）－アルカリ性の各種溶液に浸漬した締固めベントナイトの鉱物学的な変質と透水性の変化－

タイトル（英文）

Alteration behavior of bentonite barrier of radioactive waste disposal by alkaline solutions -part 1-- Permeability change of compacted bentonite immersed in alkaline solutions -

概要 （図表や脚注は「報告書全文」に掲載しております）

アルカリ溶液との反応による締固めベントナイトの変質挙動と透水性の変化を評価するために，初期乾燥密度1.6Mg/m3の締固めベントナイトと6種類の異なる組成の溶液を用いて，浸漬試験（60℃，4週間）と透水試験（23℃，1週間）を交互に11回実施した．その結果、イオン交換水、pH12のNaOH、Ca(OH)2を用いた試験では、顕著な透水係数の増大は認められなかった。pH12のKOHを用いた試験では、ベントナイトの交換性陽イオン組成が変化し，モンモリロナイトの一部が膨潤性の乏しい物質に変化したために透水係数が経時的に増大した。pH14のNaOH、KOHを用いた試験では、構成鉱物の溶解や二次生成物の沈殿等が認められ、これらの変質が複合して作用したために透水係数が経時的に増大した。以上より，pH14程度またはKOHに富む溶液は締固めベントナイトの透水性に影響を与える可能性があることが明らかとなった。

概要 （英文）

Permeability tests using the compacted bentonites and alkaline solutions were carried out to estimate of alteration behavior and the change of permeability during the alteration reaction. The permeability tests of the compacted bentonites were carried out at 23 C for one week after they were immersed in alkaline solution at 60 C for four weeks (immersing test). After permeability tests, the compacted bentonites were repeatedly tested as the same procedure (i.e. repetition of permeability test and immersing test) at 11 cycles. The compacted bentonites with initial dry density of 1.6 Mg/m3 were reacted with the different type of the alkaline solutions (deionized water, NaOH (pH = 12 and 14), KOH (pH = 12 and 14) and Ca(OH)2 (pH = 12)) in each experiments.
In the case of deionized water and alkaline solutions of pH12, the mineral compositions of altered bentonite were similar to original bentonite while the exchangeable cations of altered bentonites were changed. No changes of the mineralogical features of montmorillonite in altered bentonites (i.e. illitization, baideritization and increasing of layer charge) were observed in the case of deionized water, pH12-NaOH and pH12-Ca(OH)2. The montmorillonite was changed to the illite/smectite interstratified mineral containing about 40% illite like component during the reaction with pH12-KOH. In the case of alkaline solutions with pH14, the component minerals of bentonite (e.g. montmorillonite, quartz and clinoptilolite) were dissolved, consequently secondaly minerals (e.g. analcime and phillipsite) were crystallized during experiments. Furthermore, the mineralogical features of montmorillonite were changed as illitization (pH14-KOH), beidellitization (pH14-NaOH and pH14-KOH) and increasing of layer charge (pH14-NaOH and pH14-KOH).
No increasing of permeability were observed during the experiment using pH12-NaOH and pH12-Ca(OH)2 as well as the case of deionized water. In the case of pH12-KOH, the permeability continually increased from 4.5*10-13 to 1.2*10-12 by 11 cycles. This result is probably caused by the changes of exchangeable cations in the altered bentonite and the formation of the illite/smectite interstratified mineral. In the case of the alkaline solutions with pH14, the permeability increased with the dissolution of the component minerals of bentonite, the precipitation of the secondly minerals (e.g. analcime and phillipsite) and the change of the mineralogical feature of montmorillonite as illitization, beidellitization and increasing of layer charge.

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