報告書番号

U99013

タイトル（和文）

高レベル放射性廃棄物処分のための緩衝材・埋戻し材の膨潤評価式の提案 --砂とベントナイトの配合割合およびベントナイト中の陽イオンの種類・組成の影響--

タイトル（英文）

Evaluation for swelling characteristics of buffer and backfill materials for high-level nuclear waste disposal - Influence of sand-bentonite content and cation compositions in bentonite-

概要 （図表や脚注は「報告書全文」に掲載しております）

高レベル放射性廃棄物処分に係わる研究の一つとして、処分孔や坑道を埋戻す「緩衝材」や「埋戻し材」の材料設計・開発がある。緩衝材や埋戻し材には、高い膨潤性や止水性が要求されており、ベントナイトを主体とした材料が有望と考えられている。本研究では、粘土鉱物結晶間の反発力や引力を評価できる理論式と、著者らの提案するパラメータ「モンモリロナイトの膨潤体積ひずみ」の算出式を組み合わせることにより、ベントナイトを含有する緩衝材・埋戻し材の膨潤特性の評価式を提案した。この膨潤評価式を砂・ベントナイト混合材料および海外産ベントナイトを含む各種ベントナイトの膨潤特性に対し適用したところ、良好な精度を持って適用可能であることが分かった。提案した膨潤評価式によれば、地下水が浸入した後の緩衝材や埋戻し材の圧力を算出でき処分施設の力学的安定性の検討が可能となる。

概要 （英文）

Compacted bentonite and sand-bentonite mixture are attracting greater attention as buffer and backfill materials for disposal pits and access tunnels in the underground facilities for repositories of high-level nuclear waste. Buffer and backfill materials must have the swelling characteristics and are expected to fill up the space between these materials and surrounding ground by swelling. This role is called as "Self-sealing." To design the specifications, such as dry density, bentonite content and size, of buffer and backfill materials for the disposal facilities of high-level nuclear wastes described above, we must evaluate the swelling characteristics of compacted bentonite and sand-bentonite mixtures.For this purpose, this study proposed the evaluation formula for swelling characteristics of buffer and backfill materials containing bentonite. This study derived new equations for evaluating the relationship between the swelling deformation of compacted bentonite and sand-bentonite mixtures, and the swelling behavior of montmorillonite minerals, which are swelling clay minerals. This study also proposed new equations for evaluating the ion compositions of bentonite, ion concentration of pore water and the specific surface of bentonite, which significantly influence the swelling characteristics of buffer and backfill materials. The evaluation formula proposed in this study is presented by combining the above-mentioned new equations with theoretical equations, of which are the Gouy-Chapman diffuse double layer theory and the van der Waals force, of repulsive and attractive forces of montmorillonite minerals.The applicability of the evaluation formula proposed in this study has been confirmed by comparing the predicted results with laboratory test results on the swelling deformation and swelling pressure of compacted bentonites and sand-bentonite mixtures. The evaluation formula proposed can obtain the maximum swelling pressure and the maximum swelling strain of buffer and backfill materials at various dry densities, kinds of bentonite, and bentonite contents. Therefore, the evaluation formula can be used for selecting a kind of bentonite which is suitable for buffer and backfill materials, and for designing the bentonite content and compaction density from the viewpoint of "Self-sealing."

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