緩衝材の鉱物学的長期変遷に対する二次鉱物の種類、反応速度の影響評価

An impact statement of secondary minerals assemblage and minerals reaction rate for the long-term alteration of the buffer material

佐々木 良一; 小田 治恵  ; 本田 明

Sasaki, Ryoichi; Oda, Chie; Honda, Akira

本検討においては、地球化学計算コードPHREEQC(ver.2.10)を用いて、セメント領域1m、ベントナイト混合土1mの単純化した系での拡散による物質移行を想定し、モンモリロナイトの変質鉱物の組み合わせを変更した場合の人工バリア長期安定性を検討した。モンモリロナイトの溶解は、飽和度の増加とともに減少することが知られており、本検討では瞬時溶解平衡に加えて,Camaの経験式(Jordi Cama et al., 2000)を用いて計算した。 FRHPとSRHPにおいて,鉱物の組み合わせ,モンモリロナイト熱力学データ,溶解速度等を考慮した計算を実施した結果,鉱物の組み合わせを考慮しても,数万年以上の期間,モンモリロナイトが十分残存し,緩衝材の機能を維持する可能性が示唆された。モンモリロナイトを瞬時溶解平衡としたケース,logKを変更したケースでは,特にSRHPにおいて溶解が早く進展する結果となったが,天然事例との整合性を考慮すると現実的な結果ではない。したがって,モンモリロナイトの溶解速度を考慮すること及び現在のJNC-TDB.TRUの熱力学データが妥当であることが示された。

Radioactive waste which includes TRU nuclide (TRU waste) possesses wide radionuclide concentration distribution. Some kinds of TRU wastes (hulls-endpieces, asphalt solidification package, spent ioine adsorber etc.) should be disposed in the geological repository because of relatively high radioactivity. Most waste package will be filled with cementitious material. It is considered that waste packages will be isolated to the concrete pit constructed in a drift and filled space with cementitious material filler. I-129 and C-14 cannot be expected solubility limits and storong adsorption to the cementitious material, therefore, the bentonite-sand mixture which possesses very low permeability will be employed as a component of multi-barrier system which installs. The function of engineered barrier system(EBS) which consists of waste package, cementitious filler and buffer material are required to be maintained for a long term. Hence, it is important to analyze the EBS alteration based on available knowledge coming from both experiments and natural analogue. EBS alteration analyses were inplemented with the simplified diffusion transport system which consisted of cement region and bentonite buffer using PHREEQC code (the ver.2.10). In the analysis, variation of secondary mineral assemblage is considered. It is known that the dissolution rate of montmorillonite decreaes as its saturation index increase, so instantaneous dissolution equilibrium and the empirical equation for the dependency of dissolution kinetics on S.I. which was proposed by Cama et al. (2000) were assurned. In most cases, calculation results showed that the funcion of EBS will be maintained over ten southand years. In some cases (instantaneous equilibrium case, modified logK case, SRHP case), significant montmorillonite dissolution was shown. But, such results are inconsistent with the natural analogue knowledge which shows natural bentonite exists for 2 million years under seawater similar ...