Effects of cement and iron corrosion products on the durability of buffer material in geological disposal of HLW

为对高放废物地质处置进行安全评价，从钠基膨润土在处置后会发生的变化角度出发，对膨润土缓冲液的长期耐久性进行了基础性研究。本研究以钠基土为原料，在无氧手套箱中与FeCl2溶液接触，制得了铁基膨润土，并用选择性溶解、穆斯堡尔谱、X-射线衍射法等方法对其进行了表征。将铁基膨润土暴露于空气中，制备了氧化铁基膨润土样品，并对其进行了表征。结果表明，铁基膨润土中的主要交换性阳离子是Fe~(2+)和lt；2~(2+)，当样品暴露在空气中时，它们逐渐被氧化。在间歇吸附实验中，铁膨润土和氧化铁膨润土对Se(IV)和Se(VI)的吸附系数高于Na型膨润土。另一种蚀变产物是Ca~(2+)-膨润土，它是由废旧水泥材料中的Ca~(2+)和G~(2+)离子溶解生成的。测定了钠钙膨润土混合样品在不同扩散温度下Na~+和Ca~(2+)的表观扩散系数。研究了盐度对钠基膨润土中钠离子扩散系数的影响。结果表明，钙基膨润土和盐度对表观扩散的影响不显著。然而，盐度对扩散活化能有显著的影响。活化能的变化可以用扩散模型来解释，在扩散模型中，主要的扩散过程随着盐度的增加而改变。

For the safety assessment of the geological disposal of high-level radioactive waste, a fundamental study on the long-term durability of bentonite buffer was made from the viewpoint of the alterations of Na-bentonite that would occur after disposal.Fe-bentonite is one of alteration products caused by the corrosion of overpack. In this study, Fe-bentonite was prepared from Na-bentonite in an oxygen-free glove-box by contacting Na-bentonite with FeCl_2 solution, and characterized by various analytical methods such as selective dissolution method, mossbauer spectroscopy and X-ray diffraction method. An oxidized Fe-bentonite sample was prepared by exposing Fe-bentonite to air, and also characterized in the same way. It was found that divalent cations, Fe^<2+>, were the predominant exchangeable cations in Fe-bentonite, and they were gradually oxidized when the sample was exposed to air. In batch sorption experiments, the higher sorption coefficients of Se(IV) and Se(VI) were obtained for Fe-bentonite and oxidized Fe-bentonite than for Na-type.Another alteration product is Ca-bentonite that would be generated by Ca^<2+> ions from the dissolution of cement materials in a waste repository. In this study, apparent diffusion coefficients of Na^+ and Ca^<2+> were determined for the mixture samples of Na-and Ca-bentonite at various diffusion temperatures. The salinity effect on the diffusion coefficient of Na^+ ions was also studied for Na-bentonite. It was indicated that the effects of Ca-bentonite and salinity on the apparent diffusion were not remarkable. However, significant effects of the salinity on the activation energy for diffusion were found. The change in the activation energy can be explained by the diffusion model in which the predominant diffusion process is considered to change as the salinity increases.