Solubilities of TRU Element in the Pore Water of the Compacted Bentonite Used for the Buffer Material of the High-level Radioactive Waste Disposal

核素在压实膨润土中的溶解度是降低高放废物处置放射性影响的关键因素之一。尽管迄今为止已经完成了积累实验证据和热力学数据的努力，但对压实膨润土中不溶性元素（如Am， Pu和U）的溶解度限制浓度的直接测量尚未报道。本研究的目的是推导出适用于高压实膨润土中强液固相互作用条件下的离子活度校正方法。1. 离子强度：采用透析法测定膨润土压实后孔隙水中离子浓度。结果表明，主阳离子Na的浓度在1 ~ 10 mol/L范围内，强调了采用适当的活度校正方法计算溶解度的必要性。2. 热力学数据：利用□G和□H之间的相关性或与镧系元素的相似性，应用系统学方法推测未纳入本数据库的U、Am、Pu的热力学数据。3. 分析代码：利用扩展泊松-玻尔兹曼方程或超网状链近似开发了计算溶解度的数值分析代码，主要针对活度校正。通过比较扩散实验对U的溶解度的估计，验证了程序的适用性。结果表明该方法是有效的。部分研究成果已于2002年在日本原子能学会年会上发表。

The solubility of radionuclide in the compacted bentonite is one of the key factors to reduce the radiological impact derived from the high-level radioactive waste disposal. Although efforts to accumulate the experimental evidence and thermodynamic data have been accomplished so far, direct measurements of the solubility-limited concentration of insoluble elements such as Am, Pu, and U in compacted bentonite have not yet been reported. The objective of the research is to derive the method of the ion activity correction applicable under the condition of the strong liquid/solid interaction in highly compacted bentonite. 1. Ionic Strength : The dialysis technique was employed to estimate the ionic concentration in the pore water of the compacted bentonite. The data indicated the concentration of the main cation, Na, were in the range of 1〜10 mol/L and the result emphasizes the necessity of the appropriate activity correction method for the solubility calculation. 2. Thermodynamic data : Thermodynamic data of U, Am, Pu not included in the present database were speculated by applying the systematics using the correlation between □G and □H or similarities to lanthanides. 3. Analysis code : Numerical analysis code for calculating the solubilities mainly focused on the activity correction were developed using the extended Poisson-Boltzman equation or Hyper-Netted Chain Approximation. The applicability of the code was checked by comparing the estimated solubilities of U from the in-diffusion experiments. The results indicated the validity of the present activity correction method. A part of the research outputs was presented in the annual meeting of the Atomic Energy Society of Japan in 2002.