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

放射性核素在压实膨润土中的溶解度是减少高放废物处置产生的放射性影响的关键因素之一。尽管到目前为止已经完成了积累实验证据和热力学数据的努力，但尚未有直接测量压实膨润土中不溶元素Am、Pu和U的溶解度限制浓度的报道。本研究的目的是推导出适用于高致密膨润土中强液/固相互作用条件下的离子活度校正方法。1.离子强度：采用透析法测定压实膨润土孔隙水中离子浓度。数据表明，主要阳离子Na的浓度在1~10mol/L范围内，这一结果强调了在计算溶解度时采用适当的活度修正方法的必要性。2.热力学数据：利用-G和-H之间的相关性或与稀土元素的相似性，应用系统学方法推测了未列入本数据库的U、Am、Pu的热力学数据。3.分析程序：采用扩展的Poisson-Boltzman方程或超网链近似，编制了以活度修正为主的溶解度数值分析程序。通过比较由内扩散实验估算的铀的溶解度来检验该程序的适用性。结果表明，本文提出的活度校正方法是有效的。日本原子能学会2002年年会上介绍了部分研究成果。

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.