高温、高压和pH值多种因素作用下蒙脱石层间Cs+/UO2+2水化和扩散行为的分子模拟

作为核废物地质处置库缓冲回填材料的主要成分，蒙脱石在特殊环境（高温、高压和pH多种因素作用）下吸附阻滞核素的行为对缓冲回填材料的性能评估至关重要。为从微观分子尺度探究特殊环境下蒙脱石层间核素离子的吸附扩散行为，本文采用蒙特卡罗（MC）和分子动力学（MD）方法，分别研究了Cs⁺/UO²⁺₂在蒙脱石层间的吸附行为，以及高温、高压和pH值多种因素作用下的水化和扩散动力特征。MC结果表明：蒙脱石层间的Cs⁺、UO²⁺₂周围分别会形成1层和2层水化壳，且层间水分子与四面体中氧原子之间形成了明显的氢键；当c=1.25 nm时，蒙脱石层间最多可吸附285个水分子。MD结果表明：高温、高压和pH值都会对Cs⁺、UO²⁺₂的水化和扩散产生影响。常温、常压下pH值分别为11.85、12.15时，Cs⁺、UO²⁺₂水化壳中的水分子数最多；pH值分别为12.15、11.85时，Cs⁺、UO²⁺₂的扩散系数最大，分别为5.31×10^-13 m²/s和1.11×10^-12 m²/s。与常温、常压相比，高温、高压下Cs⁺、UO²⁺₂水化壳中的水分子数最多时，pH值分别为7.00、12.15；而Cs⁺、UO²⁺₂扩散系数最大（1.12×10^-12 m²/s、1.01×10^-12 m²/s）时的pH值均为12.15。

Molecular Simulation of Cs+/UO2+2 Hydration and Diffusion Behavior in Interlayer of Montmorillonite under High Temperature,High Pressure and pH Value

Montmorillonite is the main component of the buffer backfill material for geological disposal repository which is used to bury high level nuclear waste. The adsorption and blocking performance to radionuclides by montmorillonite in special environment (the effects of high temperature, high pressure and pH value) is very important for the evaluation of the properties of the buffer backfill material. In recent years, molecular simulation methods play important roles in investigating the adsorption and diffusion behavior of radionuclides in the interlayer of montmorillonite. In this paper, the Monte Carlo (MC) method was used to study the adsorption behavior of Cs⁺ and UO²⁺₂ in the interlayer of montmorillonite, and the molecular dynamics (MD) technique was employed to investigate the hydration and diffusion dynamic characteristics of Cs⁺ and UO²⁺₂ in the interlayer of montmorillonite under the effects of high temperature, high pressure, and pH value. The model system consisted of 8a×4b×1c unit cells which corresponded to a typical 2∶1 type moderate-charge montmorillonite (05 e/unit cell). The charge sites in the tetrahedral and octahedral sheets of montmorillonite were replaced and the charge in the octahedral layer was more than that of the tetrahedral layer. 8 Cs⁺ and 4 UO²⁺₂ were adsorbed in the interlayer of montmorillonite to balance substitution charge, meanwhile, different numbers of OH^- were introduced to control the pH value. A Clayff force field suitable for the clay minerals was used to interact with each other between nuclides, water molecules and external clay surface during the simulations. MC calculation results show that Cs⁺ and UO²⁺₂ are distributed nearby at the tetrahedral and octahedral charge substitution sites. In addition, one- and two-layer water hydration shells can be formed around Cs+ and UO²⁺₂ in the interlayer of montmorillonite, respectively. Moreover, hydrogen bonds are easily existed between water molecules and oxygen atoms in the ［SiO₄］ tetrahedral sheet. The highest numbers of 285 water molecules are adsorbed when 8 Cs⁺ and 4 UO²⁺₂ contained in the interlayer of montmorillonite with c=1.25 nm. MD results show that the hydration and diffusion of Cs⁺ and UO²⁺₂ could be affected by the high temperature and high pressure as well as the pH value. Under the normal temperature and pressure, the maximum numbers of water molecules in the water hydration shells of Cs⁺ and UO²⁺₂ are obtained when the pH value is 11.85 and 12.15, respectively. And the highest diffusion coefficients of Cs⁺ and UO²⁺₂are 5.31×10^-13 m²/s and 1.11×10^-12 m²/s when the pH value is 12.15 and 11.85, respectively. In comparison with that of the normal temperature and pressure, when the number of water molecules in the water hydration shells of Cs⁺ and UO²⁺₂ reaches the maximum, the pH value is 7.00 and 12.15 at high temperature and pressure, respectively. Meanwhile, the pH value is all 1215 when the maximum diffusion coefficients (1.12×10^-12 m²/s and 1.01×10^-12 m²/s) of Cs⁺ and UO²⁺₂ are achieved.