高放废物深地质处置库蒙脱土对铀酰的吸附阻滞行为:分子模拟研究

Radionuclide adsorption mechanism in buffer materials in high-level radioactive waste container: MD study

核废料处置库在长期运行过程中,地下水渗流会引起膨润土缓冲层水化学环境改变。运用分子动力学模拟方法,揭示含蒙脱土的膨润土缓冲材料对放射性核素铀酰的吸附机理及吸附复合物微观结构。模拟得到的复合物结构展现了铀酰与蒙脱土面的3种主要吸附模式:外球吸附、单键内球吸附、双键内球吸附。定量分析了在复杂水化学环境下3种铀酰种态与蒙脱土表面形成的复合物微观结构组成。通过计算复合物与蒙脱土表面的吸附能,发现高价阳离子和碳酸根离子的存在可以促进铀酰与缓冲材料表面形成稳定吸附。

The buffer material plays a decisive role in preventing the radionuclide to enter into the host rock, as it is the last defense of engineered barrier system. Under very high groundwater pressure, a large amount of cations percolate through the barrier with underground water, resulting in a complicated chemical condition. Molecular dynamics simulation is performed to deeply investigate the adsorption mechanism of radionuclide species onto substituted montmorillonite(001) surface in the presence of different counterions. MD simulations exhibit three typical adsorption modes: outer-sphere complex, monodentate inner-sphere complex and bidentate inner-sphere complex. With the presence of carbonate ions and covalent cations, the U atom in uranyl can coordinate with carbonate oxygen in connection with cations to form an intensive adsorption complex with MMT surface. The thermodynamic work of adhesion between the complexes and the MMT surface is calculated to evaluate the adsorption interaction. The complexes with the carbonate and covalent cation components exhibit a relatively high adhesion with the buffer material surface.