多因素耦合作用下缓冲材料对铀的长期阻滞效应数值模拟北大核心CSCD

近场环境条件下核素在缓冲材料中的迁移扩散受控于温度场、渗流场、膨胀应力场和化学吸附场的耦合作用,其对核素的阻滞特性将影响到核素随地下水向处置库围岩迁移并返回生物圈的能力,开展多因素耦合作用下缓冲材料对铀的长期阻滞效应研究,对地质处置库的长期安全性评价具有重要的意义。本研究基于混合物理论、连续介质理论、质量守恒、动量守恒、能量守恒及溶质扩散的Fick定律,推导出饱和缓冲材料中核素迁移扩散的热-水-力-化耦合控制方程,并借助于COMSOL Multiphysics软件的直接全耦合求解优势,以自主研制的缓冲材料长期阻滞性能Mock-up实验装置为几何模型,采用内置接口和添加热-水-力-化耦合控制方程中的耦合项作为源项相结合方式,实现了多物理场耦合作用下铀在饱和缓冲材料中迁移扩散行为的直接耦合分析,其长期阻滞特性数值模拟结果表明:初期阶段铀在缓冲材料中迁移扩散较缓慢,迁移距离随时间增幅在1 m左右;中后期阶段,随缓冲材料对铀的吸附容量逐渐趋于饱和后,其迁移距离较初期阶段增加更为明显,迁移距离随时间增幅为3 m左右。多因素耦合下核素在饱和缓冲材料中迁移扩散的热-水-力-化耦合控制方程构建、求解及长期阻滞性能模拟研究的方法,能够为我国高放废物深地质处置库地下实验室开展1∶1工程尺度的工程屏障设计与安全性能评价提供技术参考。

Numerical Simulation of Long-Term Retardation Effect of Buffer Material for Radionuclide Uranium Under Coupling Effect of Multiple Factors

In the near field environment, the migration and diffusion of radionuclides in the buffer material is controlled by the coupling effect of temperature field, seepage field, expansion stress field and chemical adsorption field. The blocking characteristics of the buffer material will affect the ability of the radionuclides to return to the biosphere with the migration of groundwater to the surrounding rock of the disposal reservoir. It is of great significance for long-term safety assessment of geological disposal repository to study the long-term retarding effect of buffer material on nuclide uranium under the coupling effect of multiple factors. Based on mixture theory, continuum theory, mass conservation, momentum conservation, energy conservation and solute diffusion of Fick’s law, thermal-hydro-mechanical-chemical coupling control equation of nuclide migration and diffusion in saturated buffer materials was constructed. At the same time, with the help of the direct full coupling solution advantages of COMSOL Multiphysics software, using the self-developed Mock-up experimental device for long-term retarding performance of buffer materials as a geometric model, the direct coupling analysis of the migration and diffusion behavior of uranium in saturated buffer materials under the action of multiple physical field coupling is realized by using the built-in interface and the coupling term in THM coupling control equation as the source term. The numerical simulation results of long-term retardation characteristics show that the migration and diffusion of uranium in the buffer material is slow at the initial stage, and the migration distance increases by about 1 m with time. However, in the middle and late stage, as the adsorption capacity of buffer material for uranium tends to be saturated, the migration distance increases more obviously than that in the early stage, and the migration distance increases by about 3 m with time. The method of constructing and solving thermal-hydro-mechanical-chemical coupling control equation for the migration and diffusion of nuclides in saturated buffer materials under multi-factor coupling and simulating the long-term retarding performance can provide technical reference for the 1∶1 engineering barrier design and safety performance evaluation of underground laboratories of deep geological disposal repository for high-level radioactive waste in China.