核废料处置库缓冲材料受围岩裂隙水侵蚀的致灾机制与灾害控制研究

The performance of the bentonite buffer in nuclear waste repository concept relies to a great extent on the buffer surrounding the canister having sufficient dry density. Loss of buffer material caused by erosion remains as the most significant process reducing the density of the buffer. In the worst case the process is assumed to last as long as the free volume between the pellets in the pellets filled regions is filled with groundwater. Based on the experiments of pinhole piping erosion, Comsol Multiphysics code, hydromechanics and fractal model for bentonite gels, a mechanical and chemical erosion model is proposed. Erosion rate and mass erosion are calculated based on the erosion model, and the measures are presented to prevent the geological disaster due to bentonite erosion. With fixed environmental and material parameters a set of experiments of piping erosion are performed, testing the erosive properties of the buffer and backfill materials in different groundwater conditions. The groundwaters may solubilise the smectite particles in the bentonite and carry them away as colloidal particles. A dynamic model is developed for sodium gel expansion in fractures where the gel soaks up groundwater as it expands. The model is based on a force balance between and on smectite particles, which move in the water. Attractive van der Waals forces, repulsive electric diffuse layer (DDL) forces, gravity and buoyancy forces and forces caused by the gradient of chemical potential of the particles act to move the particle in the water. The effect of the fracture width and the frictions between particles and water and surrouding rock is analysed based on erosion model. The DDL forces strongly depend on the type of clay minerals and the type of ion and concentration in the water surrounding the particles. The measures are presented to prevent the geological disaster due to bentonite erosion through changes in the measures are presented to prevent the geological disaster due to bentonite erosion. The project is not only academic innovation, but also has a large practical significance.

在核废料处置库安全使用的设计年限（数万年至十数万年）内，受地下水动力和化学作用，膨润土缓冲材料被侵蚀，导致缓冲层致密性降低、渗透性增加，危及核废料处置库安全，引起核元素扩散和对流迁移，甚至造成灾难性后果。本项目基于申请者对膨润土分形结构的研究成果，采用渗流冲刷试验、Comsol Multiphysics软件和流体力学理论，分析膨润土凝胶形成和扩散迁移机理，建立膨润土侵蚀的分形模型；建立缓冲材料层侵蚀的THMC耦合模型，分析膨润土性状、水溶液离子类型和浓度、pH值、渗流速度、裂隙特征对膨润土渗流侵蚀的影响，计算膨润土侵蚀速率和侵蚀质量，揭示膨润土的侵蚀致灾机制，建立核废料处置库缓冲层的安全评价体系，提出核废料处置库侵蚀灾害的控制方法。本项目既是地质工程的基础理论研究，又解决环境工程的实际问题，不仅具有创新的理论意义，保障核废料处置库安全，也具有保护环境、造福子孙的社会意义和经济价值。

在核废料处置库中的膨润土缓冲材料受地下水动力和化学作用被侵蚀，导致缓冲层致密性降低、渗透性增加，无法满足处置库设计年限（数万年至十数万年）的安全。同时，膨润土裹挟着放射性元素运移，引起核元素向外界的扩散和对流迁移，可能对环境和人类安全造成灾难性后果。本项目基于膨润土垫层的膨胀、挤出及运移过程研究，揭示了膨润土的侵蚀特性及其致灾机理；基于膨润土的表面分形模型，建立了膨润土化学侵蚀中膨胀挤出过程的分形模型；基于膨润土凝胶结构的分形模型，提出膨润土凝胶侵蚀的临界剪切应力和侵蚀质量计算方法；基于膨润土凝胶的粒度分布的分形特性，模拟了膨润土在地下处置库渗流作用下的侵蚀过程；基于建立的侵蚀模型和侵蚀过程的影响因素分析，提出了缓冲层侵蚀灾害的控制措施。取得以下成果：.（1）提出了膨润土在盐溶液中的广义有效应力，建立了膨润土的化学侵蚀模型.根据膨润土表面分形模型，导出了盐溶液中膨润土的统一有效应力和吸附方程的统一形式，实现了将盐溶液对膨润土的力学作用统一量化。根据膨润土孔隙水的分形吸附方程，建立了描述膨润土挤出距离与广义有效应力之间关系的化学侵蚀模型，并设计实施了化学侵蚀试验进行了验证。.（2）建立了膨润土的物理侵蚀模型.根据膨润土凝胶体的分形模型，基于van der Waals长程吸引力，提出膨润土凝胶侵蚀的临界剪切应力和侵蚀质量计算方法，建立膨润土物理侵蚀理论，并得到了物理侵蚀试验数据的验证。.（3）模拟了膨润土在渗流作用下的侵蚀过程.裂隙水作用下膨润土的侵蚀过程受到渗流和化学作用的耦合影响。从微观角度，结合膨润土侵蚀机理和凝胶粒度分布的分形特征，构建膨润土垫层侵蚀过程的数值模型，分析了凝胶结构、流体流速、溶液环境等因素对垫层挤出距离和侵蚀速率的影响。.（4）提出了侵蚀控制措施.基于盐溶液中膨润土自由膨胀特性、凝胶稳定性、凝胶侵蚀量和膨润土强度特性，提出膨润土凝胶侵蚀的控制措施。