湿化膨胀与掺砂率对混合型缓冲材料THM耦合过程的影响分析

Analysis of the influence of wetting expansion and sand mixing rate on the THM coupling process of hybrid buffer material

加入辅助骨料后的混合型缓冲材料在保留了材料密封和防渗能力的同时,克服了纯膨润土导热系数低、施工性差的缺点,作为高放废物处置库缓冲材料的备选材料正在成为新的研究热点。基于室内试验数据,以多孔介质非饱和渗流理论为基础,考虑了膨润土湿化膨胀及密度、饱和度、导热系数等材料物理性质参数的实时变化,利用COMSOL Multiphysics软件建立了包含一条巷道及单个井孔的3D网格模型,计算模拟了100 a间屏障系统中膨润土-石英砂混合型缓冲材料的THM多场耦合演化过程,分析了各物理量的时间演化及空间分布规律,并通过不同工况分析了材料湿化膨胀以及掺砂率对屏障系统演化过程的影响。系统中材料温度、饱和度与距固化体、岩壁的距离呈相关性,应力整体以受压为主,变形呈先压缩后膨胀的趋势。其中,膨润土基材料的湿化膨胀作用对温度演化的影响很小,但会略微加速饱和进程,并使材料的应力-应变出现明显的时间演化及区域分布差异。井孔和巷道中靠近岩壁区域的应力上升较快,并且在巷道底板与井孔交界处出现了显著的竖向位移。提高掺量可以有效降低罐体表面温度,增强屏障系统散热能力,降低缓冲材料的历史最大应力,有效控制井孔轴线上的竖向位移,但也会削弱系统的防渗能力。

The hybrid buffer material added with the auxiliary aggregate not only retains the material’s sealing and anti-seepage ability, but also overcomes the defect of low thermal conductivity and inferior construction performance of pure bentonite blocks. As an alternative for the buffer material of high-level radioactive waste repository, it is becoming a new research hotspot. Based on the previous research data and the permeation theory of unsaturated porous media, a 3 D calculation model containing one tunnel and a single wellbore is established using COMSOL Multiphysics, which considering the wetting expansion of bentonite and the real-time change of material physical parameters(density, saturation, thermal conductivity, etc.). By simulating the THM coupling process of the hybrid buffer material(bentonite-sand mixture) in the barrier system for 100 years, the time evolution and spatial distribution of each physical quantity are analyzed, and the influences of wetting expansion and sand mixing rate on the evolution process of the barrier system are discussed. The temperature and saturation of the material in the system are related to the distance from the vitrified HLW and the rock wall. The stress in the tunnel and wellbore is mainly compressive stress, and the deformation tends to compress first and then expand. The wetting expansion of bentonite-based materials has little effect on temperature evolution, but it will slightly accelerate the saturation process and cause significant time evolution and regional distribution differences in the stress and strain of the material. The stress in the area close to the rock wall in the wellbore and roadway rises quickly, and significant vertical displacement occurs at the junction of the roadway floor and the wellbore. Increasing the mixing rate can reduce the surface temperature of the tank effectively, enhance the heat transfer capacity of the barrier system, reduce the maximum historical stress of the buffer material, and control the vertical displacement on the axis of the borehole. On the other side, it will also weaken the antiseepage capability of the system.