损伤演化对非饱和高放废物处置围岩气体渗透特性影响规律研究

For the high-level radioactive waste (HLW) geological disposal project, gas generation due to metal corrosion, organic degradation processes and waste radiolysis will lead to gas transfer in the HLW disposal repository. It is crucial to study the gas transfer process, because it may make it easier for radioactive contaminants to escape from the HLW disposal repository. Gas permeability of the surrounding rock (particularly nonsaturated rock of the excavation damaged zone) is a critical parameter to understand transport characteristics and it is not well investigated in the research on China HLW disposal. Furthermore, damage and water saturation, which are two important influencing factors on gas permeability of surrounding rock, are rarely studied together. Therefore, this work focuses on the gas permeability of surrounding rock and its influencing factors - damage and water saturation. The surrounding rock is deep granite taken from the Beishan site, where is considered as one of the otential sites for China HLW disposal. In the present study, the gas permeability of granite under different water saturation levels will be first measured and the variation of gas permeability with water saturation will be explored based on the test results. Then, the damage evoluation of nonsaturated granite will be experimentally analyzed and the gas permeability of nonsaturated granite under different damage levels will be simultaneously measured. Based on the experimental results, the coupling effect of damage and water saturation on gas permeability will be investigated and a relative permeability model, which can reflect the coupling effect, will be developed. The proposed model will be implemented in the numerical tool to simulate the hydraulic behaviors of the Beishan granite. At last, the gas migration in the whole surrounding rock will be .simulated and its effect on the long-term safety of HLW disposal will be discussed. The research work studies the critical indexes of gas migration process and provides their mathematical models for the long-term safety analysis of the HLW disposal project. The achievements can also be extended to other geomaterials and engineering fields.

高放废物地质处置工程中气体的产生和运移，是放射性核素向外迁移的重要途径。作为从工程屏障到天然屏障的过渡区域，围岩开挖损伤区是有害气体渗流路径上的薄弱环节，损伤演化与饱和度是影响其渗透特性的两个关键因素。由于试验技术和相关理论发展的局限，损伤演化与饱和度共同作用下岩石气体渗透率演化规律的研究目前比较匮乏。本项目拟采用先进的低渗材料气体渗透率测试技术，以中国高放废物地质处置甘肃北山预选区的深部花岗岩为研究对象，开展非饱和损伤围岩的渗透特性试验研究，深入剖析损伤演化、饱和度与岩石气体渗透率之间的相关关系，建立能够反映损伤演化与饱和度耦合影响的相对渗透率模型。基于本研究建立的模型，对围岩开挖损伤区中的气体运移过程进行数值模拟研究，分析其对高放废物地质处置工程长期安全性的影响。研究成果可以进一步加深对损伤岩石渗透特性及其演化机理的认识，并为高放废物地质处置工程长期安全性评价提供更为可靠的渗流模型。

高放废物处置库中气体的产生和运移，可能造成有害核素向外迁移，危害生物圈。围岩开挖损伤区是有害气体向外迁移的主要途径，其损伤程度与饱和度水平是影响气体渗透特性的两个关键因素。本项目以中国高放废物处置甘肃北山预选区的花岗岩为研究对象，进行了花岗岩解湿和吸湿过程监测，分析了花岗岩水分特征曲线形态；开展了损伤花岗岩气体渗透率试验，分析了气体渗透率与损伤变量以及微裂纹密度的相关关系；进行了损伤花岗岩解湿过程监测，考察了损伤演化对水分特征曲线形态的影响；开展了非饱和损伤花岗岩气体渗透率试验，分析了饱和度与损伤共同作用下相对气体渗透率的变化特征，确定了描述花岗岩相对渗透率的数学模型和参数；初步开展了高放废物处置预选区深部岩体机械法开挖模拟，评价了机械开挖对围岩渗透特性的影响；测试了钻爆法开挖硐室围岩的气体渗透率，评价了钻爆法开挖对围岩渗透特性的影响。上述研究表明，甘肃北山花岗岩材料的孔隙中存在“瓶颈”结构，使得材料吸湿和解湿都较为困难，形成了解湿-吸湿滞回圈；控制花岗岩气体渗透率的微裂纹尺度相对较大，可在92%相对湿度环境中完成干燥；孔隙率并不是花岗岩气体渗透率的唯一控制因素，孔隙结构差异等因素同样可能造成气体渗透率的较大变化；在相同的相对湿度环境中，损伤花岗岩相比于完整花岗岩更容易解湿，继而说明损伤花岗岩的微裂纹开度更大；本项目负责人提出的双孔隙模型适合描述此类型花岗岩的水分特征曲线和相对渗透率曲线；数值模拟结果表明，在北山预选区实际地质条件下，机械法开挖不会引起围岩气体渗透性能的显著变化；试验测试表明，钻爆法开挖形成的硐室顶壁损伤区会放大平行于硐室轴线方向的气体渗透率；此外，矿物颗粒相对较大的花岗岩其气体滑移现象不明显，而矿物颗粒相对较小的花岗岩整体上具有明显的气体滑移效应。上述研究成果可以为我国高放废物处置地下实验室和处置库的围岩长期性能评价提供基础参数和依据。