处置库内混凝土衰解作用下高压实膨润土缓冲性能演化机理研究

In China, the Gaomiaozi (GMZ) bentonite has been selected as the first choice of buffer/backfill material and the Beishan area in Gansu Province has been considered as the most potential repository site for high-level waste (HLW) disposal. Near the repository, the concrete degradation is expected to play an important role in long-term safety assessment of compacted bentonite used as buffer/backfill material. In order to investigate the effects of concrete degradation on the adsorption, diffusion and hydro-mechanical behavior of compacted GMZ bentonite, three types of solutions with different chemical composition will be prepared to simulate the chemical environment of surrounding rock and different concrete degradation stages, and laboratory tests and numerical simulations will be performed to evaluate the long-term performance of engineering barrier system (EBS). Firstly, the batch adsorption and diffusion tests will be conducted to study chemical retardation behavior of EBS for radionuclide ions, such as europium and thorium. Secondly, based on the multi-phases suction controlling technique for unsaturated soil, a constant-volume cell for high-pressure swelling-infiltrate test will be improved to test hydro-mechanical behavior of EBS under different chemical environment. Thirdly, the mercury intrusion porosimetry (MIP) and environmental scan electronic microscope (ESEM) will be applied to quantify the microstructure characteristics of GMZ bentonite hydration in chemical solutions, and clay mineralogy and chemical analysis will also be conducted to investigate the chemical mechanism for the effects of concrete degradation on the buffer property of GMZ bentonite. Finally, in order to get a better understanding of the coupled hydro-mechanic-chemical (HMC) process in the EBS, a constitutive model for chemical induced HMC behavior of GMZ bentonite will be presented, and numerical simulations will be conducted to evaluate the long-term performance of the EBS. The expected results will provide a theoretical basis and technical support for design and construction of the EBS in China.

针对高放废物处置库内混凝土衰解对工程屏障缓冲性能的影响，以北山预选处置场和高庙子膨润土为背景，通过系统试验和数值分析，揭示混凝土衰解化学作用下高压实膨润土化学、力学和水力学性能演化规律，研究缓冲材料性能衰变机理，建立工程屏障长期安全性评价方法。人工配置3种溶液模拟近场围岩和混凝土不同衰解阶段地下水化学环境；开展批次吸附和扩散试验，揭示膨润土对核素离子的化学屏障特性；基于多相吸力控制技术，改造恒体积高压膨胀力-渗透仪，进行不同化学作用下膨胀力-渗透性试验，揭示膨润土水-力学性能演化规律。基于压汞法、扫描电镜等微观结构测试，研究化学效应引起缓冲性能演化的微观机制；结合粘土矿物学、分析化学原理，阐明缓冲性能衰变的化学机理。由此建立化-水-力耦合效应缓冲性能演化模型，采用数值方法模拟高压实膨润土屏障性能演化过程，评价混凝土衰解影响下工程屏障的长期安全性。预期成果将为我国处置库建设提供科学依据。

针对高放废物深地质处置库内混凝土衰解对工程屏障缓冲性能的影响，以北山预选处置场和高庙子（GMZ）膨润土为背景，通过模拟不同混凝土衰解阶段的地下水化学环境，开展了北山围岩地下水和混凝土衰解条件下高压实GMZ膨润土对模拟核素离子的吸附与扩散试验；改造了温湿控制式膨胀力-渗透仪和固结仪，实现了碱性溶液入渗条件下膨胀力、膨胀变形及渗透性能的测定；研制了高放废物处置库内混凝土-高压实膨润土组合体系性态模拟试验装置，揭示了混凝土/高压实膨润土界面作用下地下水化学成分及组合体系膨胀力等缓冲性能的响应规律；结合化学成分、矿物成分分析及MIP、ESEM微观结构测试，阐明了混凝土衰解液对高压实GMZ膨润土缓冲性能的影响机理；在此基础上，建立了考虑化-水-力耦合效应的高压实膨润土水－力学性能演化模型，评估混凝土衰解液作用下工程屏障系统缓冲性能。研究表明，混凝土衰解作用下高压实GMZ膨润土具有较好的吸附性能，表面络合反应为主要吸附机制；混凝土衰解液入渗时膨润土的渗透性能增大而膨胀性能降低，表现为饱和渗透系数显著增大、膨胀力大幅下降以及膨胀变形在次膨胀阶段发生衰减；早期及后期混凝土/膨润土相互作用释放Mg、Si，吸收Na、Ca，引起溶液pH值升高，并导致组合体系膨胀力衰减；从膨润土矿物成分、孔隙水-土颗粒作用以及膨润土微观结构演化三个方面阐明了混凝土衰解液对高压实GMZ膨润土缓冲性能的影响机理。本项目研究成果揭示了处置库近场混凝土衰解条件下高压实GMZ膨润土化学阻滞和水-力性能演化特性，为进一步评估工程屏障服役性能和安全防控提供了理论基础，对科学选取材料参数、合理设计工程屏障、有效保障处置库安全具有重要的科学意义和应用价值。