Na-K-Ca盐梯度循环作用下高压实GMZ膨润土水－力学性能衰变机理研究

Highly saline solution are expected to play an important role in long term safety assessment of engineering barrier system (EBS) for storage of high level radioactive waste (HLW). 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 HLW disposal. In this study, laboratory tests and numerical simulations will be performed to investigate the effects of chemical cycles near the repository site on the hydro-mechanical (HM) behavior of compacted GMZ bentonite, and then to evaluate the long term performance of EBS. Firstly, based on the multi-phase suction controlling technique for unsaturated soil, a constant-volume cell for high-pressure swelling-infiltrate test will be improved to produce a Na-K-Ca salinity gradient cycling for chemical test. Then, laboratory tests like swelling, confined deformation, 1D consolidation and infiltrate tests will be conducted to study the HM behavior of compacted GMZ bentonite. Secondly, in order to research the effects of chemical cycles on HM properties due to microstructure changes, the mercury intrusion porosimetry (MIP) and environmental scan electronic microscope (ESEM) will be applied to quantify the microstructure characteristics of GMZ bentonite hydration in salt solution. Thirdly, the changes of mineralogical and chemical compositions in GMZ bentonite will be investigated by X-ray diffraction, energy spectrum analysis and chemical analysis before and after the tests, and the chemical mechanism for the effects of saline solution on the HM behavior of GMZ bentonite will be determined. 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 HM behavior damage in bentonite will be presented, and numerical simulations will be conducted using CODE_BRIGHT to model the evolution of the long-term performance of the EBS. The expected results will provide a theoretical basis and technical support for the design, construction, operation and management of EBS in China.

针对处置库近场化学环境对缓冲/回填材料长期性能的影响，以北山预选处置库场地和高庙子(GMZ)膨润土为研究背景，通过系统试验和数值分析，探索高压实GMZ膨润土在近场循环化学作用下水－力学性能衰变机理，建立工程屏障系统长期安全性评估方法。基于非饱和多相吸力控制技术，改造恒体积高压膨胀－渗透仪，开展Na-K-Ca盐梯度循环下的膨胀力、固结、有限变形和渗透性等试验，全面揭示膨润土水-力学性能衰变规律；利用压汞法和环境扫描电镜，观察膨润土水化过程中的微观结构变化，定量研究化学效应引起性能衰变的微观机制；借助X衍射、能谱分析和化学测试，查明试验中膨润土矿物相态和化学成分转变，深入阐明膨润土水-力学性能衰变的化学机理；由此建立基于化学-力学-渗流耦合效应的性能损伤本构模型，采用先进数值方法模拟膨润土水-力学性能衰变过程，评价工程屏障系统长期安全性。预期成果将为我国工程屏障系统建设提供理论依据和技术支持。

针对北山预选处置库地下水化学环境对工程屏障性能的影响，以我国首选缓冲/回填材料高庙子（GMZ）膨润土为研究对象，通过改造恒体积高压膨胀-渗透仪，实现了不同浓度的NaCl、KCl以及CaCl2溶液循环，开展了盐梯度循环作用下高压实GMZ膨润土的膨胀力、膨胀变形及水力渗透性能演化规律试验；通过化学成分、矿物成分分析及MIP、ESEM微观结构测试，研究了盐梯度循环作用对工程屏障系统水-力学性能影响的化学机理及微观机制；在此基础上，建立了基于化-力-渗耦合的高压实膨润土水－力学性能演化模型，对工程屏障系统的长期性能进行评估。研究表明，膨胀力随着入渗溶液浓度的增加而减小；浓度相同时，NaCl溶液入渗的膨胀力低于CaCl2溶液入渗的膨胀力，其差值与溶液浓度有关；膨胀力在盐化过程中减小，淡化过程中增大，其变化幅度与溶液浓度有关。体变性能方面，化学环境对膨胀变形具有明显的削弱作用；膨胀应变随着入渗溶液浓度的增加呈幂函数减小；相同浓度的Ca2+对膨胀量的削弱作用比Na+更强，但随着溶液浓度增加，差值逐渐减小；膨胀动力学过程表明，主膨胀过程与基质吸力的消散有关，发展较快，而次膨胀过程主要受盐溶液的抑制作用，发展较慢。渗透性能方面，渗透系数随着入渗溶液浓度的增加而增加；浓度相同时，NaCl溶液的渗透系数高于CaCl2溶液的渗透系数，其差值与溶液浓度有关；渗透系数在盐化过程中增大，淡化过程中减小；改变溶液浓度进行循环过程中，渗透系数不完全可逆，具有明显的回滞性。研究成果揭示了处置库近场化学条件下高压实膨润土缓冲性能的演化规律，为我国高放废物处置库工程屏障系统设计、建造及运营管理提供了理论依据。