高温高压下核废料垫层材料在含盐水溶液中膨胀性衰减规律研究

Compacted bentonites are often considered as a possible buffer material in high-level deep radioactive waste disposals. After the installation of waste canisters,the engineered clay barriers are subjected to thermohydromechanical action in the form of aqueous solution infiltration from the geological barrier, heat dissipation from the radioactive waste canisters, and stresses generated by clay swelling under almost confined conditions. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term. In this project, the absorbed volume of aqueous solution can be expressed using the surface fractal dimension accroding to the fractal model. From the thermodynamic viewpoint, a balance exists between the decrease in free energy of aqueous solution and the increment in the work of vertical overburden pressure, a correlation bewteen the absorbed volume of aqueous solution and the osmotic suction is deduced. Swelling deformation can be calculated from the the absorbed volume of aqueous solution. In the expression of swelling deformation, the surface fractal dimension, absorbed volume of aqueous solution and osmotic suction are the function of the temperature, pressure and ion concentration. The bentonite stability is characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), and the surface fractal dimension of bentonite is measured using the single adsorption isotherm. The microfabric morphology of absorbed aqueous solution is detected using X-ray tomography and scanning electron microscope. The swelling tests are performed under high temperature and pressure using the modified odometer with the temperature controlling system. A simplified method (volume balance in a saturated medium) is applied to evaluate the swelling capacity evolution of a bentonite barrier because of their geochemical transformations by using a thermokinetic hydrochemical code. The project is not only academic innovation, but also has a large practical significance.

核废料的理想垫层材料膨润土中的蒙脱石受到水溶液、核废料放射热和遇水膨胀产生膨胀力的热?水?力学耦合作用，产生离子交换，引起蒙脱石矿物相变，导致膨胀性发生衰减，影响垫层材料的性能。本项目根据矿物表面的分形模型，建立吸附水溶液体积与表面分维的理论关系；根据吸附过程的能量守恒方程，导出吸附水溶液体积与溶质吸力间的理论公式，提出膨胀变形和膨胀压力的计算方法。采用红外光谱傅立叶变换技术和X射线衍射分析蒙脱石相变特征，采用分子吸附法确定矿物表面分维变化规律，采用扫描电镜分析吸附层厚度，验证膨胀变形的计算方法；利用改装的高温高压膨胀变形试验装置进行膨胀变形试验，采用一维动力反应和质量迁移程序预测核废料垫层材料的膨胀性衰减程度，评价膨润土的膨胀性能。本项目既是有关工程地质的基础理论研究，又解决工程实际问题，不仅具有创新的理论意义，也具有保护环境、造福子孙的社会意义和经济价值，研究成果具有广阔的应用前景。

深层地质处置是国际上普遍采用的高放射性核废料的最终处置方案。膨润土因具有高吸水膨胀性、低渗透性和强核素吸附性等优点被选作深层地质处置系统中缓冲回填材料。膨润土受到地下水溶液、核废料放射热和围岩应力的化学−热−水−力学的耦合作用，产生离子交换，引起蒙脱石矿物相变，导致膨胀性能衰减，影响缓冲回填层的性能。因此，准确计算膨润土在地下水溶液中的膨胀特性、预测膨润土膨胀性衰减规律是亟待解决的关键问题。本课题通过室内膨润土的膨胀试验和理化测试，结合理论分析和数值模拟，研究了膨润土在地下水溶液中的膨胀特性，提出了膨润土在盐溶液中的膨胀模型和膨胀性的计算方法，分析了膨润土膨胀性衰减机理，模拟了膨润土在核废料处置库中的膨胀特性和衰减规律。取得以下主要成果：.（1）采用N2吸附法测量了膨润土的表面分维，建立了膨润土表面的分形模型。膨润土粉末的分维与压实膨润土的分维相同，盐溶液对膨润土表面分维影响很小。.（2）完成了膨润土在不同浓度盐溶液中的膨胀变形试验，建立了膨润土膨胀的分形模型。基于膨润土吸水膨胀过程的表面吸附作用，根据膨润土表面的分形模型，建立了膨胀吸水体积的表达式，导出膨胀变形和膨胀力的理论公式。在膨润土的渗透膨胀阶段，结合双电层理论给出分形模型中的蒙脱石膨胀系数K。基于膨润土表面的分形模型，推导出盐溶液中渗透吸力引起的有效应力，提出了盐溶液中膨润土的膨胀变形理论方法。得到了膨润土在盐溶液中膨胀变形试验数据验证。.（3）揭示了GMZ01膨润土在氯盐溶液（NaCl、CaCl2、MgCl2）和NaOH溶液中膨胀性衰减机理。膨润土与氯盐溶液混合后，阳离子交换是主要的化学反应过程，Ca2+和Mg2+与Na基蒙脱石中的Na+反应生成膨胀性较低的Ca基或Mg基蒙脱石，引起膨胀性衰减。与NaOH混合后，高碱性溶液对蒙脱石有强烈的溶蚀作用，蒙脱石发生溶解生成非膨胀性矿物，产生相变，导致膨胀性衰减。基于膨润土膨胀性衰减机理，建立了渗透吸力、离子交换反应和相变引起膨胀性衰减的预测方法。.（4）模拟膨润土膨胀变形的衰减过程。基于核废料处置库中缓冲回填材料的THMC四场耦合作用，建立膨润土的多场耦合方程，模拟了核废料处置库缓冲层中膨润土长时间多场耦合作用下的膨胀性衰减过程，分析了膨润土吸水膨胀随时间的演化规律，提出膨润土膨胀性衰减的定量评估方法。