如何安全处置高放废物是科学、技术以及工程界面临的世界性难题。目前，高放废物地质处置是一种被广泛认可的处置手段。但是由于高放废物具有放射性强、毒性大、半衰期长的特点，为了保证高放废物可以在上百万年与生物圈充分隔离，从而不对人类活动造成影响，高放废物地质处置的安全性评价是一项非常重要的研究工作，而数值模拟是预测和评价其安全性的唯一手段。目前，我国正处在地下实验室与示范处置阶段，并开展了相应的野外调查与实验研究等实质性工作。但是，如何更准确地研究和预测核素在高放废物地质处置中的迁移始终是高放地质处置安全性评价研究领域的热点和难点。 我国高放废物地质处置的选址已经经历了技术准备阶段、选址与场址评价阶段、现场实验阶段，通过调查、对比、筛选以及评价等技术手段，初步认为甘肃北山为我国高放废物处置库的候选地区之一，且正在进行地下实验室的建设工作。因此本次研究将根据现有的资料，围绕北山地区进行场址安全性评价，采用多尺度（区域尺度、预选区尺度、库区尺度）、多场耦合（温度场、渗流场、应力场、化学场）的研究方法，利用TOUGH2-MP、TOUGHREACT、FLAC3D多种模拟软件将水文地球化学、热力学、水动力学等进行多学科交叉，涉及的领域较广。本研究从饱和-非饱和带的区域地下水流场开始分析，通过缩小研究区范围、筛选典型核素、到最后较为系统地对典型核素在高放废物处置环境中迁移进行模拟预测研究。获得的主要研究成果如下： （1）在区域尺度上，采用TOUGH2-MP/EOS3模块，并利用等效连续介质法模拟区内存在的区域性断层和裂隙，建立了三维饱和-非饱和带的区域地下水流数值模型，经过地下水位、钻孔垂向液体饱和度的变化、区域沟系的形态分布、地下水总储量变化的识别，证明了区域数值模型的合理性，并更加精细地反映了区域地下水流场形态、地下水均衡、断层和不同气候条件对区域地下水流场的影响。根据模型结果可知，研究区北部大部分地区与预选区处于完全不同的地下水流动系统中，因此可将研究区范围大幅度缩小，为重点考虑预选区的核素迁移模型奠定基础。 （2）在预选区尺度上，采用TOUGH2-MP/EOS7R模块，考虑了地下水对流、扩散、吸附以及核素衰变的影响，建立了预选区核素迁移的数值模型，通过假设在不同的位置投放核素，分析了四种核素（90Sr, 137Cs, 238U, 239Pu）在万年尺度下的迁移规律，并对岩石渗透率、分配系数以及扩散系数进行了敏感性分析，从而进一步筛选出对模型参数最敏感的典型核素—238U。其中，分配系数对238U迁移的影响最大。该结论可为高放废物安全性评价提供初步的参考，并为下一步库区尺度的典型核素迁移的多场耦合数值模拟奠定基础。 （3）在库区尺度上，采用TOUGHREACT和FLAC3D软件耦合的模拟方法，利用线弹性的力学模型描述膨润土的应力和应变关系，建立了库区尺度的THMC（温度场-渗流场-应力场-化学场）多场耦合模型，并重点研究了应力变化对缓冲材料（膨润土）THC场的影响。研究结果发现，在模拟时长1000年内，考虑库区应力变化对膨润土中其它三场的影响较小，从而分析该影响对核素在膨润土中的迁移影响不明显。基于该评估结果，构建了THC多场耦合数值模型，采用双位点质子迁移非静电配位/离子交换模型（2SPNC/CE）重点考虑了上万年尺度下，U(VI)在膨润土中的吸附解吸作用、阳离子交换反应以及矿物的溶解沉淀等复杂化学反应，并预测了U(VI)在膨润土中的迁移规律。研究结果发现，绝大部分的U(VI)将被蒙脱石和伊利石吸附，约0.1%左右的U(VI)会以液态和交换态的形式存在。经过10万年的模拟预测，U(VI)无法迁移出约0.7m厚的膨润土防护层，证明了膨润土作为缓冲材料的有效性。最后，假设该迁移规律为预选区中核素泄露的浓度曲线，预测核素在预选区尺度的迁移范围。研究结果发现，若花岗岩不存在裂隙，由于考虑了缓冲材料的吸附作用，并且预选区地下约500m深度的地下水流速很慢，核素的迁移距离比较有限，进而说明了考虑缓冲材料对核素迁移研究的必要性。 本研究通过多尺度、多场耦合的数值模拟方法，系统地研究了高放废物地质处置中核素的迁移过程。在目前主要是孤立进行核素在膨润土或花岗岩中迁移的研究背景下，提供了系统化模拟高放废物地质处置核素迁移的方法。通过模拟研究，缩小了我国高放废物地质处置的研究范围，证明了膨润土作为缓冲材料对铀的阻滞效果，提出了考虑缓冲材料对核素迁移研究的必要性，并为最终的高放废物地质处置提供技术支持。

How to deal with the High-level radioactive waste (HLW) is a highly challenging work for all over the world. So far, High-level radioactive waste disposal is a widely accepted method. However, due to the strong radioactivity, high toxicity, and long half-life of the HLW, the safety evaluation of geological disposal which could ensure the HLW can be fully isolated from the biosphere in millions of years, and the human activities are not affected, is a very important research work. Numerical simulation is the only way to predict and evaluate the safety. At present, China is in the stage of establishing the underground laboratory, and the substantive work such as field investigation and experimental research have been carried out. However, how to study and predict the migration of nuclides in the HLW geological disposal more accurately is always a hot and difficult issue in the field of safety assessment for the HLW geological disposal. In China, the HLW site selection has undergone the preparation stage, field investigation and evaluation stage, and on-site experimental stage. The preliminary conclusion reveals the Beishan area, Gansu, is the prospective HLW disposal site after investigation, comparison, screening and evaluation, and then an underground laboratory is under constuction now. Therefore, the main research work such as analysis on the saturated-unsaturated regional groundwater flow field, the nuclides migration in the prospective site of the disposal repository will be studied deeply and systematically. In this study, the site safety assessment has been carried out based on the available data, which is studied by multi-scale and multi-field (Thermal-Hydrogeological-Mechanical-Chemical field) coupling models. Then, a number of programs considering hydrogeochemistry, thermodynamics, dynamics and other multidisciplinaries, such as TOUGH2-MP, TOUGHREACT and FLAC3D, are also used in the present study. The main conclusions are as follows: (1) At the regional scale, a saturated-unsaturated groundwater numerical model is established by the TOUGH2-MP/EOS3 module. In the model, the significant faults and fractures in the area are simulated by the equivalent continuous medium method. The reliability and accuracy of this model are verified by measuring the groundwater level of open wells, the vertical variation of liquid saturation and the total groundwater reserves. It proves the rationality of the parameters about the regional numerical model and also reveals the regional groundwater flow field more accurately. In addition, the results suggest the groundwater balance, the influence of rainfall and faults for the regional groundwater flow field. In all, according to the model results, the groundwater flow system in the most northern part of the study area is totally different from the prospective site of the disposal repository. Therefore, the scope of the research area can be greatly reduced for the nuclide migration simulation. (2) At the prospective site scale, a nuclide migration model is established by TOUGH2-MP/EOS7R, considering the groundwater convection, diffusion, the sorption of granite and decay of nuclides. The migration process for four kinds of nuclids (90Sr, 137Cs, 238U, 239Pu) in the prospective site is analyzed firstly. Then, the sensitivity analyses of parameters, including the permeability, distribution coefficient and diffusion coefficient are carried out to screen out the most sensitive nuclide for the model parameters. The results show 238U is the most sensitive nuclide among four radionuclides, and the distribution coefficient is the most sensitive factor. These findings will provide a preliminary reference for the radionuclide migration process at a prospective HLW disposal site, which could be informative for the safety analyses and further transport in and around the repository system. (3) At the local scale, a full THMC coupling model is set up first by TOUGHREACT and FLAC3D simulation software, and a linear elastic model was established to describe the stress and strain relationship of bentonite and the granite. This model focuses on the impact of stress field on the thermal, hydrogeological and chemical field of bentonite. The results found that within the simulated time (1000 years), the influence is unobvious, and it is inferred that this effect also has little influence on the migration of nuclide in the bentonite. Based on that, a THC coupling model described by two-site proton migration non-electrostatic coordination/ion exchange method (2SPNC/CE) is set up. The adsorption and desorption for U(VI), ion exchange and the dissolution and precipitation of minerals are all considered in this model within 100,000 years. The results reveal that the most of the U(VI) is adsorbed by montmorillonite and illite, and approximately 0.1% of U(VI) exists in the form of liquid and exchangeable state. Then, after 100,000 years, U(VI) could not migrate out from the bentonite with the thick of 0.7m, which suggests the bentonite is useful for the release. Then, the migration rules of U(VI) in bentonite could be predicted by the coupling model. This rule is assumed as the releasing source, which is applied to calculate the migration distance at the prospective site scale. If there is not any fractures in the granite, the migration distance of U(VI) at the prospective site scale is limited (31m) in 100,000 years. There are two reasons for that: firstly, the sorption of the bentonite is considered in this mode; secondly, the groundwater velocity of the prospective site with the depth of 500m is rather small. These findings suggest the bentonite plays an important role in the migration process of nuclides, which are informative for the safety evaluation for HLW geological disposal in China. In this study, a multi-scale and multi-field coupling numerical simulation method is used to systematically study the migration process of nuclides in the geological disposal environment for HLW. It provides a new way to systematically calculate the migration distance of nuclide under the background of independent research on natural system and engineered barrier system. According to the study, on one side, the research scope has been greatly reduced; on the other side, it proves the effectiveness of bentonite for the retardation and puts forward the necessity of considering bentonite for the migration. The simulation method promotes the study of geological disposal programs in China and also provides the technical support for the final disposal.