【作者】 钱丽鑫；

【导师】 叶为民； 陈宝；

【作者基本信息】 同济大学 ， 地质工程， 2007， 博士

【摘要】 随着核科学的发展,核技术广泛应用于国防、工业、农业、医学等各个领域。核工业发展的同时也产生了大量的核废物,或称放射性废物。其中,高放废物的放射性水平高、毒性大、发热量大,若不加以安全处置,会对自然环境和人类社会产生巨大的破坏作用。因此,高放废物的处置是核废物管理中最为重要的课题,已引起国际社会的广泛关注。目前,深地质处置被国际上公认为处置高放废物最有效可行的方法。中国深地质处置的概念模型采用多重工程屏障系统（包括废物固化体、废物容器、外包装、缓冲材料）和适宜的地质体共同作用来确保高放废物与生物圈的安全隔离。膨润土由于具有极低的渗透性和优良的核素吸附等性能,被国际上多个国家选作缓冲材料。根据矿床位置、矿区地质特征、矿床成因、矿床储量、矿区自然地理、开采技术条件与交通等因素的综合对比研究,内蒙古兴和县高庙子膨润土矿床被确定为我国高放废物地质处置库缓冲材料供给基地的首选矿床。本文以非饱和土理论为依据,利用先进的非饱和土试验方法,对高庙子钠基膨润土的基础特性进行研究,为深地质处置库的设计提供理论依据。本文的主要研究内容及取得的研究成果包括:1.综述了高放废物深地质和缓冲材料的研究进展。阐述了非饱和土的水力一力学行为的相关概念,包括:吸力的定义和组成;持水曲线的概念和特点,以及各种拟合公式;非饱和渗透性函数的特点及公式;非饱和土中的有效应力和净应力;膨润土的微观结构研究及膨胀力等。介绍了与本论文有关的试验方法,试验技术和试验装置,包括吸力的量测方法、持水曲线的确定、微结构试验方法、非饱和渗透系数的测定、和膨胀力的测量方法等。2.分析归纳了高庙子钠基膨润土的一些基本性质特征:蒙脱石含量较高,阳离子交换容量和比表面积较大,液限较高,具有很好的不渗透性、高膨胀性和良好的导热性能;含水量相同时,无侧限抗压强度和弹性模量随初始干密度的增加而增大;干密度相同时,导热系数随着膨润土含水量的增加而增加;饱和渗透系数随温度的增加而增加,也随干密度的增加而增加。标准试样和膨胀试样的压汞试验结果表明,膨胀引起了孔隙量的增加,土中存在小孔和大孔两种孔隙,膨胀使膨润土中的大孔隙减少,小孔隙增加,膨润土的微结构趋于均匀化;膨胀力和干密度之间存在指数关系,干密度是影响膨胀力的一项重要外部因素。3.采用汽相法和渗析法,首次获取了自由和侧限条件下高庙子膨润土的持水曲线。结果表明:高吸力范围内,自由和侧限条件下的持水特征无明显不同,含水量随吸力的变化不大;在低吸力范围内,自由条件下含水量随吸力的降低产生突变,侧限条件下含水量随吸力降低的变化较为平缓。压汞试验和环境扫描电镜试验结果表明:自由状态下,膨润土集合体内吸收的水量非常有限,大量的水进入了集合体间的孔隙中。侧限条件下,随着吸力的降低,内部膨胀力不断发展,集合体间的孔隙被大量压缩,转变成了较小的孔隙,膨润土的孔隙出现了均质分布。考虑到压汞仪自身的限制,提出了一种计算残余孔隙量的新方法。采用双电层理论对干密度为1.7g/cm³的高压实高庙子膨润土在自由条件下的体变特征进行计算,结果与实验数据吻合较好。4.采用瞬时截面法,完成了侧限条件下高压实高庙子膨润土的非饱和渗透试验。结果表明:干密度为1.7g/cm³的高庙子膨润土在侧限条件下的水力渗透系数介于1.13×10^-13m/s和8.41×10^-15m/s之间。水力渗透系数并不是随着吸力一直减小,当吸力为65MPa左右,渗透系数最小;当吸力大于65MPa时,渗透系数随着吸力的增加而增加;当吸力小于65MPa时,渗透系数随着吸力的增加而减小。5.采用CODE_BRIGHT（考虑温度和吸力的剑桥模型有限元程序）软件,模拟分析了侧限条件下高庙子膨润土的非饱和渗透特性。结果表明:吸力的数值解与实测值吻合较好,吸力值从试样的顶部到底部逐渐减小,并随着水化时间的增加而减小;试样底部的饱和度明显高于试样顶部的饱和度,底部试样刚注水0.5个月就基本饱和,而整个试样在注水4个月后基本饱和。综上所述,本文首次对我国高放废物处置库首选缓冲材料——高庙子膨润土的基本特性进行了系统研究,获取了自由和侧限条件下高压实高庙子膨润土的持水曲线并分析了曲线特征;研究了高压实高庙子膨润土水化后的微结构变化特征,提出了计算残余孔隙量的新方法;采用瞬时截面法试验获取了侧限条件下高压实高庙子膨润土非饱和渗透系数,并完成了数值模拟,数值模拟结果与试验实测值吻合较好。本文所取得的研究成果,对于我国高放废物处置库工程屏障材料选用与工程设计具有重要的理论与工程实践意义。更多还原

【Abstract】 With the development of nuclear science, nuclear technique has been widely used in many fields, such as national defense, industry, agriculture and medicine. Large amount of high-level radioactive waste （HLW） has been produced, which can make great harm to natural environment and human society. Attentions have been devoted in recent years to the challenge of how to safely store nuclear waste. The deep geological disposal is regarded as the most reasonable and effective way to safely dispose HLW in the world. The conceptual model of HLW geological disposal in China is based on a multi-barrier system that combines an isolating geological environment with an engineered barrier system including the vitrified HLW, canister, and backfill and buffer material. The bentonite is selected as base material of the buffer material in HLW repositories, due to its very low permeability and excellent retardation of nuclides from migration, etc. According to the ore deposit position, traffic, genesis, reserve, geological feature of ore district and physic-geography etc, GMZ bentonite ore deposit located in Xinghe county, the Inner Mongolia Autonomous Region, is considered as the first choice of Chinese buffer materials providing base for HLW geological repository. In this paper, based on soil mechanics for unsaturated soils, advanced lab tests were conducted to study the fundamental properties of GMZ bentonite.The main contents in this paper are as follows:1. The research and development of deep geological disposal of HLW and buffer material are summarized. Theories relevant to the hydro-mechanical behavior of unsaturated soils are described, including the concept of suction, soil water retention curves （SWRC） and their equations, hydraulic conductivity, effective stress, microstructures and swelling pressure. A literature review on experimental techniques that are relevant to this study is presented, including measurement of suction and SWRC, microstructure technique （MIP and ESEM）,measurement of hydraulic conductivity and measurement of swelling pressure. 2. Some basic properties of GMZ bentonite are described: GMZ bentonite is characterized by high content of montmorillonite （about 75%）, high exchangeable cation content, specific surface area and liquid limit, low permeability, high swelling, and good heat conductivity. With the same water content, a larger initial dry density leads to a higher unconfined compressive strength and a larger elastic modulus. The heat conductivity of GMZ increases with the rising of water content in bentonite under the same compact density. Saturated hydraulic conductivity of GMZ increases with temperature and dry density. The results of MIP test indicate that swelling induces an increase in pore volume. There are two levels of pores （micro- and macro-pores）. Swelling of GMZ leads to the homogenization of the microstructure. There is exponential relationship between dry density and swelling pressure.3. The SWRC of GMZ are obtained using vapor equilibrium technique and osmotic technique. The results indicate: in high suction range, whether under confined or unconfined condition, the water content of compacted GMZ changes little with suction, and the data under two conditions are almost the same. However, in low suction range, the water content change with suction of the two curves indicates different behavior. Under unconfined condition, water content increases quickly with suction decreasing. MIP and ESEM test results show that, under unconfined condition, the amount of water absorbed by aggregates is very limit, and much water enters the inter-aggregate pores. Under confined condition, with the decrease of suction, the generation of swelling pressure compresses the macro-pores; the amount of micro-pores increases. Considering the limitation of MIP machine, a new method which can calculate the residual pore volume is put forward. Using double layer theory, the calculated volume change of GMZ under unconfined condition has good fit to the experiment data.4. Unsaturated hydraulic conductivity test is conducted on GMZ with 1.7g/cm³ dry density under unconfined condition. Results show that the hydraulic conductivity of GMZ is between 1.13×10^-13m/s and 8.41×10^-15m/s. The hydraulic conductivity is lowest at the suction point of 65MPa. When suction is higher than 65MPa, the hydraulic conductivity increase with suction increase; it is reverse when suction is below 65MPa. 5. CODE_BRIGHT （a finite element program of Cambridge model considering temperature and suction） is applied to perform numerical analysis for unsaturated hydraulic conductivity of GMZ under confined condition. Conclusions: the calculated suction is basically fit to experiment data. The saturation degree of the bottom specimen is obviously bigger than that of the top specimen. The whole specimen is almost saturated after 4 months.更多还原