A Fundamental Study Of GMZ Bentonite As Buffer Material In Deep Geological Disposal For High-level Radioactive Waste

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.