Unsaturated Property Of Compacted Bentonite-sand Mixtures As Buffer Backfilling Material

High-Level Radioactive Waste (HLW) needs to be isolated from the living environment, because it contains strong radioactive, heat, poisonous, and long half-life nuclides. The deep geological disposal is currently believed to be the best feasible disposition, which means that the waste is deeply buried under ground. A multiple barrier system is accepted as the repository conceptual, in which the buffer backfilling material is the most important engineered barrier to prevent the migration of radionuclides by groundwater.Much research has been done on the selection of Chinese buffer backfilling material during the past20years. So far Na-bentonite compacted to very high density has been considered as the ideal buffer backfilling material in HLW repositories due to its high absorption, high swelling property and low permeability. However, the heat conductivity of pure bentonite is very low. If the heat produced by radioactive waste is not able to be transported out to the surrounding rock promptly and effectively, the temperature of the buffer layer would rise correspondingly. When the temperature exceeds100℃, liquid gasification will produce too high water vapor pressure that destroy the buffer layer structure and the integrity of the buffer backfilling material. Because of the very high plasticity, pure bentonite tends to be pelletoid during bentonite-water mixing process and lead to inhomogeneous wetting of dry bentonite powder. It is the poor workability that hinders the pure bentonite to be compacted to a high density. However, putting some additional material, such as sand, into bentonite, not only can improve its mechanical strength, heat conductivity and minimize long-term creep deformation without obvious high absorption, high swelling property and impermeability, but also can optimize the design and construction of deep geological disposal of HLW, reduce costs. Lanzhou University proposed an innovative research project to develop bentonite-sand mixtures as buffer backfilling material following the research trend in developed countries.The final goal to the research on compacted bentonite-sand mixtures is to investigate the optimum sand ratio, under which the mixtures will reach the best state of adsorption to radionuclides, mechanical strength, impermeability, heat conductivity and swelling sealing simultaneously. As known, the buffer layer is difficult to be fully saturated even after1000years. The bentonite swells during the infiltration process of the pore water in the host rock, resulting in the pressure development and consequent changes in dry density distribution. Besides, the water-holding capacity and unsaturated permeability are affected by the saturation degree, pore solution and temperature. Thus, this thesis is to study the unsaturated property of bentonite-sand mixtures. More specifically, the influence of sand ratio, dry density, solute and temperature on the unsaturated property of bentonite-sand mixtures has been explored, and hence providing a basal data for final optimum sand ratio.Gaomiaozi (GMZ) bentonite is selected as major research material in this study. With different quartz sand ratio, four different compaction methods for buffer backfilling materials have been developed. The static compaction with variable energy is selected for sample preparation. The unsaturated property is studied by soil water characteristic curve test and infiltration test. The tests are made on samples with different dry densities (1.50to1.90g/cm3) and sand ratios (0%to50%), the salinity of hydration water in this case is0,6and12g/L (pH=7.1), obtained by mixing NaCl and Na2SO4in the weight ratio2/1. The test temperature is ranging from20to60℃. The nature of unsaturated property changing is revealed with the consideration of mentioned factors, and the result can provide support to the optimum buffer backfilling material design of China’s HLW disposal repository. It is found that:According to the mixing-wetting-curing-compaction process, the small-scale buffer backfilling material sample preparation in laboratory is proposed. To begin with, Quartz sand is added into bentonite in step-by-step by using mixing method. Then the spray-on process is performed to achieve the objective water content, preventing the generation of "aggregates". After unloading, the compacted sample trace will rebound. The rebound phenomenon would become more obvious with increasing dry density. It is found that the sample dry density at which near compacting end is slightly higher than its average value. Finally, the four compacting methods are developed:dynamic compaction with constant compaction energy, dynamic compaction with changing compaction energy, static compaction with constant compaction energy and static compaction with changing compaction energy. Among them, static compaction with changing compaction energy is a favorate one, by which samples can be compacted to desired parameters with highest density.When the suction keeps constant, the water content of sample under confined condition is lower than that under unconfined condition. The water-holding capacity of sample under confined condition becomes lower with the sand ratio or dry density increasing, while becomes higher with the augment of salt content decreasing, and nearly independent of the temperature. The water-holding capacity of sample under unconfined condition becomes lower with the boost of sand ratio as well, but it is nearly independent of the dry density. Based on the conception of composite soil, the effective clay density and effective water content are introduced to determine and modify soil water characteristic curves (SWCCs). It is found that the normalized SWCCs of samples under confined condition is mainly controlled by the effective clay density, while the normalized SWCCs of samples under unconfined condition is largely affected by the sand ratio. This thesis selects the best calculation method of osmotic suction. With the aid of ESEM pictures, it is also found that the samples under confined condition rarely crack after losing water, while the samples under unconfined condition always break after losing water. Specifically, most of the fracture is distributed near quartz sand, and the width of fracture amplifies with increasing sand ratio.The increase in sand ratio or the decrease in dry density leads to the reduction of effective dry density and the expansion of micro-pore. As a result, the value of unsaturated hydraulic conductivity increases. The thickness of diffused double layer (DDL) decreases with the distilled water changing into the solution containing12g/L concentration of Na+. The higher the Na+concentration in the pore fluid, the thinner the thickness of DDL is. As a result, the value of unsaturated hydraulic conductivity also increases. The viscosity coefficient of infiltrate water decreases with the temperature increasing from20to40℃, as a result, the value of unsaturated hydraulic conductivity increases. The sample absorbes water to produce pressure during infiltration process. The swell stress value near water inletting end develops faster than that near the outletting end. With the elevation of dry density or the decrease of sand ratio, the stress value of sample increases at the end of water infiltration. After the tests, it is found that the dry density in the middle position nearby water inletting end of sample is lower than those found before water infiltration. The dry density through sample is more homogeneous as well as the water content at the end of the infiltration test. The infiltration process could help the dry density distribution in the sample tend to become more uniform.As a precondition of the long-term stability of the buffer backfilling material, the parameters, namely that hydraulic conductivity, cohesion, friction angle, swelling pressure, swelling strain, liquid limit and plastic limit are evaluated, by investigate the engineering properties, including compaction property, physical property, swelling property, permeability property and shear property, respectively. By using methods of fuzzy opinion concentration, the data thus far supports the idea that the GMZ bentonite-sand mixtures with sand ratio of20%～30%and dry density of1.60～1.80g/cm3would be an effective buffer backfilling material. That is, the compaction property and workability are improved, permeability remains basically unchanged, and excessive swelling pressure from absorbing water, which may be harmful to the container, is lessened.