Swelling Characteristics Of Compacted Bentonite-Sand Mixtures As Buffer/Backfill Materials For HLW Disposal

High-Level Radioactive Waste (HLW) is a waste very difficult to be disposed, which contains strong radioactive, heat, toxicity, and long half-life nuclides. It must be long-term and reliably isolated with the living environment of the human. The deep geological disposal is currently believed to be the best feasible disposition way, which means that the waste was deeply buried in the ground. A multiple barrier system was accepted as the repository conceptual, in which the buffer/backfill material is the most important barrier to prevent the migration of radionuclides by groundwater.In the past20years, many researches have been done on the select of backfill/buffer materials in China, and GMZ bentonite is considered to be a reliable buffer/backfill material in HLW repositories because of its high absorption and high swelling properties and low permeability. However, because the plasticity of pure bentonite is very high, a tendency of "pelletization" will present during the process of water-soil mixed, this will cause an obvious inhomogeneous wetting of bentonite, and then give trouble to the block making process of buffer/backfill material. In addition, the heat conductivity of pure bentonite is very low, if the heat produced by radioactive waste can not be sent out to the surrounding rock promptly and effectively, the temperature of the buffer layer will rise. When the temperature exceeds100℃, liquid gasification will produce too much water vapor pressure and affect the stability of the repository. Moreover, montmorillonite in bentonite will be converted into illite when the temperature exceeds100℃, so that the function of the buffer/backfill material changes. For bentonite-and, bentonite-crushed rock or bentonite-ballast mixtures possess higher structural integrity, thermal conductivity. As a modern trend in backfill/buffer material development, bentonite is optimized by addition of certain content of quart sand to improve the strength and heat conductivity without obvious lowering of permeability. Following the recent development trend in deep geological disposal of HLW, compacted bentonite-sand mixtures is recommended by Lanzhou University, China as an innovative buffer/backfill material for its enhanced thermal conductivity and workability.The final research aim to the compacted bentonite-sand mixtures is to investigate the optimum sand addition, under which the mixtures will provide a best adsorption to radionuclides, impermeability, swelling sealing at same time. As a specific research, objectives of this study is to reveal the swelling characteristics, study on the swelling behavious of compacted GMZ bentonite-sand mixtures with different initial dry density and sand content with different pore water, providing a basal data for final optimum ratio of sand content of the mixtures.GMZ bentonite-sand mixtures as backfill/buffer material in China for high level radioactive waste (HLW) were mixed with quartz sand in a ratio of0,10,20,30,40and50%(w/w) and compacted to a series of specimens in different dry density. By distilled water as pore water, liquid limit,,plastic limit and swelling tests were conduced on bentonite-sand mixtures pressure and the influence of sand content, initial dry density on the characteriscice of compacted mixtures were analyzed and reported. Liquid limit, plastic limit and the swelling behaviors of compacted GMZ bentonite-sand mixtures inundated in NaCl-Na2SO4solutions are investigated and the influence of chemical solutions on the swelling behaviors of GMZ bentonite-sand mixtures is investigated. The sand content ratios of the bentonite-sand mixtures are0,20,30and50%, and the total dissolved solids (TDS) of the NaCl-Na2SO4(NaCl:Na2SO4=2:1by mass) solution are0.5,1.0,3.0,6.0and12.0g/L (pH=7.1). The specimens of bentonite-sand mixtures for swelling tests are prepared by static-compaction to various dry densities, ranging from1.50-1.90g/cm3.Results received by these tests were as follows:Liquid limit and plastic limit of the mixture decrease linearly with the increase of sand addition percentage. Swell occurs in three distinct phases:inter-void swelling, primary swelling and secondary swelling, both swelling pressure and swelling strain follow a sigmoid relationship with time. With constant initial water content, the maximum swelling pressure presents an exponential increase with increased initial dry density, and the maximum swelling strain increases linearly. With the increase of sand content ratio, the maximum swelling pressure decreases exponentially and the maximum swelling strain follow a quadratic decrease.Liquid limit (wL), plasticity limit (wP), swell time, maximum swelling pressure and maximum swelling strain decrease with the increase of TDS for GMZ bentonite-sand mixtures. All of the LL, PI and maximum swelling strain are decreased exponentially with TDS increase:very quickly as TDS<3.0g/L, slowly as TDS=3.0-6.0g/L and almost stabilized as TDS>6.0g/L. The maximum swelling pressure shows a linear reduction with the TDS increasing, but the pure bentonite indicates a high sensitivity than the bentonite-sand mixtures with30%sand content ratio. Compared with the pure bentonite, bentonite-sand mixtures show a better tolerance withstanding the chemical attack to ground water chemistry because of the replacement of some quantity of expansive clay by quartz sand in the mixtures.As NaCl-NaoSO4(TDS=0.5g/L) solution was used according to the ground water, with initial dry density of1.70g/cm3, the maximum swelling pressure of specimens decrease exponentially while the maximum strain decrease linearly with the increase of sand addition. With30%sand addition in0.5g/L NaCl-Na2SO4solution, the maximum swelling pressure increase exponentially while the maximum strain increase linearly with the increase of initial dry density. This means that the increase of sand would restrain the swelling properties of bentonite while the increase of initial dry density can enhance that.By the introduction of the concept, effective clay density pb, equations include both initial dry density and sand content ratio are proposed, they produce a good satisfaction with the experimental data. And these equations can predict the swelling characteristics of GMZ bentonite-sand mixtures with sand content ratio and initial dry density designed in certain range.Based on the data obtained from experimental tests, considering both swelling pressure and swelling strain comprehensively, the optimal sand content ratio and initial dry density are20-30%and1.60-1.80g/cm3.