Fuzzy Comprehensive Performance Evaluation Of Bentonite-sand Mixture As HLW Buffer/Backfill Material

With the depletion of fossil fuels and corresponding environmental pollution involved with fossil fuels utilization, nuclear power proportion is increasing in the whole energy consumption because of the nuclear power advantages compared with other regular generation like thermal power, water power, and wind power, etc. Nuclear power plants will produce a little amount of radioactive wastes that have to be handled safely and properly. Under the current technical conditions, deep geological disposal is widely acceptied approach for the safe disposal high-level radioactive wastes(HLW), simply, by closing the HLW in the stable strata at the depths of 500-1000 m underground to keep away from the surface environments. Backfill/buffer material is the most important part of engneering barrier in the HLW deep geological repository for the safe disposal. Bentonite is selected as a backfill/buffer material by many countries because of its good properties. Pure bentonite is good at impermeability and hydrated expansion, but poor at thermal conductivity and workability. Domestic and foreign scholars, therefore, tried to improve these disadvantages by add auxiliary materials to the pure bentonite. After a large number of experimental and theoretical studies, it has been found that bentonite-sand mixture has a better terminal conductivity and workability compared to pure bentonite, and the permeability and adsorption the mixture are slightly lower than pure bentonite, but still meet the design requirements.Composition study about the bentonite-sand mixture as HLW backfill/buffer material is to determine optimum content of quartz sand in the mixture at which the overall performance of bentonite-sand mixture as barriers comes to its best. Researchers from Lanzhou University conducted a lot of laboratory tests on the properties of GMZ bentonite-sand mixture, such as permeability, expansion, thermodynamic properties, compaction properties, mechanical properties, adsorption properties, and determined the relationship between dry density and sand mixing ratio. Based on the previous experimental studies about the mixtures with a mixing sand ratio ranging from 0% to 5% and a dry density ranging from 1.20 to 1.80 g/cm3, this thesis is intend to introduce the fuzzy mathematical theory to establish a model for evaluatation of the comprehensive properties of bentonite-sand mixture. The conclusions as followings can be drawn.Evaluation results show that the overall performance of bentonite-sand mixture is best when the mixture has a dry density of 1.82-1.83g/cm3 and a mixing sand ratio of 29-30%. After the addition of quartz sand, the impermeability of the bentonite-sand mixture is reduced so slightly that the mixture still fit the requirements as a hydraulic barrier. The properties of the mixture except mechanical properties are all great improved, like stability, hydrated expansion, and thermal performance. In case that quartz sand is added, the bentonite-sand mixture can be compacted to a greater dry density and, therefore, to compensate the strength loss due to the addition of sand. In other words, bentonite-sand mixture presents excellent, comprehensive properties. By introduction of the concept of effective clay density, sand ratio optimization for overall performance improvement can be reasonably explained as followings. Addition of sand to bentonite means an increasing in the effective dry density of the mixture. As a final conclusion, the comprehensive performance of the bentonite-sand mixture at a constant dry density is elevated until the sand ratio reached to 30% and then decreased. In case of constant mixing sand ratio, the comprehensive performance of the mixture will increase with the increase in dry density within the investigated range of 0-50% sand addition.