Hydro-thermal Evolution Law Of Double-layer Buffer In High-level Radioactive Waste Repository

As a new clean energy,nuclear energy has been widely used in the world.However,a large amount of nuclear waste will generate due to the utilization of nuclear energy.Nuclear waste is a product with high levels radioactivity(HLW).And HLW has characteristics of a long halflife and high toxicity.If it is not properly disposed,the safety of the biosphere will be effected.The deep geological disposal is recognized as the most feasible method to dispose HLW.The artificial barrier is the last engineering barrier of the deep geological repository.Thus,it is very important to design the artificial barrier.Bentonite is selected as the material of the buffer/backfill in the engineering barrier due to the superior swelling performance,extremely low permeability and strong adsorption performance.However,the thermal conductivity of bentonite is lower than host rock,and decay heat from HLW releases continuously,resulting in that the surface temperature of the canister will increase significantly and the function of the repository will be damaged.Therefore,it is necessary to design the buffer for the safe operation in the repository.In the last few decades,most researchers suggested that the sand,graphite and other additives with superior thermal conductivity can improve the thermal conductivity of the buffer material.Although,the addition of additives can improve the thermal conductivity of the buffer material,the hydraulic conductivity will be increased as well,which is also one of the most important properties for buffer material.Therefore,a double-layer buffer model with outer and inner layers was proposed to develop both thermal conductivity and anti-seepage performance,and the temperature and humidity effects on double-layer buffer were studied by simulation and experiment method in this paper.Firstly,the optimal thickness of the double-layer buffers was determined.And then,the advantages of double-layer buffer compared with single-layer buffer by numerical simulation method were studied,based on the optimal thickness.Whereafter,the temperature and humidity evolution and distribution in the double-layered buffer were investigated by model test,under the thermal-hydraulic coupling.Finally,the physicochemical properties of the samples before and after the thermal-hydraulic coupling were analyzed,and the influence of thermal-hydraulic coupling on the buffer material was studied.The main conclusions are as follows.(1)The single-layer and double-layer buffer models under the thermal-hydraulic coupling were established and analyzed by the numerical simulation software.The results showed that when the buffer is stratified,and the inner layer width is 2/5 of the total width of the buffer,the conduction of temperature is most conducive.Furthermore,the double-layer buffer can partly reduce the peak temperature of the repository.(2)A testing device was developed and calibrated to study the evolution and distribution of temperature and humidity in the buffer under the thermal-hydraulic coupling.The results showed that the humidity change in the buffer depends on the process of the competition between the drying effect of the heater and the saturation effect of water.Meanwhile,the temperature and humidity have some changes after applying thermal insulation materials,it indicated that the changes in the surrounding environment have an impact on the distribution of temperature and humidity in the buffer material.(3)The moisture content,micro morphology and mineral component of the samples at different parts were analyzed after the laboratory experiments,the effect of thermal-hydraulic coupling on physicochemical properties was investigated.The results showed that the moisture content in the upper part is higher than the lower part of the sample,due to the heating effect.With temperature increasing,the moisture at the bottom of the sample migrate upward due to evaporation.Moreover,the results of SEM showed that the hydration degree of the part close to the heater was lower than the part close to the water inlet.From the results of XRD,it can be inferred that ion exchange between the montmorillonite and iron occurs in the part close to the heater,resulting in that a new substance is formed.