Retardation Effect Of Buffer Material In High-level Radioactive Waste Geological Disposal

High-level radioactive waste which contains strong radioactive,heat,toxicity,and long half-life nuclides 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.Due to its very low Permeability and excellent retardation of nuclides from migration,the bentonite is selected as base material of the buffer material in HLW repositories.However the pure bentonite as the buffer material has two limitations.One is that low thermal conductivity properties is not helpful to conducting the heat of radiation into the surrounding rock in the cavern,that causes the buffer layer temperature raise to more than 100℃ and water vapor Pressure.Anther is that it has bad workability,which is difficult to compact to the maximum dry density.As a modern trend in 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.But the limitations bentonite for radionuclide adsorption capacity,quartz sand is joined into the bentonite that did not improve the adsorption ability of radionuclides and reduce the overall adsorption performance.It is proposed that a multivariate model integration buffer material of "basesupplementary-additive material for HLW disposal,its purpose is to maximize the function of blocking radionuclide migration for natural mineral materials.For this reason,adsorption characteristics,engineering properties and retardation effect are studied based on the multiple integrated methods,which include theoretical analysis,tests and numerical simulation.Meanwhile based on the theory of porous media pollutant migration,the convection-diffusion-adsorption model of radionuclide migration is established,and based on numerical simulation,the retardation effect of buffer material is analyzed under different conditions of the different factors.The major research achievements are summarized as follows:(1)A mixture of bentonite-zeolite-pyrite(abbreviated as B-Z-P)is suitable for processing the radionuclide strontium and cesium,and a mixture of bentonite-attapulgite-pyrite(abbreviated as B-A-P)is suitable for processing the radionuclide uranium by comparing the adsorption properties of single mineral materials and integrated buffer materials for strontium,cesium and uranium.(2)The experimental results of engineering characteristics and the constant source diffusion experiment for Integrated buffer material show that B7ZP(B:Z:P=63:26:10)is optimal formula for blocking radionuclide strontium and cesium,and then B7AP(B:A:P=63:26:10)is optimal formula for blocking radionuclide uranium.(3)The convection-diffusion-adsorption model of radionuclide migration is established,and based on the model,MATLAB software is applied to simulate and analy nuclide transport distance of the integrated buffer material under the different time scale degree.The results indicate that the migration distance of radionuclide strontium and cesium is shortest in B7ZP,and the migration distance of radionuclide uranium is minimum in B7AP.At the same time,the results verify the block effectiveness of the proposed optimal integration buffer material formula.Finally,When seepage velocity is less than 10-4m/a,relative to seepage velocity,apparent diffusion coefficient and retardation factor are main parameters affecting the radionuclide migration,which is obtained by analysis of radionuclide migration distance under the different seepage velocity,apparent diffusion coefficient and retardation factor.