Study On The Diffusion Behavior Of The Radionuclides ⁷⁹Se And ⁹⁹Tc In Tamusu Clay

Nuclear safety is the basis and key of China’s national security system,and the safe disposal of high-level radioactive waste is an important part of nuclear safety.Geological disposal of high-level radioactive waste is the most safe and feasible method recognized by the international community.At present,all countries in the world are actively promoting the research and development of geological disposal repository of high-level radioactive waste.The disposal library is mainly built in the geological body at a depth of500～1000 m below the surface by means of"multiple barriers"with"containment retardation"as the core.Therefore,the choice of disposal site is crucial.The pre-selection of the disposal site should be carried out from the aspects of geological structure,hydrological conditions,rock mechanics,groundwater physical and chemical properties and nuclide migration.The evaluation of geological conditions of the tamusu claystone site in China has been basically completed,but the study of nuclide migration in claystone has not been reported..In this paper,the diffusion behavior of nuclides ⁷⁹Se and⁹⁹Tc in tamusu clay and irradiated tamusu clay was simulated by capillary method in laboratory.The non-radioactive elements Se（Ⅳ）and Re（Ⅶ）were used to replace ⁷⁹Se and⁹⁹Tc,respectively,to investigate the influence of clay compaction density,solution ion strength,solution p H value,humic acid and other conditions on diffusion.Key parameters such as apparent diffusion coefficient D_a,effective diffusion coefficient D_e,porosityεand distribution coefficient K_dwere obtained.The experimental results show that the diffusion behavior of nuclides in tamusu clay can be simplified by the one-dimensional and semi-infinite medium diffusion model.The nuclides mainly diffused in the pore water of the clay,and the diffusion behavior was affected by the following conditions:1)with the increase of compaction density from 1600 kg/m³to2050 kg/m³,the small diffusion coefficients of Se（Ⅳ）and Re（Ⅶ）in the clay before and after irradiation all decreased;2)with the increase of solution ion strength from 0.01mol/L to 0.5 mol/L,the diffusion coefficients of Se（Ⅳ）and Re（Ⅶ）in the clay increased before and after irradiation;3)as the p H value increases from 4 to 10,the diffusion coefficient of Se（Ⅳ）decreases first and then increases,and is minimized when the p H value is 7.The diffusion coefficient of Re（Ⅶ）is not affected by p H value;4)humic acid can promote the diffusion of Se（Ⅳ）and Re（Ⅶ）in irradiated clay,but the diffusion coefficient has a small range;5)the diffusion coefficient of Se（Ⅳ）and Re（Ⅶ）in irradiated clay is 2～3 times smaller than that in unirradiated clay.This work also characterized the clay before and after irradiation,analyzed its composition and microstructure,and provided a basis for discussing the diffusion mechanism.As a preliminary study on the diffusion behavior of nuclides in tamusu clay,we hope that the experimental results can provide data support and theoretical reference for the site selection and safety evaluation of the geological disposal repository of high-level radioactive waste in China.