Hot Steam Effect And Thermal Conductivity Of Buffer Materials For High-Level Radioactive Waste Repository

The world is entering a new era of clean energy and nuclear energy has been widely used,during which large amount of unwieldy high-level radioactive waste(HLRW)has been also produced.At present,the deep geological repositories(DGRs)have been valued as the most feasible option to ensure the permanent isolation of nuclides from the human biosphere.Owing to its high swelling ability,extremely low permeability and high radionuclide retardation capacities,bentonite is generally regarded as an ideal matrix material for buffer and backfill layers,which serves as a strong guarantee for the long-term safe operation of the repository.However,the low thermal conductivity of pure bentonite is not conducive to the rapid dissipation of heat,and will even cause a series of adverse consequences.Therefore,it is necessary to reasonably evaluate the hazards induced by high temperature,and to study how to improve the thermal conductivity of bentonite-based materials.Based on the background of DGRs,advanced relevant tests were conducted on buffer materials,adopting the method of combining macro-micro test and theorical analysis in this paper.The research work includes: investigating the influence of hot steam on the isolating potential of nearfield bentonite,foucusing on the method of improving the thermal conductivity of bentonite with graphite,analyzing the change mechanism of thermal conductivity,and combining the therical model to predict and analyze the thermal conductivity.The main research contents and conclusions are as follows:1)A new hot steam testing device was developed and calibrated,in order to simulatively investigate the performance evolution of compacted bentonite with a dry density of 1.70 g/cm3 under hot steam conditions.The test results showed that the swelling and anti-seepage properties of bentonite undergoing hot steam sufferd an irreversible deterioration compared with the case of normal hydration at room temperature.The ion test and element tests revealed that the concentration of Mg2+,Ca2+,Si and Al in the aqueous solution decreased.Such change trends depended on the temperatue.The above results can be attributed to the mineral cementation inducing the change in microstructure and the possible mineral transformation.2)The thermal conductivity of bentonite-graphite with different particle sizes of graphite was measured by the steady-state heat flow meter.It was found that the 100 mesh graphite had the best effect on increasing the thermal conductivity of bentonite,compared with graphite having other different particle sizes.The tests of particle distribution and pore distribution characteristics indicated that the bentonite-graphite gradation was better and the accumulated pore volume and micro macropore were smaller when 100 mesh graphite was added.The reason why the graphite particle size affects the thermal conductivity of bentonite is related to the soil microstructure and pore characteristics.3)The thermal conductivity tests were performed on compacted bentonite-graphite with different void ratio,water content and graphite content(100 mesh).The results showed that the effect of temperature on the thermal conductivity of bentonite and bentonite-graphite was negligible.The thermal conductivity of bentonite-graphite increased with increasing water content and mixing ratio,while it decreased with increasing initial porosity.The change of thermal conductivity is related to the coupling of void ratio and water content,and the anisotropy of graphite flake.4)The thermal conductivity of bentonite-graphite was theorically predicted and analyzed.The results showed that the Sakashita model and Johansen model could well predict the thermal conductivity of pure bentonite.By introducing the "multi-step homogenization" processing method,the modified Cote-Konrad model,Woodside-Sass model and effective medium theory could more accurately predict the thermal conductivity of bentonite-graphite.5)In terms of improving thermal conductivity of bentonite,the graphite is obviously superior to quartz sand and rock debris,which can effectively reduce the dependence of bentonite on saturation.