Study On The Thermal Expansion Of Unsaturated Densely Compacted Bentonite And Its Effect On Block Combination

With the large-scale utilization of nuclear energy,the disposal of high-level radioactive waste has always been a crucial environmental issue.In the high-level radioactive waste repository,the buffer backfill material composed of bentonite blocks is an important barrier to block the migration of nuclides.It will be subject to the complex thermal-hydro-mechanical(THM)three-field or even multi-field coupling.The release of heat from radioactive waste will have a greater impact on bentonite(thermal expansion,shrinkage and cracking,etc.).Therefore,to effectively evaluate the engineering performance of bentonite blocks,it is necessary to study the thermal expansion properties of bentonite and blocks.In this paper,theoretical analysis,laboratory experiments and numerical simulation methods are used to study the thermal expansion properties of Gaomiaozi(GMZ)bentonite.The main research contents and results are as follows:(1)Comparing the pros and cons of different material thermal expansion measurement methods and trial conditions,the test method and test plan suitable for measuring the thermal expansion performance of unsaturated high-pressure bentonite are proposed.The test program takes into account the effects of temperature,initial compaction state(including initial moisture content and initial dry density),suction,shape,anisotropy and ambient gas.The related physical properties of GMZ bentonite were tested and a mold for preparing high-pressure bentonite test blocks was developed.(2)Using GMZ bentonite as the material,a large number of tests were carried out using the DIL806 thermal dilatometer and related test results were obtained.The influence of different influencing factors on the thermal expansion performance of the compacted bentonite is systematically analyzed,and the thermal expansion coefficient of the compacted unsaturated bentonite is obtained with the temperature,dry density,moisture content,suction,shape and environmental gas.Quantitatively described the anisotropy of bentonite and its influencing factors.(3)Considering physical phenomena such as thermal expansion and loss of water shrinkage,the micromechanics theory and equivalent inclusion theory are introduced to derive the thermal expansion coefficient model of the compacted unsaturated bentonite.The obtained model can be used to analyze and predict the bentonite under different conditions.The coefficient of thermal expansion is compared and analyzed with the test results in this paper.(4)Drawing lessons from the design experience of buffer backfill materials at home and abroad,considering a variety of block combinations,the initial numerical simulation of the repository was carried out through COMSOL software to study the effect of bentonite thermal expansion on block combinations,and the lower temperature field,The changing law of displacement field and stress field.The law of displacement and stress change between the interior of the block and the joint is comparatively analyzed,and the results show that the stress and displacement at the joint are significantly higher than that of the interior of the block.The research results of this paper are of great significance to the analysis and understanding of the thermal expansion properties of bentonite in high-level radioactive waste repositories,the evaluation of the overall buffering performance of the block-joint system and the ability to block the migration of radionuclides.