| As the safest and most reliable clean energy mastered by human beings,nuclear energy is profoundly changing people’s life and the pattern of the world.With the rapid development of nuclear energy,the total amount of radioactive waste produced and accumulated in the industry is increasing,but the disposal capacity of radioactive waste does not match the development speed of nuclear energy.The safety,social and environmental problems caused by waste overdue storage have become increasingly prominent.How to dispose of these high-level radioactive wastes safely and reliably is the most arduous task for people in the nuclear industry,and it is also one of the bottlenecks affecting the safe and stable development of nuclear energy.Geological disposal of high-level radioactive waste is recognized as the safest,reliable and accepted engineering disposal scheme,which has the characteristics of long time span,magnificent and difficult project,many uncertain factors and so on.As an important comparison and selection scheme of geological disposal of high-level radioactive waste in China,compared with granite geological disposal,there is still a certain gap in the overall progress of clay rock geological disposal.Compared with the underground laboratory construction of Beishan Granite in Gansu Province,the clay rock is still in the stage of preliminary determination of Tamusu area in Inner Mongolia as the pre-selection site and safety evaluation of pre-selection.As an important part of safety assessment of geological disposal of high-level radioactive waste,the study of radionuclide migration has a wide variety of nuclides and a large research task;the experimental period of nuclide migration and diffusion is long and the time span is large;the chemical composition of geological samples is complex and changeable,and the research results are uncertain.As the study of radionuclide migration in Tamusu clay rock has just started,the basic data such as apparent diffusion coefficient Da and effective diffusion coefficient De value lack empirical values,and the confidence interval of data is large;at present,Tamusu clay rocks are only used as far-field natural barriers for safety evaluation,lack of other engineering applications evaluation studies such as buffer backfilling materials and other.In general,the surface of clay rock is electronegative,cationic nuclides will generate electrostatic attraction in clay rock,while anions will generate electrostatic repulsion.Based on the basic principles of adsorption retarding diffusion and exclusion accelerating migration,and combined with the radius,half-life,and chemical toxicity of nuclide ions.In this paper,tritium,technetium（replaced by rhenium）,selenium,and other radionuclides with higher risk levels in the neutral or anionic state in water were selected to carry out their migration and diffusion research in the Tamusu clay rock;The column bracing method was used to modify the Tamusu claystone,and selenium was taken as a typical nuclide representative to systematically investigate the adsorption performance of the claystone to nuclides before and after the modification,providing new ideas and new materials for the engineering modification application of the natural barrier.The specific conclusions are as follows:（1）this paper takes tritium as the typical representative of the neutral nuclide,on the basis of comprehensively characterizing the microstructure and chemical composition of Tamusu clay rock,comprehensively considering the high migration characteristics of tritium and choosing penetration diffusion as the experimental means to systematically carry out the study of tritium diffusion in Talmud rock.The apparent diffusion coefficient and effective diffusion coefficient of tritium in Tamusu clay rock were obtained.The results show that the Tamusu clay rock is mainly composed of dolomite,cristobalite,and a small amount of clay（Illite,kaolin）.The clay has obvious floc granular and micro-layer microstructure,and the clay surface is negatively charged.The zero electric point is 6.90.The range of effective diffusivity and apparent diffusivity of tritium in Tamusu clay rock with compaction density of1600 kg/m3-2400 kg/m3 is 4.49×10-10m2/s～5.81×10-10m2/s and 3.38×10-10m2/s～4.62×10-10m2/s.The effective diffusivity and apparent diffusivity of tritium（HTO）decrease with the increase of compaction density.Compared with the drawing method and FDP,CⅣ has a higher degree of fit.Therefore,for the diffusion behavior of tritium and other nuclides with neutral molecules in Tamusu rock,the penetration diffusion method is recommended to study the migration law,and the CⅣ software is used to fit the test data.（2）In this paper,for anionic nuclides with only anionic repulsion effect,rhenium（instead of technetium）is selected as the representative.Firstly,the changes of microstructure and chemical composition of Tamusu claystone before and after irradiation were analyzed.Then,capillary method and penetration diffusion method are used to systematically study the migration rule of rhenium ion in Tamusu clay rock,and the influence of different external factors such as clay compaction density,ion strength and humic acid on rhenium ion migration,diffusion and migration is comprehensively investigated.The results show that the mineral composition of Tamusu clay is basically unchanged after irradiation,but the specific surface area of the clay increases,and the pore size and average particle size of its microstructure decrease after irradiation;The diffusion coefficient of Re（Ⅶ）in the clay after irradiation is less than that before irradiation,and the distribution coefficient in clay rock is greater than that before irradiation;The capillary method study shows that the diffusion coefficient of Re（Ⅶ）decreases with the increase of rock and soil compaction density,increases with the increase of solution ionic strength（within a certain range）,and decreases with the increase of humic acid.Because the core samples have not been mixed,the diffusion results are not completely consistent with the rules presented by the capillary method.After fitting the two diffusion research methods,the apparent diffusion coefficient DA and effective diffusion coefficient De of rhenium in Tamusu clay rock are calculated,and their orders of magnitude are 10-10m2/S-10-9 m2/s.According to the data fitting results,CAPILL software is more suitable for capillary method,while FDP software is more suitable for penetration diffusion method.（3）For nuclides with electrostatic repulsion and adsorption at the same time,selenium is selected as a typical representative in this paper,and the diffusion behavior of selenium in Tamusu clay rock is systematically studied by internal diffusion method and penetration diffusion method.The effects of clay compaction density,ionic strength,humic acid and other external factors on selenium diffusion and migration were comprehensively investigated.,The capillary method shows that the selenium diffusion coefficient decreases with the increase of clay compaction density,increases with the increase of solution ionic strength（within a certain range）,and decreases with the increase of humic acid;The order of magnitude of selenium diffusion coefficient calculated by the capillary method is 10-10 m2/s-10-9 m2/s,while the order of magnitude of penetration diffusion method is 10-12-10-13 m2/s;According to the data fitting results,CAPILL software is more suitable for capillary method,while FDP software is more suitable for penetration diffusion method.（4）In this paper,aiming at the weak adsorption of clay rock on selenium,hydroxyl iron oxide is used to modify the adsorption performance of natural Tamusu clay rock,and the adsorption behavior of hydroxyl iron oxide modified Tamusu clay rock on selenium is studied.The results show that the modification of hydroxyl iron oxide can promote the adsorption of Se（Ⅳ）in an aqueous solution by Tamusu clay rock.The microstructure characterization results show that hydroxyl iron oxide exists in granular form between soil layers,and the calcination treatment will destroy the original disordered accumulation structure of clay rock,promote the granulation degree of iron,and form a pillared lamellar structure.The adsorption effect of the modified clay sample is about 1.5 times that of natural clay rock.When the optimal adsorption acidity p H is 6,the saturated adsorption capacity of selenium can reach2.05 mg/g.Through isotherm,dynamics,and thermodynamics,the results show that the adsorption process is more in line with Freundlich and quasi-second-order kinetic model.The adsorption reaction belongs to the spontaneous process of entropy increase endothermic,and the adsorption process is controlled by chemical action.In a word,this paper selects tritium,rhenium,and selenium,the three most representative nuclides,to study their diffusion and adsorption behavior in the Tamusu clay rock,further enrich the empirical database of nuclide migration rules in the Tamusu clay rock,provide technical support for the safety evaluation research of the repository,and provide ideas for the construction of geological repository engineering barriers and effect evaluation. |
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