Study On Preparation And Chemical Stability Of Pyrochlore-based Borosilicate Glass-ceramic Waste Forms

High-level radioactive waste（HLW）is mainly high-level waste liquid and its waste forms produced by spent fuel reprocessing,spent fuel prepared for direct disposal（one-pass type）and other wastes with corresponding radioactive levels.The elements represented by U and Pu are extremely radioactive and must be safely and effectively isolated from the biosphere.Therefore,how to safely and effectively dispose of HLW becomes a major problem in the safe use of nuclear energy.The pyrochlore-borosilicate glass-ceramics are one of the candidate substrates for the third generation of high-level radioactive waste immobilization.They have the advantages of both borosilicate glass immobilization and Pyrochlore Ceramics immobilization,and have broad application prospects.In this study,a series of glass-ceramics samples were prepared on the basis of traditional melting-heat treatment and sintering process,through appropriate process improvement and component design,for the purpose of preparing a new type of immobilization base material of titanate pyrochlore-borosilicate glass for immobilization An.By means of XRD,SEM-EDS,FT-IR,TEM and ICP-MS,the intrinsic relationship between phase composition,microstructure and process parameters of glass-ceramic waste form was studied,and the chemical stability of the waste form was discussed.The main conclusions were as follows:1.Lu₂Ti₂O₇-based glass-ceramics prepared by nucleus incorporation-sintering method:In this paper,a new sintering process combining nucleus incorporation with traditional sintering method is adopted.By changing sintering temperature,holding time,glass/ceramic two-phase mass ratio and other parameters.Taking Er₂Ti₂O₇-based glass-ceramics as an example,a series of glass-ceramics waste forms were prepared in order to explore the optimum preparation conditions of the new process.XRD results show that glass-ceramics with single Er₂Ti₂O₇ pyrochlore phase can be obtained at different mixing ratios,sintering temperature and sintering time.The glass component has good thermal stability at high temperature and no second phase precipitation;Er₂Ti₂O₇ precursor has good phase stability and no phase decomposition occurs in the glass.Increasing sintering temperature and holding time is beneficial to enhancing the structural orderliness of Er₂Ti₂O₇,and the effect of sintering temperature is stronger than holding time.The higher the glass content,the larger the spacing between the crystal planes of Er₂Ti₂O₇,and the tensile stress of the glass on the ceramics.SEM results showed that Er₂Ti₂O₇ grew in a square shape on the glass matrix,and had a clear interface with the glass and good compatibility.The physical property test results of the waste form confirm that the proportion of glass component will directly affect the porosity and density of the waste form.The optimum preparation process of glass-ceramics containing Er₂Ti₂O₇ was determined as follows:W_G/W_P=50/50,sintering temperature 1200 C,holding time 6h.The phase and structure of Ln₂Ti₂O₇-based glass-ceramics（Ln=Sm,Eu,Gd,Lu）were further prepared and studied by optimum preparation process.XRD and SEM results show that compared with Er₂Ti₂O₇-based glass-ceramics,pyrochlore in Ln-based glass-ceramics exhibits slight phase decomposition,while the density of the samples becomes poor and porous,which will directly affect the stability of the waste form.Therefore,although nucleus incorporation-sintering method can be used to prepare glass-ceramics samples with pyrochlore as the main crystalline phase,there are some defects in the structure of As fixed substrates in Shanghai,which are expected to be improved by hot pressing technology.2.Ca_2-xU_xTi₂O₇-based glass-ceramics prepared by pretreatment-melting-heat treatment:In order to overcome the shortcomings of nucleus incorporation-sintering method,a new process of pretreatment-melting-heat treatment was developed,and the relationship between different processes and the phase structure of the waste form was discussed with U as actinide nuclide simulator.For different glass-ceramics preparation processes,the content of uranium pyrochlore in GC1 samples prepared by process A is the lowest,which is mainly attributed to the excessive precipitation of calcium-uranium oxides and calcareous feldspar.The precipitation of Ca-U oxides in GC2 and GC3 waste form was inhibited and the precipitation of uranium pyrochlore increased slightly when the mixed sample powders were pre-crystallized or treated by ball milling.When the pre-crystallization and ball milling processes are combined,the percentage of uranium pyrochlore in the crystalline phase of GC4 waste form reaches78.27%,and the composition of uranium pyrochlore formed in air is basically consistent with the design formula in this paper.A large number of directional solid solution of U in pyrochlore is realized.This shows that process D can obtain vitrified ceramics with uranium pyrochlore as the main crystalline phase,and is superior to the other three process schemes.The 14-day leaching rate of U in all samples is similar to that of HLW glass waste form.During leaching,the glass matrix dissolves slightly,but the ceramic phase does not change significantly.This indicates that the leaching of U will depend more on the stability of the ceramic phase in the waste form in a longer leaching period.A series of glass-ceramics samples containing different uranium pyrochlore were further prepared on the basis of the selected optimum process.A series of pyrochlore-based glass-ceramics with different uranium contents（6.35-25.48wt%）can be prepared by pretreatment and melt heat treatment.XRD characterization and microstructural analysis confirm that pyrochlore phase is mainly homogeneous internal crystalline phase.When uranium content exceeds 21wt.%,UTi₂O₆ and U₃O₈crystallize in glass matrix.Uranium in vitrified ceramics exists as a mixture of tetravalent and hexavalent,and can adjust its valence state spontaneously with the change of uranium content,which makes the presence of sodium and aluminium ions contribute to the formation of pyrochlore.The results of static leaching test show that the leaching rate of uranium is between 10^-4-10^-3 g·m^-2·d^-1.Especially when the uranium load reaches 16.96%,the leaching rate of GC3 at 42d is only 2.56×10^-4g·m^-2·d^-1.The results are expected to provide experimental reference and technical reserve for immobilization of high-level radioactive wastes,especially actinide-rich wastes on glass-ceramics.