Study On The Structure And Chemical Durability Of Borosilicate Glass-ceramic Waste Forms

The treatment and disposal of high-level radioactive waste?HLW?has always been a difficult problem in the world,especially about the immobilization of radionuclides in HLW,which attracted a great deal of interest from researchers.Borosilicate glass-ceramics is becoming one kind of potential candidate materials for immobilization of HLW,because the preparation process is relatively simple and glass-ceramics possess higher stability compared with glasses.In this paper,we chose the borosilicate glass with good properties by previous experiments firstly and then prepared the zirconolite borosilicate glass-ceramics by adding ZrO2 and TiO2as nucleation agents in a one-step heat-treated method.The microstructure and properties of the glass-ceramic waste forms were investigated by Differential Scanning Calorimetry?DSC?,X-ray Diffraction?XRD?,Backscattered Electron Microscope?BSE?,Energy-dispersive X-ray analyzer?EDX?,Raman spectra,Inductively Coupled Plasma-Optical Emission Spectrometer?ICP-OES?.The ratio of zirconolite phase to glass phase and the immobilization ability of zirconolite phase to simulated nuclear Ce was calculated by Rietveld structure refinement.The main contents are as follows:?1?Research on structure and chemical durability of SiO₂-B₂O₃-Na₂O-CaO-TiO₂-Al₂O₃-MgO system borosilicate baseline glass with different contents of CeO2 or Nd2O3,the effect of CeO2on crystallization behavior of the borosilicate glass.?2?Based on SiO₂-B₂O₃-Na₂O-CaO-TiO₂-Al₂O₃ system borosilicate glass,study on the effects of different contents of CaO,TiO₂ and ZrO₂ on the preparation of borosilicate glass-ceramics by different two-step heat treatment.The optimum formula of zirconolite glass-ceramic was obtained.Then simulated nuclide CeO2 was added to the formula and zirconolite glass-ceramic waste forms were prepared by slow-cooling method.The crystallization,immobilization mechanism and chemical durability of zirconolite glass-ceramic waste forms were investigated.?3?Study on the effect of CeO₂ and Nd₂O₃ codoping on the microstructure,crystallization behavior and chemical durability of SiO₂-B₂O₃-Na₂O-Al₂O₃-CaO-ZrO₂-TiO₂ system borosilicate glass-ceramics waste forms was investigated.Here are main conclusions were summarized as follows:?1?15 wt%CeO2 and 28 wt%Nd2O3 can be well incorporated into the SiO2-B2O3-Na2O-CaO-TiO₂-Al₂O₃-MgO baseline borosilicate glasses respectively,Ce or Nd doping leads to an increase in the concentrations of non-bridge oxygen bonds and a decrease of B-O-B bonds in[BO₃]units,promotes the conversion of[BO₃]units into[BO₄]units.According to PCT standard,after leaching 28 days in 90°C deionized water,the normalize leaching rates of the B,Si elements remain in the order of magnitude of 10^-3 g·m^-2·d^-1 and actinide surrogate Ce element keeps in the order of magnitude of 10^-8 g·m^-2·d^-1;the normalize leaching rates of actinide surrogate Nd element keeps in the order of magnitude of 10^-7 g·m^-2·d^-1,which indicated the borosilicate glass waste forms have good chemical durability.Moreover,glass transition activation energy and crystalline activation energy decrease with the addition of CeO2 in the glass forming range,and surface crystallization is the main crystallization process for the CeO₂ doped borosilicate glass while annealing,and CeO₂ crystalline phase is detected when the glasses with CeO₂ are annealed at 750?.?2?Main crystalline phase or single crystalline phase zirconolite-2M can be precipitated by slow-cooling method in the SiO2-B2O3-Na2O-CaO-TiO2-Al2O3 borosilicate baseline glass with 40 wt%CaO,TiO₂ and ZrO₂?the molar ratio of Ca:Zr:Ti is 1:1:2?.The quantitative fractions of the zirconolite phase is about 30 wt%,the chemical composition of zirconolite crystals grown from a glass matrix is determined by Rietveld refinement to be Ca0.96Zr0.72Ce0.32Ti1.95Al0.05O7,Ca0.93Zr0.76Ce0.31Ti1.95Al0.05O7,Ca0.97Zr0.75Ce0.28Ti1.97Al0.03O7for M1,M2 and M3,respectively and up to 84.53%Ce can be incorporated in zirconolite crystals.According PCT standard,the normalized leaching rates of Si,Ca and Ce of the glass-ceramic waste forms are about 4×10^-4 g·m^-2·d^-1,1×10^-4 g·m^-2·d^-1 and 8×10^-7 g·m^-2·d^-1respectively after 56 leaching days in 90°C deionized water,which indicated the glass-ceramic waste forms have good chemical durability.?3?The borosilicate glass-ceramics containing zirconolite as main crystalline phase were prepared in SiO₂-B₂O₃-Na₂O-Al₂O₃-CaO-ZrO₂-TiO₂ borosilicate glass-ceramic with 10-40wt%CeO2 and Nd2O3.The phase transformation of zirconolite-2M to zirconolite-4M occurs when increasing the contents of CeO₂ and Nd₂O₃ is 15 wt%.Moreover,high doping level of CeO₂ and Nd₂O₃ to 40 wt%results in the formation of oxyapatite phase.The grain size of crystalline phase decreases with the addition of CeO₂ and Nd₂O₃ and the doped Ce and Nd mainly distribute in zirconolite and oxyapatite phases.The aqueous chemical durability test shows that their normalized leaching rates of Ca,Si and Nd are in order of 10^-3 g·m^-2·d^-1,10^-3g·m^-2·d^-1 and 10^-4 g·m^-2·d^-1 respectively after 28 MCC-1 test leaching days.