Study On The Composition,Structure And Chemical Stability Of Mo-Containing Glass-Ceramics

The high-level waste resulting from spent nuclear fuel reprocessing contains a large amount of alkali/alkaline-earth,lanthanide and transition metal elements,which are highly hazardous to human beings and the environment.The safe dispose such high-level waste has attracted great public and government attention.At present,vitrification is the only method with engineering prospects for high-level waste solidification,while borosilicate glass is the most widely studied matrix material.However,the low solubility of MoO₃ in conventional borosilicate glass limits the total waste loading.Therefore,it is imminent to adjust glass formula or develop new waste form to increase the waste loading,to accommodate the development of nuclear energy.This paper is based on the solidification of high-level waste from power reactor in our country.Adjust the borosilicate glass components to improve the solubility of MoO₃,and develop a powellite-zirconolite borosilicate glass-ceramic to solidify the fission products in high-level waste.The main findings obtained are as follows:?1?The solubility of MoO₃ in borosilicate glass?59.97 SiO₂-14.15 B₂O₃-11.51 CaO-2.91Na₂O-0.15 Gd₂O₃-11.46 Al₂O₃?system is about 1.5 mol%,exceeding the solubility can lead to glass phase separation.When the MoO₃ content is 2³ mol%,the spherical CaMoO₄is homogeneously distributed in the glass matrix.When the MoO₃ content is 4 mol%,Na₂MoO₄ comes into being,which forms from the surface of the high-temperature melts.Moreover,the grain size of CaMoO₄ gradually increase with the MoO₃ content and with the slow cooling rate decrease.?2?The V₂O₅ doped in borosilicate glass can effectively enhance the solubility of MoO₃ in glass.The solubility of MoO₃ in glass can be increased up to 2.8 mol%in V₂O₅ containing borosilicate glass system?58.76 SiO₂-13.86 B₂O₃-11.30 Na₂O-2.85 Al₂O₃-11.23 CaO-2.00 MoO₃?.The results reveal that the addition of V₂O₅ can effectively decrease the charge density of alkali/alkaline-earth ions around[MoO₄]^2-,and enhance the disorder of[MoO₄]^2-in glass matrix.As a result,the phase separation was effectively inhibited and the solubility of MoO₃ in glass was improved.Leaching tests show thatV₂O₅ addition has no obvious influence on the chemical stability of the waste forms.?3?The powellite-zirconolite borosilicate glass-ceramic system?44.66 SiO₂-10.54 B₂O₃-8.57 Na₂O-2.17 Al₂O₃-14.52 CaO-11.96 TiO₂-5.98 ZrO₂-1.60 MoO₃?can simultaneously solidify the Mo and Nd,which are the most abundant fission products in high-level waste.When the Nd₂O₃ content is 0³ mol%,only two of the targeted crystal phases,powellite and zirconolte,were observed.And these crystals distributed homogeneously in the glass matrix.The EDX results show that the Mo and Nd can be successfully incorporated in two target crystal phases respectively.When the Nd₂O₃ content is 4 mol%,apatite phase was observed.The bulk density of the glass-ceramics increases with the Nd₂O₃ content.Leaching tests show that these Nd-containing borosilicate glass-ceramics possess good chemical stability.?4?Fission products can be incorporated in zirconolite borosilicate glass-ceramic systerm?45.39SiO₂-10.71B₂O₃-8.71Na₂O-2.20Al₂O₃-14.75CaO-12.16TiO₂-6.08ZrO₂?.Compared with conventional borosilicate glass waste forms,the waste loading of this glass-ceramic increased by about twice.Only two homogeneously distributed target phases:powellite and zirconolite,were observed for the glass-ceramics with 15³5 wt%waste.As the amount of waste addition reaches 45 wt%,apatite and perovskite phases appear.With the waste content increase?0⁴5 wt%?,the glass transition temperature gradually decreases,whereas the volume density gradually increases,and the sample with 35 wt%waste possess the maximum density.The leaching results indicate that the glass-ceramic waste forms possess good chemical stability.