Study On Preparation Process Of Pyrochlore-borosilicate Glass-ceramic Waste Forms

Glass-ceramic waste forms are potential hosts for immobilization of high-level nuclear wastes?HLWs?and considered as the third generation of the treatment for high-level radioactive wastes,leading the development direction of the waste forms disposal.In this work,pyrochlore-borosilicate glass-ceramics with stable structure and properties were synthetized by melting-heat treatment method.The glass-ceramic samples were analyzed by DSC,XRD,SEM and SEM-EDXS method and density test,and the leaching behaviors of the samples were studied by PCT method.The research results as follows:?1?Effects of glass-ceramic samples formula on waste forms showed that:Samples prepared were melted firstly and then cooled in the furnace,it was showed that the synthesis of titanate and zirconate pyrochlore glass-ceramic needs higher melting temperature than niobate pyrochlore glass-ceramic.When ratios of SiO₂:Na₂O:B₂O₃:CaF₂:Nb₂O₅:TiO₂:Nd₂O₃:Al₂O₃ mass fractions?wt%?are 22.74:18.38:6.63:8.16:21.75:4.72:13.77:3.86,pyrochlore?Ca,Na?₂?Nb,Ti?₂O₆F and CaNdNb₂O₇ can be obtained from the glass-ceramic sample.The normalized leaching rate of the elements in the sample is equivalent to the similar glass or glass-ceramic,indicating that the chemical durability of waste form sample is good.?2?The effects of different initial temperature and cooling rate on the waste form samples were studied.The samples prepared were melted,then rapidly cooled and then slowly cooled and finally annealed.That is,the raw material was put in alumina crucible and moved to a furnace,and heated with a rate of 5°C·min^-11 to1300°C and hold at this temperature for 1.5 h,then the samples were quickly transferred to a new furnace with different initial temperatures?1100,1050,1000or 900°C?,and then cooled to 500?at different cooling rates(5,7.5,10 or15?·min^-1).After annealing at 500°C for 2 h to eliminate the residual stress and cooling in furnace,the glass-ceramic samples were obtained.When the initial cooling temperature is 1050?and cooling rate is 10?·min^-1,the pyrochlore in the sample shows continuous and regular growth,the crystallinity of sample is the highest and the chemical durability is the best.?3?The effects of different melt temperature and Nd₂O₃ content on the waste form samples were studied,the results showed that the main crystalline phases in the waste form samples were crystalline phase?Ca,Na??Nb,Ti?₂Nd_x O₆F?x>0?and?Na,Ca??NbO₃?.The amount of crystal phase?Ca,Na??Nb,Ti?₂Nd_x O₆F?x>0?increased first and then decreased with an increase of melt temperature or Nd₂O₃content.The Nd₂O₃ content of 14 wt%and the melt temperature of 1325?were most conducive to the single and uniformly distributed pyrochlore?Ca,Na??Nb,Ti?₂Nd_0.67O₆F.