Nepheline crystallization behavior in simulated high-level waste glasses

Abstract The U.S. Department of Energy immobilizes legacy nuclear high-level waste (HLW) in borosilicate glass. HLW glass with elevated Na 2 O and Al 2 O 3 concentrations have a demonstrated propensity for precipitating nepheline (NaAlSiO 4 ) during cooling, which adversely affects the waste form performance. Currently, nepheline is suppressed by controlling the HLW glass composition, but this can limit the waste loading and result in longer mission life and cost. Correlation between composition and thermal history experienced during processing with time-temperature relationships that enable nepheline crystal growth would facilitate a more realistic prediction of nepheline crystallization in HLW glass and allow for a wider range of compositions to be accessed. In this work, crystallization behavior in simulated HLW glass was investigated to identify key attributes that lead to nepheline formation, with results indicating that the rate of diffusion for crystal growth may be controlled by glass structure connectivity.