Crystallization on surface layer of high level waste glass

Surface crystallization was investigated on boro-silicate glass with simulated waste of 26.4 wt% in the temperature range of 550 to 800 deg C. The crystals deposited on the surface layer of glasses were mainly composed of molybdate and molybdenum oxide, the chemical forms of which were identified as K2MoO4, CaMoO4, KMoO4, Nd2MoO6, Na2MoO4 and MoO3. The saturated volume fraction of each deposition except KMoO4 was the maximum at 800 deg C, decreasing with lowering temperature. In the case of KMoO4, the maximum deposition was observed at 600 degC. An analysis of deposition rate, based on JMA equation, suggested that the surface crystallization was controlled with nucleation rate but not with growth rate. The activation energies of nucleation were estimated to be ranged between 50 and 60 Kcal/mol for K2MoO4, CaMoO4 and Nd2MoO6, and approximately 28 Kcal/mol for Na2MoO4. (author)