Devitrification behavior in a zinc borosilicate nuclear waste glass

Abstract

Experimental studies of thermally induced changes in a simulated nuclear waste glass were conducted to define the composition and concentration of all phases formed over a broad range of time (up to one year) and temperature (<1200/sup 0/C) conditions. Depending on time and temperature, a steady-state condition is achieved in which a number of crystalline phases coexist with a glass phase which is partially depleted of some elements. Concentrations of the phases increase with decreasing temperature but usually reach only a fraction of their maximum theoretical concentration. Considering the major phase formed (Zn/sub 2/SiO/sub 4/), this fractional concentration is about 10% at 900/sup 0/C and 45% at 700/sup 0/C, when equilibrium is achieved. Under these unfavorable time/temperature conditions, the glass is about one-third crystalline. Because of melt insolubles (RuO/sub 2/,Pd) the most homogeneous glass will contain approximately 3% crystalline phases. Crystallization occurs at rates in agreement with those estimated from theory, based on a knowledge of the glass viscosity and an estimated heat of crystallization for the Zn/sub 2/SiO/sub 4/ phase. The times to reach steady-state concentrations range from a few hours at 900/sup 0/C to approximately 1 year at 700/sup 0/C. No crystallization at 500/sup 0/C was observed after one year. The crystal ingrowth rates follow a reasonable 1/T dependence for Zn/sub 2/SiO/sub 4/ and give an activation energy for the process of approximately 40 kcal/mole. Growth rates for the other phases are of the same order of magnitude as for Zn/sub 2/SiO/sub 4/. Results of the present work are used to evaluate processing/storage conditions needed to maintain a vitreous product. It is shown that crystallization over very long time periods (10/sup 6/ years) will likely occur only if ambient temperatures exceed approximately 225/sup 0/C for the entire time period.