The Effect of Temperature on the Redox Constraints for the Processing of High-Level Nuclear Waste into a Glass Waste Form

ABSTRACTThe oxidation-reduction equilibria of selected multivalent elements in an alkali borosilicate glass melt (Savannah River Laboratory frit #131) were measured as a function of the imposed oxygen fugacity over the temperature range from 950°C to 1350°C. Redox constraints on the processing of high-level nuclear waste into the glass melt require that the prevailing oxygen fugacity be about 10−5 to 10−12 Zatm at 950°C, about 10−2 to 10−9 atm at 1150°C, and about 100 to 10−7 atm at 1350°C. Such conditions circumvent foaming under oxidizing situations and metal/sulfide precipitation if the system becomes too reducing. The defined oxygen fugacity ranges correspond to the previously prescribed range of 0.1 to 0.5 for the [Fe2+]/[Fe3+] ratio in the resulting glass, independent of the processing temperature from 950°C to 1350°C.