PNNL

Abstract

We investigated volatilization of Re, S, Cs, and I during the course of conversion of high-level waste (HLW) melter feed to glass and compared the results for Re volatilization with those in low-activity waste (LAW) borosilicate glasses. Whereas Re did not volatilize from HLW feed heated at 5 K min-1 until 1000°C, it began to volatilize from LAW borosilicate glass feeds at ~600°C, a temperature ~200 K below the onset temperature of evaporation from pure KReO4. Below 800°C, perrhenate evaporation in LAW melter feeds was enhanced by vigorous foaming and generation of gases from molten salts as they reacted with the glass-forming constituents. At high temperatures, when the glass-forming phase was consolidated, perrhenates were transported to the top surface of glass melt in bubbles, typically together with sulfates and halides. A report of Re loss from LAW iron phosphate glass melt with bubbling indicated that a fraction of Re resisted to extended bubbling while sulfur could be removed completely.