Evolution of chromium, manganese and iron oxidation state during conversion of nuclear waste melter feed to molten glass

Abstract

Oxidation state and chemical conversion of three surrogate nuclear high-level waste melter feeds heat treated to temperatures between 500 and 1200 °C were studied using X-ray near-edge structure, feed expansion tests, X-ray diffraction, and evolved gas analyses. For the high-chromium feed (3.00 mass% Cr2O3 in glass), the average Cr oxidation state was predominately Cr3+ at the lowest and highest heat-treatment temperatures and Cr6+ at intermediate temperatures while the Fe coordination shifted from octahedral to non-centrosymmetric tetrahedral at higher temperatures with Fe oxidation state unchanged. For the high-manganese-high-iron feed (3.23 mass% MnO and 16.01 mass% Fe2O3 in glass), the average Mn oxidation state decreased from 4 to ~2 while the Fe oxidation state shifted towards increasing ferrous content and changed from five-fold coordination to tetrahedral. For the high-alumina feed (24.02 mass% Al2O3 in glass), average Fe coordination did not change, but Fe oxidation state shifted towards higher ferrous content with increasing temperature.