Microstructures and Leachability of Vitrified Radioactive Wastes

Metal oxide precipitates (primarily iron oxide compounds) will form in the glass matrix of some compositions of vitrified nuclear waste, during cooling of the melt, whenever solubility limits are exceeded. These precipitates, containing part of the cesium and strontium radionuclides from the waste, are more resistant to leaching by water than the as-cast glass matrix. Some of the glass matrix compositions devitrify during heating for 1 month at 600°C with the formation of equal amounts of NaAlSiO₄ (nepheline or carnegieite) and (Ca, Mn) (Mg, Fe, Mn) Si₂O₆, plus a small amount of Ca₄Fe₁₄O₂₅. The leachability of devitrified glass can be up to 100 times greater than the leachability of as-cast glass. The appearance and structure of the metal oxide precipitates are unaffected by the temperature conditions that caused devitrification of the glass matrix. The metal oxide precipitate particles are less leachable in water than any of the phases in the devitrified matrix.