Séminaire/Seminar - Incorporation of volatiles in nuclear waste glasses: partitioning between silicate and salt phases - John McCloy

Vendredi 29 juin à 10 h 30

Sorbonne Université - Campus P. et M. Curie - 4 Place Jussieu - 75005 Paris - Barre 23-33 - 2e étage, pièce 207

John S. McCloy - Washington State university

Abstract 

The incorporation of volatile elements and anions in silicate glass is a scientific consideration for both volcanology and radioactive waste vitrification. For volcanology, outgassing on depressurization during magma rise has a strong influence on eruptive behavior. For waste vitrification, fission products and process chemicals having low solubility in silicate/ borosilicate glasses are often present in high concentrations. Special considerations must be made to ensure sufficient retention in glass, and complex off-gas systems are required when vitrifying mixed legacy wastes or fuel reprocessing wastes. Some examples of problematic volatile components in nuclear waste treatment include oxyanions (sulfate, molybdate, chromate, pertechnetate) and halides (fluoride, chloride, iodide), the concentration of which varies greatly depending on the waste. When these components are present in high concentrations, they can condense during melting and form an ionic molten salt phase, usually combining with alkali elements, which separates from the silicate melt. This molten salt phase can cause processing problems, volatilization, and creation of water-soluble salts on cooling. In this talk, the salt phase formation issues will be summarized specifically for US nuclear wastes, and a few brief examples of recent experimental work will be given on systems of sulfate-chloride, silicate-sulfate-chromate, and silicate-sulfate-chloride/fluoride. In the latter example, distinctive differences are seen in the fluoride and chloride case, both in terms of their effect on sulfate retention and their incorporation into the glass. Measurements – including electron microprobe, F-19 and Al-27 nuclear magnetic resonance, X-ray diffraction, and nano-computed tomography – will illustrate the incorporation, phase separation, and crystallization in these systems.