Basic study of formation of acicular crystals by chemical reactions between RuO2 and NaNO3 in a waste vitrification process

Abstract

ABSTRACT Aggregates of acicular RuO2 occur in high-level waste glass. Large aggregates may settle rapidly in the glass, providing a more efficient electrical path on the melter bottom and interfering with the Joule heating of the glass. To clarify the formation mechanism of aggregates and to develop countermeasures to such formation, chemical reactions between RuO2 and NaNO3 with/without borosilicate glass were observed. Sodium ruthenates were formed from a RuO2–NaNO3 mixture heated without the glass or on a flat glass. On the flat glass, aggregates of acicular crystals of RuO2 appeared with a decreasing amount of sodium ruthenates at high temperature. Neither sodium ruthenates nor acicular crystals of RuO2 were formed from the RuO2–NaNO3 mixture well mixed with powdery glass and heated. Results of a series of experiments suggest that acicular crystals of RuO2 and their aggregates formed through the formation and decomposition of sodium ruthenate when the dissolution of sodium into the glass was slower than its reaction with RuO2. Promoting the dissolution of sodium into a glass or reducing the amount of sodium in liquid waste is expected to suppress the formation of aggregates and their sedimentation.