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

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.

KEYWORDS:

• Vitrification
• high-level radioactive waste
• ruthenium oxide
• sodium ruthenate
• cold cap
• glass melter
• reprocessing

Acknowledgements

This work was carried out as a part of the basic research programs of vitrification technology for waste volume reduction supported by the Ministry of Economy, Trade and Industry, Japan.

Disclosure statement

No potential conflict of interest was reported by the authors.