Towards an understanding of the behaviour of ruthenium during vitrification of highly active waste: A study of the volatilisation of RuO2

Abstract Recycling nuclear fuel to recover materials such as uranium and plutonium involves high temperature processes to treat the resulting highly active (HA) waste. This waste contains ruthenium, an important fission product due to its tendency to form volatile compounds and the presence of the radioactive isotope 106 Ru. These properties, combined with the elevated temperatures during HA waste treatment, necessitate a deeper understanding of ruthenium volatilisation mechanisms. Key ruthenium species present or potentially formed in HA waste include nitro and nitrosyl complexes, and ruthenium dioxide (RuO 2 ). This study focuses on the behaviour of RuO 2 at high temperatures, due to its relevance to the vitrification process used to immobilise HA waste. Preliminary thermogravimetric analysis of commercially available RuO 2 revealed a dehydration process under N 2 , O 2 , air atmospheres, and volatilisation under oxidising conditions. Post-heating analyses using scanning electron microscopy and X-ray diffraction identified a concurrent, thermally induced recrystallisation process, most prominent under non-oxidising conditions. Graphical abstract