Thermodynamic assessment of the rhodium-ruthenium-oxygen (Rh-Ru-O) system

• Published in: Journal of nuclear materials 2018-03, Vol.500, p.252-264
• Main Authors: Gossé, S., Bordier, S., Guéneau, C., Brackx, E., Domenger, R., Rogez, J.
• Format: Article
• Language: English
• Subjects: Calphad; Chemical elements; Computer simulation; Dioxides; Engineering Sciences; Fission product; Fission products; Fuels; Fugacity; Heavy metals; High temperature; Level (quantity); Low temperature; Metal particles; Micro and nanotechnologies; Microelectronics; Nuclear energy; Nuclear fission; Nuclear fuels; Oxidation; Oxides; Oxygen; Phase diagram; Phase equilibria; Phase transitions; Phases; Platinum; Precipitates; Radioactive wastes; Rhodium; Ruthenium; Ternary systems; Thermochemistry; Thermodynamic properties; Thermodynamics; Vitrification; Volatile
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2017.12.034

Ruthenium (Ru) and rhodium (Rh) are abundant platinum-group metals formed during burn-up of nuclear fuels. Under normal operating conditions, Rh and Ru accumulate and predominantly form metallic precipitates with other fission products like Mo, Pd and Tc. In the framework of vitrification of high-level nuclear waste, these fission products are poorly soluble in molten glasses. They precipitate as metallic particles and oxide phases. Moreover, these Ru and Rh rich phases strongly depend on temperature and the oxygen fugacity of the glass melt. In case of severe accidental conditions with air ingress, oxidation of the Ru and Rh is possible. At low temperatures (T  1815 K for ruthenium dioxide), they may decompose into (Rh)-FCC or (Ru)-HCP metallic phases and radiotoxic volatile gaseous species. A thermodynamic assessment of the Rh-Ru-O system will enable the prediction of: (1) the metallic and oxide phases that form during the vitrification of high-level nuclear wastes and (2) the release of volatile gaseous species during a severe accident. The Calphad method developed herein employs a thermodynamic approach in the investigation of the thermochemistry of rhodium and ruthenium at high temperatures. Current literature on the thermodynamic properties and phase diagram data enables preliminary thermodynamic assessments of the Rh-O and Ru-O systems. Additionally, select compositions in the ternary Rh-Ru-O system underwent experimental tests to complement data found in literature and to establish the phase equilibria in the ternary system. •A thermodynamic assessment of the Rh-O and Ru-O systems are proposed using the Calphad method.•Equilibrium vapor pressures of main gaseous ruthenium oxides RuO2(g), RuO3(g), RuO4(g) are provided.•Using experiments performed in Rh-Ru-O, the rhodium solubility in the ternary (Ru,Rh)O2 rutile phase is evaluated.•The influence of Rh solubility on the Ru/RuO2 Red/Ox couple is calculated at 1473 K.