Separation of ultrafine uranium oxide particles from zirconium alloys in remelting processes with fluoride and oxide slags: thermodynamic analysis

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This paper examines how ultrafine particles of uranium oxide interact with zirconium alloys in the presence of oxide and fluoride slags. The authors analyzed the probability of the components of Zr – 1% Nb alloy contaminated with ultrafine uranium oxide particles transitioning into slag phase during the process of remelting fuel element claddings using different fluxing agents. By means of thermodynamic modelling, the authors looked at the interaction of the cladding material of fuel elements used in thermal reactors (zirconium-niobium alloy) contaminated with traces of nanosized uranium oxide with molten oxide and fluoride fluxes in the temperature range of 1,000 to 2,000 oC. It was found that, for the interaction of Zr – 1% Nb alloy with fluoride and oxide melts, the metal should be slightly dissolved in the slag phase. Analysis of the interactions of ultrafine uranium compounds with slag mixtures for the temperature range of 1,000 to 2,000 oC showed a predominant transition of uranium, when reacting with fluoride fluxes, into the metal phase in its elementary form – U and into slag phase (about 4%) in the form of UF3 and UOF2 compounds. During reactions with oxide fluxes, uranium mainly distributes in the metal in its elementary form — U. At the same time, 18 to 19% of uranium can transition into slag phase in the forms of UO, UO2, and UOF2. It should be noted that that fact that uranium oxide particles have an ultradispersed structure should contribute to their transition into slag and thus help speed up the treatment process.This research was carried out in the framework of the Single Industry-Specific Plan by Rosatom; Project No. EOTP-TTsPM-263. Equipment owned by the shared knowledge centre Ural-M was utilized for this research.The authors would like to thank T.V. Osinkina and A.S. Russkikh, junior researchers at the Laboratory of Rare Refractory Metals of the Institute of Metallurgy at the Ural Branch of the Russian Academy of Sciences, for their contribution.