Extraction du zirconium dans le réacteur à sels fondus

International audienceThe Molten Salt Fast Reactor is a GEN IV reactor which uses a liquid fuel salt constituted of a mixture of fluoride salts: LiF-ThF4-UF4 (77.5-18.5-4 mol%). The working temperature is close to 650°C. The design of MSFR integrates a continuous reprocessing in order to prevent the precipitation of insoluble compounds (fluorides, oxides and oxy-fluorides). In the previous European project EVOL, a reprocessing scheme has been recommended to treat the liquid fuel of the Molten Salt Fast Reactor but this reprocessing scheme do not propose any option to remove zirconium fluoride from the salt whereas this element is one of the more important produced in the core. The aim of this paper is to present the chemical behavior of zirconium in the fuel salt. The thermodynamic stability diagram of zirconium in a fluoride salt is calculated from the thermochemical data of pure compounds. This diagram shows two stable soluble compounds of zirconium, ZrF2 and ZrF4. On the contrary, the experimental study evidences that only ZrF4 is stable in the salt, probably due to the low activity coefficients of zirconium fluoride which was determined equal to-4.33. The reduction of ZrF4 to Zr metal occurs at a potential of 0.377 V against the potential ThF4/Th which constitutes the limit of the electroactivity range of the fuel salt. Then, the electrochemical reduction on solid cathode is retained to perform Zr extraction from the salt