Fission products behavior in molten fluoride salts: Speciation of La3+ and Cs+ in melts containing oxide ions

In this paper we address the effects of fission products on the speciation in molten fluoride salts. Numerous systems with cross-connections have been investigated in order to better identify the influence of CsF in a fluoride melt containing rare earth and oxides : LaF3–AF (A = Li, Na, K, Rb and Cs), LaF3–LiF–CsF, LaF3–LiF–CaF2, LaF3–LiF–CaO, LaF3–LiF–CaO–CsF. In this goal, we performed high temperature NMR experiments and followed in situ the evolution of 19F, 23Na, 85Rb, 133Cs and 139La NMR chemical shifts. In LaF3–AF–CsF and LaF3–AF–CaF2 systems, the coordination number of lanthanum cation ranges from 6 to 8 depending on the LaF3 concentration and on the polarizability of the other cations. The addition of oxide (CaO) in the latter mixtures leads to the formation of lanthanum oxyfluoride species that precipitate in LaOF when CaO concentration is increased. The addition of CsF to LaF3–LiF–CaO yields to a displacement of the dissolved versus precipitated LaOF proportion.