The oxide chemistry of protactinium in molten fluorides

Abstract

The solubility behavior of an insoluble oxide phase of protactinium(V) has been studied in molten LiFBeF 2ThF 4 (72-16-12 mole%)-the proposed solvent salt for the Molten Salt Breeder Reactor (MSBR)-as a function of temperature and redox potential. The stoichiometry of the precipitation indicated the 0 Pa ratio to be 2·5 in this oxide phase; however, it could not be identified as Pa 2O 5. Moreover, the apparent entropy of solution of the precipitate, as indicated by the high temperature dependence of its solubility, is higher than expected for Pa 2O 5 by ∼23 entropy units per mole of protactinium. For these reasons, this oxide phase is thought to be an addition compound PaO 2·5. nLiF. It has such a low solubility − log(X PaF 5 ·X O 2− 5 2 ) = 0·91-12·76 ( 10 3 T ) —that the PaF 5 concentration in solution can be reduced by the addition of oxide to ∼ 1 ppm at 550°C before ThO 2 saturation is reached. With UF 4 present at concentrations typical of an MSBR fuel (0·3 mole%), the PaF 5 concentration can be lowered to ∼ 10 ppm (550°C) before a UO 2-rich solid solution with ThO 2 is precipitated. From equilibrium under a hydrogen atmosphere, which solubilizes the Pa 2O 5 phase as PaF 4, the equilibration quotient Q = X PaF 5 · P 1 2 H 2 X PaF 4 P HF was estimated as log Q = −3·92 + 5·16 ( 10 3 T ) . Entropies and formation free energies are estimated for PaF 4 and PaF 5 in solution and for the Pa 2O 5 phase.