The exchange of U 4 + and Th 4 + between molten LiF-BeF 2-ThF 4-UF 4 and (U-Th)O 2 solid solutions

Abstract

The exchange of uranium and thorium between LiF-BeF 2-ThF 4 melts and (U-Th)O 2 solid solutions is relevant to the oxide chemistry of Molten Salt Breeder Reactor fuel, to the thermodynamics of UO 2-ThO 2 solid solutions, and to a two-phase “solvent extraction system” in reprocessing reactor fuels. Exchange equilibrium at 580–780 °C usually was obtained after 40–120 h of vigorous stirring of the two phases. The equilibrium quotient of mole fractions, Q = (X UO 2 X ThF 4 ) (X UF 4 X ThO 2 ) , was determined at various compositions of the oxide phase and of the fluoride phase and at different temperatures. The results obtained for 72-16-12 mol % LiF-BeF 2-ThF 4 were fitted by least squares to an equation containing only two parameters: log 10 Q = (2101 + 550 X UO 2 ) T. Other results indicated that Q did not vary over a fairly wide range of composition of the fluoride phase. The expression for Q gives for the oxide solid solution log 10 γ UO 2 = (275 T) X 2 ThO 2 and log 10 γ ThO 2 = (275 T) X 2 UO 2 , and for the heat of mixing ( ΔH m ) 0.5 = 315 cal/ mol.