The oxide chemistry of plutonium in molten fluorides and the free energy of formation of PuF 3 and PuF 4

Abstract

The behaviour of plutonium in molten fluorides in the presence of oxide ion has been studied in order to establish, in a molten salt reactor (MSR) fuel salt, the conditions of redox potential and oxide concentration necessary to avoid the precipitation of plutonium oxides. Three salt mixtures, LiFBeF 2 (66·7−33·3 mole %), LiFBeF 2ZrF 4 (65·7−29·3−5·0 mole %), and LiFBeF 2ThF 4 (72−16−12 mole %), containing PuF 3 have each been equilibrated with the oxide that is stable in that media (BeO, ZrO 2, and ThO 2, respectively) under various redox conditions and different temperatures. Only in the ThF 4 − containing melt, equilibrated with ThO 2 and NiO, did the PuF 3 concentration decrease. This was confirmed to be the result of the following reaction: 1 2 NiO(c)+ 3 4 ThO 2(ss)+PuF 3(d)⇌PuO 2(ss)+ 3 4 ThF 4(d)+ 1 2 Ni°(c) where c, ss, and d denote respectively pure crystalline and solid solution phases, and components dissolved in the molten fluoride phase. The free energy of formation of PuF 3 was determined from the measured equilibria quotient of Reaction (1), the solubility of PuF 3, and the formation free energies available for ThO 2( c), NiO( c), PuO 2( c and ThF 4( d): δG f PuF4( c) =−334·7±2·6 and −333·1±2·6kcal/mole at 615° and 715° C. By combining this with literature data on the reaction of O 2 with PuF 3 we have obtained: δG f PuF4( c) =−385·4+8·7( T/1000)±2·8kcal/mole. From the results obtained we may conclude that no plutonium oxide would precipitate from the fuel of an MSR under normal operating conditions.