Determination of the E-pO2− stability diagram of plutonium in the molten LiCl–KCl eutectic at 450 °C

Plutonium trichloride solution in the molten LiCl–KCl eutectic was prepared by carbochlorination of plutonium oxide. Kinetics of this reaction was compared in different conditions in the range of 443–550 °C. Using this molten salt solution, the redox potential of the Pu(III)/Pu couple at inert tungsten electrode was measured at 450 °C by electromotive force measurement and was found to be E′○ = −2.76 V vs. the Cl2(g)(1 atm)/Cl− reference electrode (molar fraction scale). Reaction between plutonium trichloride and oxide ions was studied by potentiometric titration, using yttria stabilized electrodes. In our experimental conditions, the titration curves indicate the precipitation of the sesquioxide Pu2O3. The solubility product cologarithm calculated from these curves is found to be pKs(Pu2O3) = 22.8 ± 1.1 (molality scale). Using the experimentally obtained values for E′○, activity coefficient and pKs joined to the published thermodynamic data, the stability phase diagram of the Pu–O species was then drawn.