Electrochemical studies on plutonium in molten salts

Electrochemical studies on plutonium have been supporting the development of pyrochemical processes involving plutonium at CEA. The electrochemical properties of plutonium have been studied in molten salts – ternary eutectic mixture NaCl–KCl–BaCl2, equimolar mixture NaCl–KCl and pure CaCl2 – and in liquid gallium at 1073 K. The formal, or apparent, standard potential of Pu(III)/Pu redox couple in eutectic mixture of NaCl–KCl–BaCl2 at 1073 K determined by potentiometry is equal to −2.56 V (versus Cl2, 1 atm/Cl− reference electrode). In NaCl–KCl eutectic mixture and in pure CaCl2 the formal standard potentials deduced from cyclic voltammetry are respectively −2.54 V and −2.51 V. These potentials led to the calculation of the activity coefficients of Pu(III) in the molten salts. Chronoamperometry on plutonium in liquid gallium using molten chlorides – CaCl2 and equimolar NaCl/KCl – led to the determination of the activity coefficient of Pu in liquid Ga, log γ = −7.3. This new data is a key parameter to assess the thermodynamic feasibility of a process using gallium as solvent metal. By comparing gallium with other solvent metals – cadmium, bismuth, aluminum – gallium appears to be, with aluminum, more favorable for the selectivity of the separation at 1073 K of plutonium from cerium. In fact, compared with a solid tungsten electrode, none of these solvent liquid metals is a real asset for the selectivity of the separation. The role of a solvent liquid metal is mainly to trap the elements.