Electrochemical behaviour of neptunium in the molten LiCl-KCl eutectic

The electrochemical properties of neptunium, dissolved in the LiCl-KCl eutectic, were investigated by transient electrochemical techniques, such as cyclic voltammetry (CV) and chronopotentiometry on inert tungsten electrodes. It was shown that Np(4+) is reduced to Np(0) by a two-step mechanism corresponding to the transitions of Np(4+)/Np(3+) and Np(3+)/Np(0). In the 400-550 degrees C (673-823 K) temperature range, the diffusion coefficients of Np(3+) and Np(4+) were found to be of similar magnitude and equal to D(Np)- = 9.47x 10(-4) exp(-23.6 x 10(3) /RT(K)) and D(Np)4+ = 2.14 x 10(-4) exp(-14.1 x 10(3) /RT(K)) cm(2) s(-1), respectively. The apparent standard potentials of Np(4+)/Np(3+) and Np(3+)/Np(0) redox systems were determined to be E(*)(Np(4+)/Np(3+)) 1.250 + 6.637 x 10(-4) center dot T (K) V versus Cl(2)/Cl(-) and E(degrees*)(Np(3+)/Np(0)) = -3.250 + 7.251 x 10(-4) center dot T(K) V versus Cl(2)/Cl(-), respectively. The Gibbs free energies of formation in diluted solutions of NpCl(3) and NpCl(4) in the LiCl-KCl eutectic were determined to be Delta G(*degrees) = -936.54 + 0.20654 center dot T(K) and - 120.61 + 0.06405 center dot T (K) kJ mol(-1), respectively. Between 400 degrees C and 550 degrees C, the activity coefficients gamma(NpCl3) and gamma(NpCl4) range between 9 - 125 x 10(-6) and 2 - 10 x 10(-3), respectively. (C) 2007 Elsevier B.V. All rights reserved.