Electrochemical Studies of Uranium and Thorium in Molten LiF-NaF-KF at 500°C

Abstract

Electrochemical studies of U(IV) in molten normalLiF‐normalNaF‐KF (46.5‐11.5‐42.0 mole per cent) at 500°C reveal a two‐step reduction process at a platinum electrode. The first step is complicated by disproportionation of U(III) to regenerate U(IV); the second step involves formation of uranium metal. Standard electrode potentials with respect to a unit mole fraction Ni(II)/Ni electrode are estimated for the U(IV)/U(III) and U(III)/U couples as −1.41 and −1.81V, respectively. These values must be considered tentative because of complications in the electrode processes. The voltammetric oxidation of U(IV) at platinum and pyrolytic graphite electrodes in normalLiF‐normalNaF‐KF occurs at ∼ + 1.3V vs. a Ni(II) (saturated)/Ni reference electrode. The results point to the disproportionation of electrochemically generated U(V). In voltammetric studies of Th(IV) in molten normalLiF‐normalNaF‐KF at 500°C, a reduction wave is obtained at nickel and tungsten electrodes, with a peak potential of −2.02V (vs. a Ni(II) (saturated)/Ni reference electrode). Analysis indicated that Th(IV) is reversibly reduced to the metal with alloy formation between the deposited thorium and the nickel. A standard electrode potential for the Th(IV)/Th couple in this melt is calculated to be −2.13V vs. a unit mole fraction Ni(II)/Ni electrode.

Development of Pyrochemical Reprocessing of the Spent Nuclear Fuel and Prospects of Closed Fuel Cycle

“…The anodic peak corresponding to stripping of deposited thorium metal was detected during the back scan after the peak belonging to oxidation of alkali metal reduced during the melt decomposition. The measured voltammograms qualitatively well agreed to the data published by Clayton et al (1974) and the typical one is also at Fig. 3.…”

“…Its electrochemical behaviour was found to be similar in each melt, when a twostep reduction up to uranium metal was observed. Both reactions seem to be quite well reversible and in case of FLINAK, measured voltammograms correspond well to theoretical assumptions and they are also in very good agreement with data published by Clayton et al (1974) [4]. In case of LiFCaF 2 melt, the ratio between maximums of current densities of the peaks does not correspond to the expected value 1:3, albeit the potential difference between the peaks is almost the same in both melts.…”

Reduction process of uranium(IV) and uranium(III) in molten fluorides

“…Clayton and Mamantov [8] studied uranium electrochemical behaviour in the same bath at 500 • C. They also observed U(0) and U(III) oxidation. As the previous author, they attributed it to a reaction of U metal with potassium: U + K(I) = U(IV) + 4K.…”

“…Several authors have already reported U metal instability in LiF-NaF-KF melts (46.5-11.5-42.0 mol%) [8,31]. Young [31] worked at 525 • C and studied uranium by spectrophotometry.…”

“…The results given for the number of exchanged electrons are 1 for this first step [4,5,9] except for Clayton and Mamantov [8] in LiF-NaF-KF who obtained a range of 1.4-1.7. These authors explain this difference by a possible disproportionation reaction; 4UF 3 = U + 3UF 4 and a reaction with the solvent; U + 4K(I) = U(IV) + 4K: these reactions generating U(IV) disturb the calculation of n.…”