Studies on the Electroreduction of Uranyl (VI) in Molten Equimolar KCl-NaCl by Chronopotentiometric and Electrode Impedance Measurements

Abstract

The reduction of UO2(VI) to UO2 in molten KCI-NaCI salt at 716~ has been shown by chronopotentiometry to be diffusion controlled at a platinum electrode. Furthermore, the potential-time curves indicated that the reaction occurred in two steps at nearly the same potential. This observation was verified by measuring the pseudo capacity of the polarized cathode as a function of its potential for which a plot of the data shows two peaks in capacity. The first was found to be symmetric with a concentration-independent potential, which is indicative of an electrode reaction producing a soluble product. The second peak was asymmetric with a concentration-dependent potential, which is indicative of an electrode reaction producing an insoluble product. These observations suggested that the electrode reaction proceeds by a reduction of UO2(VI) to UO2(V) followed by the reduction of the UO2(V) to UO2(s) Additional verification of this mechanism was provided by the reduction of UO2 (VI) by UO2 to yield a stable solution of UO2 (V). The heterogeneous rate constant for the reduction of UO2(VI) to UO2(V) was estimated to be 2 cm sec -I, but a value for the reduction of UO2(V) could not be determined because of the solid UO2 deposit. The diffusion coefficient was estimated from the chronopotentiometric data to be 2.2 x 10 -5 em 2 sec -I.Uranyl(VI) in molten chloride salt solutions can be electrolytically or chemically reduced to UO2. The electrolytic reduction is under investigation at this laboratory for use in a process to reclaim uranium and plutonium from UO2-based nuclear fuels (1) and as a method of preparing high quality crystalline UO2 (2). Part of this research program has been a study of the mechanism and the thermodynamic properties of the reduction.The electrolytic reduction of UO2C12 to UO2 was first described by Hampe in 1888 (3). More recently, Smirnov and Ivanovsky (4) studied the anodic behavior of the UO2 electrode in the eutectic LiC1-KC1 melt. They found only hexavalent uranium in the melt after anodization and determined that the reaction proceeded by direct electron loss rather than through dissolution by anodic chlorine. Their estimate of the oxidation potential of UO2, compared to a chlorine electrode, was 0.57v. Hill, Perano, and Osteryoung conducted a polarographic investigation of the UO2(VI) reduction, also in the eutectic LiC1-KC1 melt (5). They found that the polarographic wave did not conform to a reversible twoelectron process, but that there was a linear relationship between wave height and UO2(VI) concentration. They plated UO2 on the microelectrode and measured the open-circuit potential as a function of concentration. The data obtained in this fashion did suggest a reversible two-electron transfer process, and their estimate of the emf for the reaction was close to that obtained by Smirnov and Ivanovsky. Wilks (6) reported a study of the reactions occurring in the UO2C12-KC1-NaC1 melt which would result in stoichiometric and nonstoichiometric UO2 deposits on electrolysis. He suggested the pres...

On the formation of uranium(V) species in alkali chloride melts

“…The main discrepancy in the literature data [4,5,[9][10][11] concerns the possibility of the formation of uranium(IV) ions as a product of decomposition of uranyl(VI) chloride. Absorption spectroscopy offers a convenient way for identifying uranium ions in different oxidation states.…”

“…Based on literature reports [4,5,[9][10][11], the following reactions, describing the thermal decomposition of uranyl(VI) chloride dissolved in a chloride melt, can be proposed: …”

“…According to Wenz and Adams [4], uranyl(V) ions are formed in the melt below 750 °C: at higher temperatures UO 2 is precipitated and a small amount of U(IV) ions is formed in the melt. Adams [5] and Stromatt [10] attributed the formation of UO 2 + ions in the melt to the reaction between UO 2 2+ and uranium dioxide. Somewhat later, Smirnov et al [11] reported that upon removing Cl 2 from the atmosphere above a uranyl(VI)-containing melt, U 3 O 8 and uranium tetrachloride are formed.…”

The Effect of Fission Product Elements on the Behavior of Uranyl Species in Alkali Chloride Melts: a Contribution towards Reprocessing Spent Oxide Fuels

“…, both UO 2 and MoO 2 are cathodically deposited as individual phases. Some interaction of uranyl(VI) ions with metallic platinum in a NaCl-KCl melt was noted by Smirnov and Skiba but not by Stromatt (4,26). The reaction between UO 2…”

Electrochemical separation of uranium in the molten system LiF–NaF–KF–UF4

“…Electrochemistry of uranium was mainly investigated in various chloride systems. Hill et al [11] dealt with MgCl 2 -NaCl-KCl, Stromatt [12,13], Flengas [14], Serrano and Taxil [15] examined uranium in NaCl-KCl and Poa et al [16] used LiCl-NaCl-CaCl 2 -BaCl 2 systems. However, a majority of authors scrutinized system LiCl-KCl [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36].…”