Thermodynamics and the Separation Factor of Dy/U Couple on Ga and Ga–Al Electrodes in Fused LiCl–KCl Eutectic

Abstract

The radiological danger of spent nuclear fuel (SNF) mainly depends on the presence of minor actinides (Np, Am, Cm) and certain long-lived fission products (Cs, I, Tc). If these elements are effectively separated from SNF and converted into short-lived or stable isotopes, the radiological danger of SNF storage can be significantly reduced. Pyrochemical reprocessing of spent nuclear fuel represents a promising alternative to the existing hydrometallurgical technology (solvent extraction PUREX-process) with subsequent transmutation of long-lived radionuclides. Pyrochemical separation methods in molten salts have some potential advantages over hydrometallurgical processes for separating actinides from fission products present in irradiated fuels. High radiation stability of molten salts and liquid metals as well as absence on neutron moderators make possible reprocessing highly active fuel after a short cooling time.In this study the principal thermodynamic properties and the separation factor of U/Dy couple were determined on gallium and gallium-aluminum electrodes in molten LiCl–KCl eutectic at 723–823 K using open-circuit potentiometry.The equilibrium electrode potentials of metallic dysprosium, uranium and their alloys with Ga and Ga-Al eutectic in molten LiCl–KCl–MeCl3 solutions were determined by open-circuit potentiometry. The potential-time dependencies of Me–Ga–Al and Me–Ga alloys (Me = Dy, U) after cathodic polarization are presented in Fig. 1.The electrochemical behavior of the alloys is characterized by the apparent standard potential of the alloys, . Temperature dependencies of the apparent standard potentials of Me(Ga–Al) and Me(Ga) alloys were fitted to the following expressions using the Origin Pro software (ver. 7.5):E** Dy(Ga-Al) = –(2.705±0.009) + (3.5±0.1) ·10–4∙T ±0.005 V [1]E** U(Ga-Al) = –(2.688±0.013) + (6.4±0.1) ·10–4∙T ±0.005 V [2]E** Dy(Ga) = –(2.726±0.007) + (3.4±0.2) ·10–4∙T ±0.009 V [3]E** U(Ga) = –(2.565±0.004) + (4.2±0.1) ·10–4∙T ±0.003 V [4]Activity coefficients of Dy and U in liquid Ga and Ga–Al alloys were calculated from the following equation:log(γMe(alloy)) = [nF(E* Me(III)/Me – E** Me(alloy))]/2.303RT [5]Values of Dy(III)/Dy and U(III)/U equilibrium electrode potentials were taken from our previous works (1, 2).E* Dy(III)/Dy = –(3.401±0.009) + (6.2±0.1) ·10–4∙T ±0.007 V [6]E* U(III)/U = –(3.051±0.006) + (7.6±0.1) ·10–4∙T ±0.003 V [7]Activity coefficients of solid U and Dy in liquid Ga and Ga–Al alloys as a function of temperature were fitted to the following equations:log(γDy(Ga)) = 4.24 – 10224/T ±0.12 [8]log(γDy(Ga-Al)) = 4.09 – 10542/T ±0.12 [9]log(γU(Ga)) = 5.15 – 7361/T ±0.14 [10]log(γU(Ga-Al)) = 1.16 – 4962/T ±0.17 [11]Partial excess Gibbs free energy change of Dy and U in liquid Ga and Ga–Al alloys was calculated according to eq. [13] and previously obtained expressions [8–11]:ΔGex Me(alloy) = ΔHMe(alloy) – TΔSex Me(alloy) [12]ΔGex Me(alloy) = 2.303RTlog(γMe(alloy)) [13]ΔGex Dy(Ga) = –195.4 + 0.081· T ±2.8 kJ/mol [14]ΔGex Dy(Ga-Al) = –201.5 + 0.078· T ±2.8 kJ/mol [15]ΔGex U(Ga) = –140.7 + 0.098· T ±3.1 kJ/mol [16]ΔGex U(Ga) = –140.7 + 0.098· T ±3.1 kJ/mol [17]When the separation of lanthanides and actinides in the spent nuclear fuel is studied, the effectiveness of electrochemical separation methods is usually described by the value of the distribution or separation factor. The separation factor for uranium and dysprosium on liquid gallium and gallium–aluminum based alloys was calculated using the following equation:log(θ) = [(n–m)FE + mFE** Dy(alloy) – nFE** U(alloy)]/2.303RT [18]The temperature dependence of the apparent standard potentials of dysprosium [1, 3] and uranium [2, 4] alloys were used to derive the following expressions for the separation factor of Dy/U couple:log(θ(Ga)) = 1.21 + 2438/T ±0.04 [19]log(θ(Ga-Al)) = 4.39 + 275/T ±0.6 [20]The separation factor of Dy/U couple in molten LiCl–KCl eutectic calculated according to the above formula indicates that dysprosium will be concentrated in the molten salt phase and uranium in the liquid metal phase. Analysis of experimental data shows that these systems are interesting for application in pyrochemical methods of nuclear waste disposal.The study was carried out with the financial support of the RFBR in the framework of the scientific project # 20-03-00743.References А. Novoselova, V. Smolenski, and V. A. Volkovich, J. Electrochem. Soc. 167, 112510 (2020). A. Novoselova and V. Smolenski, J. Radioanal. Nucl. Chem., 326, 621 (2020). Fig. 1. The potential-time dependencies of Me–Ga and Me–Ga–Al alloys (Me = Dy, U) after cathodic polarization in LiCl–KCl–MeCl3 solutions at 726 K. Polarization current 50–150 mA, duration 20–40 s. [U(III)] = 4.7·10-2 mol·kg-1. [Dy(III)] = 5.8·10-2 mol·kg–1.1 – The equilibrium potential of U(Ga-Al) alloy. 2 – The equilibrium potential of U(Ga) alloy.3 – The equilibrium potential of Dy(Ga-Al) alloy. 4 – The equilibrium potential of Dy(Ga) alloy. Figure 1