• Measured relative equilibrium potentials of rare earth metals in molten LiCl-KCl • Established rare earth metal nobility order without thermodynamic reference electrode • Derived relative activity coefficients for select rare-earth chlorides in LiCl–KCl Electrorefining of metallic spent nuclear fuel (SNF) has been demonstrated using irradiated fuel from Experimental Breeder Reactor-II at Idaho National Laboratory and is being considered for recycling SNF. Metal-chloride reduction potentials are key to predicting constituent metal behavior during SNF electrorefining. Rare earths are of particular interest, as major fission products in SNF are known to have very close equilibrium reduction potentials. Different sources of data on these reduction potentials indicate conflicting sequences. A comparative study was performed in which Gd, Nd, Ce, and La metal rods were equilibrated in molten eutectic LiCl–KCl with 1 wt% LaCl 3 at 773 and 873 K. Equilibrium potentials of Gd, Nd, and Ce were measured versus La/La³⁺, and concentrations of GdCl 3 , NdCl 3 , and CeCl 3 were measured via inductively coupled plasma mass spectrometry. All reported equilibrium potentials and derived activity coefficients are relative to the La/La³⁺ redox couple. The order of equilibrium potentials was Gd > Ce > Nd > La. Measured potential differences ranged from 2.76–32.12 mV. Relative activity coefficients for GdCl 3 , NdCl 3 , and CeCl 3 with respect to that of LaCl 3 were calculated and varied from 0.18–11.68. The order, by magnitude, of the activity coefficients is CeCl 3 > LaCl 3 > GdCl 3 ≈ NdCl 3 . This proved to be an effective method for measuring relative potentials and activity coefficients.
González et al. (Fri,) studied this question.