Influence of Nd(II) and Nd(III) Ions on the Speciation and Kinetic Dynamics of Radiolytic Transients in Molten LiCl-KCl Eutectic Salt Mixtures

Abstract

In the pyrochemical reprocessing of used nuclear fuel, understanding the effects of ionizing radiation on the chemical behavior of fission products in molten chloride eutectic salt is crucial. This study investigates the impact of radiation on the chemical and kinetic speciation of Nd ions in molten LiCl-KCl eutectic, which are challenging to separate due to their multivalent oxidation states and comproportionation reactions. Electron pulse radiolysis techniques were used to determine transient absorption spectra, chemical kinetics, and activation parameters for the reaction of Nd ions with the primary products of molten LiCl-KCl eutectic radiolysis: the solvated electron (e_S^-) and dichlorine radical anion (Cl₂^•-). Initially, Nd(III) reacted rapidly with e_S^-, forming Nd(II) with a second-order rate coefficient of k = (4.54 ± 0.07) × 10¹⁰ M^-1 s^-1 at 673 K. The resulting Nd(II) ions then reacted slowly with Cl₂^•-, regenerating Nd(III) with an estimated rate of k = (1.72 ± 0.04) × 10¹⁰ M^-1 s^-1 at 673 K. Additionally, the comproportionation reaction of Nd(III) and Nd(s) to form Nd(II) was monitored chemically and electrochemically. This work suggests that the comproportionation reaction of Nd(III) with Nd(s) to form Nd(II) is unstable in molten LiCl-KCl eutectic.