Kinetics of radiation-induced Cr(ii) and Cr(iii) redox chemistry in molten LiCl-KCl eutectic

Chromium (Cr) is a frequent constituent of the metal alloys proposed for molten salt nuclear reactor (MSR) applications, and is typically the least noble metal ion present. Consequently, chromium is preferentially corroded into molten salt solutions. The redox poise and redox cycling of chromium ions in the salt can greatly influence its corrosivity towards structural alloys, ultimately impacting the longevity of MSR systems. Radiation-induced chemistry is expected to play a significant role in determining the chromium oxidation state distribution during MSR operations. In the present research, electron pulse radiolysis techniques were employed to characterize the reactivity of Cr(ii) and Cr(iii) ions with primary radiolysis products in molten lithium chloride-potassium chloride (LiCl-KCl) eutectic over a temperature range of 400-600 °C. Both chromium oxidation states were found to rapidly react with the primary products of molten chloride salt radiolysis, i.e., the solvated electron (e_S⁻) and the dichlorine radical anion (Cl₂˙⁻). For reactions with the e_S⁻, second-order rate coefficients (k) of k = (4.1 ± 0.2) and (6.1 ± 0.3) × 10¹⁰ M⁻¹ s⁻¹ at 400 °C for Cr(ii) and Cr(iii), respectively, were determined. Temperature-dependent measurements allowed for the derivation of activation parameters for electron capture by Cr(ii) and Cr(iii). Both chromium ions also react with Cl₂˙⁻, k = (7.2 ± 0.3) and (1.4 ± 0.1) × 10⁹ M⁻¹ s⁻¹ at 400 °C for Cr(ii) and Cr(iii), respectively.