Electrochemical redox reactions of chromium and iron ions in molten NaCl–2CsCl eutectic for pyro-reprocessing of nuclear fuels

Abstract

Basic electrochemical and spectroscopic properties of Cr 3? , Cr 2? , Fe 3? , and Fe 2? were studied to analyze the cyclic redox reactions of Cr and Fe, which may decrease the current efficiency of the electro-winning method using NaCl-2CsCl melts. The formal redox potentials of the Cr 3þ jCr 2þ and Fe 3þ jFe 2þ couples, E 0 Cr 3þ jCr 2þ and E 0 Fe 3þ jFe 2þ , in NaCl-2CsCl melts at 923 K were spectroelectrochemically determined to be -0.648 ± 0.005 V and À0:140AE 0:010 V vs: Cl 2 jCl À , respectively. These values were determined by measuring electromotive force and UV-VIS absorption spectra at varying concentration ratios of trivalent and divalent ions. Cyclic voltammetry was also carried out to examine the characteristics of the voltammograms for the Cr 3þ jCr 2þ and Fe 3þ jFe 2þ couples in NaCl-2CsCl melts. The E 0 Cr 3þ jCr 2þ determined by the spectroelectrochemical method was close to that determined by cyclic voltammetry ðÀ0:651 AE 0:006 V vs: Cl 2 jCl À Þ. The effect of temperature on the E 0 Cr 3þ jCr 2þ in NaCl-2CsCl melts was studied by cyclic voltammetry in the range from 823 to 1,023 K ðE 0 Cr 3þ jCr 2þ ¼ 0:00143T À 1:971 AE 0:005 V vs: Cl 2 jCl À Þ. Diffusion coefficients of Cr 3? and Cr 2? , D Cr 3þ and D Cr 2þ , were determined between 823 and 1,023 K to be D Cr 3þ ¼ 2:23  10 À3 exp ðÀ4;135=TÞ and D Cr 2þ ¼ 3:34  10 À3 expðÀ4;106=TÞ, respectively. Molar absorptivities of Cr 3? and Cr 2? in NaCl-2CsCl melts at 923 K were determined to be 77.8 ± 2.4 M -1 cm -1 at 17,670 cm -1 and 48.0 ± 1.4 M -1 cm -1 at 9,170 cm -1 , respectively. In addition, the effects of these ions on the cyclic redox reaction of the pyro-reprocessing process were discussed.

Molten Salt Reactor Engineering Study for Off-Gas Management

“…Electrochemistry is one method of monitoring molten salt speciation using electrodes capable of withstanding the highly corrosive nature of molten salts. Electrochemistry can, for example, be used to detect species such as chromium in the salt produced by the corrosion of containment materials (Cotarta et al, 1997;Martıńez et al, 2000;Uehara et al, 2009). Another important aspect of preventing corrosion in MSRs is the ability to control the U(III)/U(IV) ratio, which can be accomplished by electrochemical methods (Prabhakara Reddy et al, 2004;Kim et al, 2012).…”