In Situ Spectroscopy and Spectroelectrochemistry of Uranium in High-Temperature Alkali Chloride Molten Salts

Abstract

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.

Electronic absorption spectra of U3+and U4+in molten LiCl-RbCl eutectic

“…The characteristic large electronic absorption bands of U 3+ in LiCl-RbCl eutectic at the UV/Vis region of over 14000 cm -1 were observed as shown in figure 4 (a). These bands are quite similar to those observed in LiCl-KCl [19] and NaCl-CsCl [9] eutectics, and are due to the 5f 3 -5f 2 6d 1 transitions according to the comparison with those of U 3+ doped Cs 2 NaYCl 6 and Cs 2 LiYCl 6 single crystals [20]. These absorption peaks at the UV/Vis region are different from the melt composition as shown in table 1.…”

Electrochemical behavior and electronic absorption spectra of uranium trivalent ions in molten LiCl–CsCl mixtures

“…6a and b show absorption spectra of U 3+ in molten LiCl-CsCl mixtures at 973 K. The characteristically large electronic absorption bands of U 3+ were observed at the UV/Vis region over 14,000 cm À1 . The similar absorption properties are confirmed for U 3+ in LiCl-KCl [13], LiCl-RbCl [5], and NaCl-CsCl [8] eutectics. These bands are assigned to be the 5f 3 -5f 2 6d 1 transitions from the resemblance to spectra measured for single crystals of U 3+ doped Cs 2 NaYCl 6 and Cs 2 LiYCl 6 [14].…”

Electronic Absorption Spectra of Niobium Species in Halide Melts

“…The details of the spectroscopy experimental procedures and preparation of solventsalts were described elsewhere (10,11). All preliminary manipulations with oxygen and moisture sensitive compounds were carried out in an argon filled dry box with strictly controlled moisture and oxygen content (MBraun).…”