Alternate Anodes for the Electrolytic Reduction of UO 2
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TRODUCTION
A key component in the Integral Fast Reactor fuel cycle is the electrometallurgical reprocessing process, also known as pyroprocessing, developed at Argonne National Laboratories.[1,2] Pyroprocessing offers numerous advantages over PUREX and other conventional aqueous reprocessing methods of which most notable are proliferation resistance and increased separation efficiency of actinides.[3,4] Current supplies of UO2 need to be reduced to metallic uranium before they can be pyroprocessed. An electrolytic reduction process for UO2 in molten salt has been developed and successfully demonstrated by multiple research groups.[5–11] The current process utilizes platinum as the working anode and a SS 316 basket that contains the UO2, as cathode.[6] Platinum undergoes corrosion in this electrolyte system and thus there is a need to investigate alternative anode materials that can maintain their stability for a significant amount of time before replacement is essential. The electrochemical half-cell reactions and standard cell potential of the electrolytic reduction process under standard conditions in LiCl at 923 K (650 C) were calculated using thermochemical software by researchers at Idaho National Laboratories and Park et al.,[6,12] the results of which are summarized below.
AUGUSTUS MERWIN, Doctoral Student, and DEV CHIDAMBARAM, Professor, are with Materials Science and Engineering, University of Nevada, Reno, 1664 N. Virginia St. MS 0388, Reno, NV 89557-0388. Contact e-mail: [email protected] Manuscript submitted May 28, 2014. Article published online November 12, 2014 536—VOLUME 46A, JANUARY 2015
Reduction: UO2 þ 4e ! UðmÞ þ 2O2 Oxidation: 2O2 ! O2ðgÞ þ 4e Cell: UO2 ! UðmÞ þ O2ðgÞ
Eo ¼ 2:396V vs SHE ½6; 12
The electrolyte used in this process is a mixture of LiCl and Li2O in a temperature range of 923 K to 1023 K (650 C to 750 C) with the concentration of Li2O varying up to its solubility limit of 8.8 wt pct.[5,13,14] In the LiCl-Li2O electrolyte, additional oxidation reactions include: 2 Cl fi Cl2 + 2 e and the oxidation of the anode. In these highly oxidizing conditions at the required anodic potential, corrosion of the anode material emerges as a technical challenge.[15] Platinum has been shown to be susceptible to lithium attack, anodic dissolution and is known to form oxide compounds such as Li2PtO3 when anodically polarized in a LiCl-Li2O electrolyte.[16–22] The consumption and degradation of the platinum anode is not desirable, as it decreases the economic viability of the process and could potentially lead to the failure of the electrolysis circuit by breaking electrical contact between the external power supply and the salt electrolyte. Recent research has also shown that underpotential deposition of Li on the cathode could assist the kinetics of UO2 reduction; however, this leads to higher current densities at both electrodes.[19] Furthermore, as soluble fission products accumulate in the salt, the rate of UO2 reduction has been shown to decrease, resulting in longer electrolysis times.[9] Since
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