Elecrochemistry of Uranium and Plutonium in Ionic Liquids

Abstract

Actinides can exist in many oxidation states. The redox potentials for various oxidation states of actinides strongly depend on their electronic structures, but are also affected by the medium properties. The differences between the redox potentials of actinides, most notably uranium and plutonium, have been utilized in the traditional PUREX process (Plutonium Uranium Reduction EXtraction) developed during the 1947. PUREX is based on a nitric acid dissolution of used nuclear fuel followed by liquid-liquid extraction and utilizes hazardous chemicals such as concentrated nitric acid and volatile and hazardous organics like tributyl phosphate and dodecane or kerosene. In spite of the differences in redox chemistries of the actinides, electrochemical methods have never been applied for large scale used nuclear fuel processing. Molten salts (in particular halides) have been demonstrated to be better suited for actinide electrochemistry; however, the practical application of pyroprocessing is limited by the high temperatures and strongly corroding properties of molten salts. Ionic Liquids (ILs) are relatively new media that offer the benefits of molten salts, e.g., high ionic conductivity required for practical operations and wide electrochemical windows, but are also free of the molten salt shortcomings such as the high temperature and corroding properties. Their high ionic conductivity and low reactivity make them especially well-suited for electrochemical applications. While the high conductivity enables for electrochemical reactions at the high current densities required for practical operations, the low reactivity typically translates into a wide potential window, which allows access to electrochemical processes that cannot be accomplished in conventional media. Moreover, ILs offer a wide range of tunable physical properties, which could potentially be utilized not only in the electrochemical but also in numerous other separation techniques relevant to the nuclear industry, including solvent extraction, ion exchange, dissolution, crystallization, etc. The development of separation technologies for spent nuclear fuel in ILs could complement or replace current PUREX and/or high-temperature pyrochemical processing technologies. The current state of understanding of the electrochemical behavior of actinides in ILs is far from satisfactory. As part of our multi-faceted approach to study the structural, thermodynamic, and kinetic behavior of actinides, we undertook electrochemical studies of uranium and plutonium in selected chloride and bistrifluorosulfonamide based-ILs in order to demonstrate the feasibility of using ILs for electrochemical actinide separations. In this presentation we will summarize our most recent results of those studies.