Thermochemical Investigations in the System Cadmium-Praseodymium Relevant for Pyrometallurgical Fuel Reprocessing
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INTRODUCTION
NOWADAYS several countries in the world strive for strong nuclear power programs to satisfy their demands of energy. As a consequence, according to the World Nuclear Association* around 12,000 tons of *http://world-nuclear.org/.
high-level nuclear waste is produced annually. Currently, a considerable amount is stored intermediately in on-site water pools, waiting for long-term deposition in geological repositories. For an adequate and economic utilization of nuclear fuels, reprocessing of spent nuclear fuels is an option which is currently adopted only in a few countries. Conventionally, reprocessing is practiced by means of solvent extraction of actinides using tributyl phosphate (TBP), known as hydrometallurgical technique or aqueous reprocessing. Unfortunately, this technique has to deal with several problems like radiation and temperature instability of the various solvents used in the process. In addition, a huge amount of liquid waste is produced by this method. This problem makes it reasonable to investigate another type of reprocessing technique, called the pyrometallurgical technique. Pyrometallurgical reprocessing overcomes some of the major
THOMAS L. REICHMANN, Project Worker, and HERBERT IPSER, Professor, are with the Department of Inorganic Chemistry (Materials Chemistry), University of Vienna, 1090 Wien, Austria. Contact e-mail: [email protected] Manuscript submitted May 6, 2013. Article published online October 18, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
problems of aqueous reprocessing, as described by Olander.[1] The central part of pyrometallurgical reprocessing is the electrorefining step, which is carried out in an ‘‘electrotransporter’’ cell. The theoretical feasibility has been described repeatedly in the literature, see e.g., References 2 through 5. In particular, electrotransport and reductive extraction are applied to separate actinides and lanthanides from high level radioactive waste (HLW). The respective electrochemical vessel contains a liquid metal pool at the bottom, covered by a molten salt solution serving as electrolyte. One basket of anode, containing chopped nuclear fuels, and at least two cathodes are inserted into the liquid salt. During this process, especially uranium, plutonium, and minor actinides are transported whereas rare-earth (RE) elements, alkaline, and alkaline earth elements remain in the liquid salt. Additional reductive agents are added to the salt which promotes the extractability of RE elements into the liquid metal pool at the bottom by forming intermetallic compounds. Moriyama et al.[6,7] determined that the separation factors, which are an indicator for extractability, are quite different between actinides and lanthanides. In principle, actinides have the higher affinity for extraction into a metal phase, a fact that is preferable, considering the chemical similarity to lanthanides. The extraction behaviors of different elements between a molten chloride salt phase and a liquid metal strongly depend on the standard free energy
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