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REPROCESSING OF IONIC MELT
BATH CONTAINING HIGH-LEVEL
RADIOACTIVE WASTES BY COUNTERCURRENT
ELECTROMIGRATION Haruaki MATSUURA1,
Masahiko MATSUMIYA1 Ryuzo TAKAGI1* and Reiko FUJITA2 1 Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan TEL: +81-3-5734-3076, FAX: +81-3-5734-3057, e-mail: hmatsuur@nr.titech.ac.jp 2 Nuclear Chemical System R&D Dept., Power ＆ Industrial Systems R＆D Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan TEL: +81-44-288-8153, FAX: +81-44-270-1807 * diseased After the electrorefining of fuel materials such as U and transuranium elements, all alkali and alkaline-earth fission products are still staying in the melt bath, since these decomposition potentials are more negative than that of melt bath elements. Thus, in order to reuse the melt bath repeatedly, we have proposed a countercurrent electromigration technique invented by Klemm. The electromigration technique has the advantage for the pyrochemical treatment due to its simple construction for being able to use the same container with the conductive melt bath. Iwasaki and Takagi [1] have already demonstrated enrichment of La in the NaCl-KCl equimolar mixture bath. Moreover, Matsuura et al. [2] have recently pointed out a possibility of enrichment of Cs, Sr,, and Gd in the LiCl-KCl eutectic mixture. In the present work, we have employed molten CsCl, SrCl2 and BaCl2 in the NaCl-KCl equimolar mixture in order to both a) check the feasibility of using NaCl-KCl melt bath and b) design a new electromigration cell for repeatable processing According to preliminary experiments, Sr and Ba can be much easier enriched than Cs. Thus, the electromigration method could be effectively used for separation of multivalent cations from monovalent cations on the conditions of containing Cs, Sr and Ba at the similar concentration. Supposing that Sr and Ba are removed in advance, Cs also can be concentrated. However, due to small difference between the mobility of solvent cations and that of Cs, much transported charge are required for enriching Cs. In order to estimate the maximum concentration of Cs in the melt bath, we have carried out also the electromigration experiments on molten CsCl + NaCl + KCl mixture. Since the isotherm of internal mobilities of Cs and solvent at 1023 K has a Chemla crossing point at xCs~0.25, Cs can be enriched at most 25 mol%. The most distinctive features of improved design for a countercurrent electromigration are: 1) The Cl2 gas from the anode was circulated to the cathode compartment, so as to oxidize Na metal deposited on the cathode to NaCl 2) we have designed a migration tube kept in the melt bath so as to take out the part of enriched salt from the column. The removed salt contains Sr and Ba more than the melt bath does. It was found that the specially designed cell could successfully remove the enriched part from the column repeatedly. This electromigration technique can be contributed to not only reduction of salt waste volume but also prolonging the electrorefining processes. We propose this technique to be one of options between top-end fuel removal process and back-end waste deposition This work was performed at Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology in collaboration with Tokyo Electric Power Company. REFERENCE [1] M. Iwasaki and R. Takagi,, J. Nucl. Sci. Technol., 31 (1994) 751-753 [2] H. Matsuura, R. Takagi, I. Okada, and R. Fujita, J. Nucl. Sci. Technol., 34 (1997) 304-309 |
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