Studies On The Electrochemical Deoxidation Of Uranium Dioxide In Molten Calcium Chloride

Abstract:

Electrochemical deoxidation of a solid metal oxide cathode in molten calcium chloride medium at 900 oC was reported about a decade ago and there has been a spurt of research activities world over to develop the process for production of metals and alloys. The process, with some modifications, was adapted to nuclear technology for consolidation of spent-oxide nuclear fuels. The electro-deoxidation process is also being developed as a head-end step in the pyrochemical reprocessing of spent oxide fuels by atomic energy programmes of USA, South Korea and Japan. In the FFC Cambridge process, molten calcium chloride is used as the electrolyte and graphite serves as the anode. However, due to issues of handling of materials with radioactivity, a lower temperature is preferred for the nuclear applications of the process and hence most of the work in this area was carried out in molten lithium chloride containing 1-3 wt.% lithium oxide at 650 oC. Platinum is used as the anode for purity concerns of the nuclear materials. We have initiated a programme on the electro-deoxidation of actinide oxides with the above stated objectives. In order to gain insight into the electro-deoxidation phenomenon at the elevated temperatures, we have carried out electro-deoxidation studies with solid UO2 in the typical FFC method, i.e with actinide oxide as the cathode, graphite as the anode and molten calcium chloride as the electrolyte. Platinum anode is also used in few experiments. Prior to the electrodeoxidation of the oxide, Cyclic voltammetry of different electrodes were carried out in the calcium chloride melt to characterise the melt and also to relate the redox potentials to the electro-reduction of the oxide. Powder compacted and sinterered UO2 pellets (sintering temperature 1550 oC) were used in the electrodeoxidation studies. The experimental results showed that the surface of thick UO2 pellets (4 mm thick)pellets was transformed to an impervious uranium metal layer, but the bulk remained as unreduced UO2. A thin UO2 disc (1 mm thick) could be completely reduced under similar experimental conditions. The experimental results indicated that it is difficult to achieve bulk reduction of thick UO2 preforms in the calcium chloride melt as the uranium metal preferentially forming on the surface of the preform sinteres as an impervious metal layer at the elevated temperature of operation (900oC) of the cell. The non-porous surface metal layer prevents the electrolyte contact to the bulk of the pellet and hence the bulk reduction. The results also indicated that it may be possible to achieve complete reduction of UO2 in molten calcium chloride provided the metal oxide is made as thin discs. The electro-reduction experiments with platinum anode showed significant mass loss of the platinum metal from the electrode. The details of the studies are presented in the paper.

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