Odourless kerosene

The control of the actinide metal ion valence state plays a pivotal role in the separation and purification of uranium and plutonium during the processing of spent nuclear fuel. Most commercial plants use the plutonium-uranium reduction extraction process (PUREX) [58], wherein spent fuel rods are initially dissolved in nitric acid. The dissolved U and Pu are subsequently extracted from the nitric solution into a non-aqueous phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent such as dodecane or odourless kerosene (OK). The organic phase is then subjected to solvent extraction techniques to partition the U from the Pu, the extractability of the ions into the TBP/OK phase being strongly dependent upon the valence state of the actinide in question. 

The TBP solvent is used as a 20 per cent solution in odourless kerosene, and extraction takes place in a simple mixer-settler where flow is controlled by gravity only, as shown in Fig. 4.30. Eight extraction stages and eight... 

Since all sources of thorium are associated with a certain amount of uranium, the first solvent-extraction cycle is designed solely to eliminate this element. The proportion of uranium to thorium can vary, but is often of the order of 5 per cent. The uranium distribution coefficient into tributyl phosphate is much higher than that of thorium under comparable conditions, e.g. 20 and 0-5 respectively for 40 per cent TBP/xylene or 6 and 0 04 respectively for 5 per cent TBP/xylene. The more highly diluted solvent is used for the uranium separation cycle in view of the higher ratio of the two distribution coefficients, or separation factor . Xylene is chosen in preference to, for example, odourless kerosene owing to the danger of formation of a third phase rich in thorium with the latter diluent. 

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