Heavy element separation for thorium-uranium-plutonium fuels

A large potential exists for resource conservation through introduction of thorium fuel cycles in CANDU (CANada Deuterium Uranium) reactors. While a number of fuel cycles have been suggested, this paper deals with one using thorium dioxide fuel topped with plutonium, and a processing scheme in which all fissile and fertile materials are separated and recycled. Although not discussed here, other cycles not involving complete separation are also being studied as part of the International Nuclear Fuel Cycle Evaluation. To test the separation of the three actinides (Th, U and Pu) from each other, a modified Thorex solvent extraction flow sheet using 30% tributylphosphate (TBP) has been developed. Considerable work has already been reported in the literature for separations in Th-U systems, but inclusion of Pu in the fuel cycle adds additional complexities. The flow sheet adopted is that in Figure 1, which also shows the relative flows (FL) of the inlet streams and concentrations (M or mol/L) of the major components. The flow ratios and acidities for each contactor were initially derived by constructing McCabe-Thiele operating diagrams based on unpublished distribution measurements made in our laboratories. Contactors I and II are used as a decontamination cycle to remove most of the fission products from the actinides. After intercycle concentration and Pu valency adjustment (to Pu(III)), the next three contactors make up the primary separation system and are used to recover Pu(III), Th and 233U. in this work, natural U was used in place of 233U. (author)