The Applications of Photocatalytic Waste Minimisation in Nuclear Fuel Processing

Abstract

Nuclear fuel processing has two main waste management requirements: (1) the disposal of waste organic solvent (secondary waste) generated by solvent extraction processes during the separation and purification of uranium and plutonium in nuclear fuel and materials processing; and (2) the management of the small fractions of U and Pu that are inseparable during reprocessing (primary waste). Environmental impact associated with fuel use and reprocessing can be minimised by addressing either of these requirements. Semiconductor particles and films may act as efficient photocatalysts for a range of environmentally and industrially useful reactions including heavy metal recovery from effluent streams by manipulation of the metal valence state. The manipulation of actinide metal ion oxidation states plays an important role in nuclear fuel and materials processing. Thus, this review explores the potential use of heterogeneous photocatalysis in actinide valence state control in the context of actinide photochemistry and minimised primary and secondary waste management requirements in the plutonium--uranium reduction extraction (PUREX) nuclear fuel processing route. Criteria are defined for the selection of heterogeneous semiconductor catalysts and sacrificial charge scavengers for use within reprocessing scenarios and two main applications discussed: (1) the photocatalytic control of the neptunium ion oxidation state and consequent separation of Np from Pu and U; and (2) the photocatalytic control of U and Pu ion oxidation states and their consequent separation from each other. A quantum efficiency, ϕ, of 0.27 is reported for the photocatalytic reduction of the Pu(IV) simulant, Ce4+ to Ce3+ at pH 0. The high value of ϕ is attributed to both the forward and reverse charge transfer processes occurring via a dynamic quenching mechanism. Yields of 100% are reported for the reductions of UO2 2+ to U4+ and Ce4+ to Ce3+.