The COEX (co-extraction of uranium and plutonium) process for reprocessing of spent nuclear fuels involves coprecipitation of plutonium(III) and uranium(IV) oxalates via the reaction between Pu/U nitrates and oxalic acid. The mixed crystals are filtered, dried and decomposed in a rotary calciner for conversion to Pu/U dioxides for the fabrication of mixed oxide fuels (MOX). The crystal morphology and size distribution influence the efficiency of these downstream processes and the performance of fuel. The coprecipitation reaction is performed in a continuous vortex reactor agitated with a magnetic stirrer. In precipitation processes, mixing of reactants is of primary importance in determining the local supersaturation, which is crucial for tailoring the crystal attributes. In a previous study, we have investigated the performance of this type of reactor for the precipitation of Ca-oxalate and Ce-oxalate as simulants. The proposed PhD project builds on this study and aims to develop a detailed understanding of how the vortex reactor configuration and operating conditions influence the coprecipitation process and to optimise the reactor performance for the synthesis of mixed actinide oxalates crystals with required properties for an efficient downstream decomposition process. The reactor operation must ensure reproducibility of the product crystals with homogeneously mixed metal ions. Measurements of critical process parameters and crystal properties will provide necessary data for process optimisation and to develop of a coprecipitation process model.
Experiments will be carried out in a laboratory-scale vortex reactor in the Nuclear Process Engineering laboratory in SCAPE, University of Leeds. The mixing and residence times in the reactor will be determined using the tracer injection technique involving conductivity measurements. Suitable metal/ actinide oxalates will be used, as simulants for Pu(III) and U(IV) oxalates, for studying the coprecipiation process as functions of reactants feed addition rates and locations, stirrer speed, temperature, reactants concentrations and pH. In situ on-line process analytical techniques (such as IR/UV-vis spectroscopy for solution concentration and video imaging for the evolution of crystal size/shape) will be used for monitoring the precipitation process and off-line techniques (including Malvern Morphologi G3 and SEM/TEM for the crystal imaging and size/shape analyses and powder X-ray diffraction for solid state structure analysis) will be used for characterising product crystals properties.
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