Uranium carbide is a candidate fuel for future nuclear reactors. However, for it to be implemented in a closed fuel cycle, an outline for its reprocessing is necessary. One proposed method is to oxidise the uranium carbide into uranium oxide which can then be reprocessed using current infrastructure. A mathematical model describing the heat and mass transfer processes involved in such an oxidation has been constructed. The available literature was consulted for reaction coefficients and information on reaction products. A stable and convergent numerical solution has been developed using a combination of finitedifference approximations of the differential equations. Completion times of approximately 3-30 hours are predicted given a spherical pellet with a radius of 9.35 under varying initial conditions. The transient temperature distribution throughout the system is predicted, with a maximum temperature of 1458° observed from an initial temperature of 500° at an oxygen concentration of 3.15 .

Highlights• Model of the oxidation of UC including an adherent U 3 O 8 product layer is presented.• Highly non-linear numerical solution to a double moving boundary problem.• Transient heat and mass transfer in expanding oxide and shrinking carbide phases• Temperature and oxygen concentration sensitivities tested and recommendations made. AbstractUranium carbide is a candidate fuel for Generation IV nuclear reactors. However, like any candidate fuel, a reprocessing route should be established before implementation. One proposed method involves a preoxidation step, where the carbide fuel is oxidised to an oxide and then reprocessed as normal. A mathematical model has been developed to simulate such an oxidation using finite difference approximations of the heat and mass transfer processes occurring. Available literature was consulted to provide coefficients for the reaction rates and importantly the diffusion of oxygen through the adherent oxide layer that forms on the carbide: the rate limiting step. The transient temperature, oxygen and carbon monoxide distributions through the system are modelled in order to predict oxidation completion times and the temperatures reached. It was found that for a spherical pellet of radius 0.935 cm, the oxidation can take between 1 h to 19 h depending on the oxidation conditions and reach temperatures of up to 1556 • C. A robust model results that offers increased understanding of a process crucial to the sustainable use of carbide fuels in energy generation.

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Abstract. The oxidation of spent uranium carbide fuel, a candidate fuel for Generation IV nuclear reactors, is an important process in its potential reprocessing cycle. However, the oxidation of uranium carbide in air is highly exothermic. A model has therefore been developed to predict the temperature rise, as well as other useful information such as reaction completion times, under different reaction conditions in order to help in deriving safe oxidation conditions. Finite difference-methods are used to model the heat and mass transfer processes occurring during the reaction in two dimensions and are coupled to kinetics found in the literature.