Modelling the Oxidation of Spent Uranium Carbide Fuel

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Abstract

Uranium/mixed carbide fuels are a candidate fuel for future nuclear reactors. However, in order to be implemented, a clear outline for their reprocessing must be formed so as to reduce the volume of nuclear waste produced as much as possible. One proposed method is to oxidise the uranium carbide into uranium oxide which can then be reprocessed using current infrastructure. A mathematical model has been constructed to simulate such an oxidation from a combination of finite-difference approximations of the relevant equations describing the heat and mass transfer processes involved. Available literature was consulted for reaction coefficients and information on reaction products, however the behaviour of the produced oxide is uncertain. The model was built accounting for this uncertainty and the resultant predictions will assist in characterising the proposed reprocessing method for carbide fuels.

Introduction

The oxidation of uranium carbide as a reprocessing step has the potential to significantly increase the efficiency of uranium (U) and plutonium (Pu) extraction from spent carbide fuel. However, it is not without its own difficulties. The oxidation in air is known to be highly exothermic (Mazaudier et al., 2010), especially if the carbide is in powder form (Le Guyadec et al., 2009), resulting in the risk of thermal run-away of the oxidation and perhaps even self-ignition. A model of this process, therefore, should define a safe operating envelope in which the exothermic nature of the reaction can be managed.

The only existing model of the oxidation of a carbide fuel pellet in the literature is that of Scott (1966), with no published literature on the simulated oxidation of carbide fuel since. Scott’s model is a shrinking core model of a spherical graphite-uranium pellet in a fixed or moving bed. The model produced in this work elaborates on Scott’s model by including a far more thorough description of the oxygen transfer through the uranium oxide (UO2) product layer, including carbon dioxide transfer and affording the oxide layer the ability to spall off. This allows the oxidation behaviour of a uranium carbide pellet to be much more realistically modelled, given that under different conditions the oxide product layer may behave differently, and produces a valuable tool for research into carbide fuel reprocessing methods.

Section snippets

The Oxidation of Uranium Carbide

Taking the oxidation to occur in air, the reaction in its simplest form can be written as below:UC(s)+2O2(g)UO2(s)+CO2(g)The main products, therefore, are gaseous carbon dioxide and a solid, powdered oxide (in reality, a further oxidation to U3O8 can occur and a number of fission products are produced). It is unclear from the literature at this stage how the oxide product will behave, i.e. whether it adheres to the surface of the oxidising carbide, slowing the reaction, or if it continually

Mathematical Model

The next stage is to characterise the physical processes involved that are to be included in the oxidation model, in particular, the heat and mass transfer processes. The pellet is assumed to be surrounded by an infinite gaseous region of stagnant oxidant. The oxidant, air in the case of the models, is held at a constant temperature. As shown in Figure 1, the pellet is assumed to be an equivalent volume sphere with a radius of 3.27 mm allowing the system to be considered in one dimension. This

Results and Discussion

A finite-difference technique known as the fully implicit backward method (Smith, 1965) was then applied to the above equations with the resulting approximations then solved in a FORTRAN code using the Thomas algorithm (Chang and Over, 1981). The results calculated were then plotted into graphs showing the heat profiles of the solid as time passes, as well as the changing radius of the carbide, r1(t). Figure 2 is such a plot for the case where no oxide layer is present. The peak temperature

Conclusions

A novel one-dimensional model of the oxidation of a spherical uranium carbide pellet has been derived for three different conditions to account for the uncertain nature of the oxide product. Notable features include non-linear boundary conditions that allow the calculation of co-dependent, transient heat and mass transfer. The model was completed using finite-difference approximations of the relevant heat and mass transfer equations as well as a first-order reaction rate of the reaction

6. Acknowledgements

The research leading to the results contained in this paper received funding from the European Union 7th Framework Programme FP7-Fission-2011–2.3.1 under grant agreement number 295825 (Project ASGARD). The paper reflects only the authors’ views and the European Union is not liable for any use that may be made of the information contained therein.

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