Mathematical modelling of the pre-oxidation of a uranium carbide fuel pellet

Abstract

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 .

Mathematical model of the oxidation of a uranium carbide fuel pellet including an adherent product layer

“…Comparison of these predictions to those obtained from a model where the U 3 O 8 layer does not adhere [20], indicates that the product layer slows the reaction. For example, at 500…”

“…The combination of the model presented in this work and that by Shepherd et al [20] therefore allows predictions to be made for both circumstances: where the oxide layer adheres and where it does not.…”

“…If the intermediate UO 2 phase immediately spalls from the reaction surface of the pellet, then the further oxidation of UO 2 to U 3 O 8 takes place in the surrounding space away from the solid system. This situation has been recently investigated [19,20] and the time to completely oxidise a spherical pellet of 0 935cm was between 3 h to 30 h depending on the oxidation conditions with temperatures reaching as high as 1458…”

“…al [20] by including the presence of an adherent oxide layer and the significant effect it has on gaseous mass transfer and hence reaction rate, allowing predictions to be made for a much broader range of oxidation conditions . However, if the UO 2 does not spall off from the pellet surface, it is quickly oxidised to U 3 O 8 and a permanent layer of this material adheres to the pellet.…”

“…Uranium monocarbide (UC) is under consideration for use in Generation IV reactors due to its higher conductivity and metal atom density making it easier to control in a reactor [12,11,3,19]. An important consideration when implementing a novel fuel type into the nuclear fuel cycle is that it must be able to be efficiently reprocessed.…”