| In the event of a nuclear fuel element developing a defect in its sheath, coolant can enter the element and negatively affect its thermal performance. In particular, the coolant will flash to steam in the fuel-to-sheath gap, resulting in a reduced gap heat transport coefficient and oxidation of the fuel to UO2+x. Oxidized fuel has a reduced thermal conductivity and lower incipient melting point which, coupled with the reduced gap heat transport coefficient, increases the potential for centreline melting.;The work is validated and compares well with laser flash experiments from open literature. Furthermore, the applicability of this work to the analysis of in-core irradiation of defected elements and direct electrical heating experiments is demonstrated. The work is then used to predict the potential behaviour of centreline melting in operational, defected nuclear fuel elements. Under the examined conditions, simulation results show that centreline melting is self-regulating such that the melting front will not reach the sheath of the fuel element.;The goal of the current work is therefore the development of a robust and versatile model to simulate the melting of nuclear fuel, particularly under fuel-failure conditions where oxidation may have occurred. A comprehensive review of recent material properties is performed in order to obtain input parameters for the simulation, and to ensure reasonable extrapolation to temperatures where material properties are not known or have a large degree of uncertainty. The modelling technique used in this work is based on the phase field (diffuse interface) approach, and is compared to a Stefan (sharp interface) formulation. Transient mass and heat transport, coupled with phase stability analysis for a non-congruent phase change, is considered as derived from the theory of irreversible processes. |
