Thermal analysis of the melting process in a nuclear fuel rod

• Melting of a nuclear fuel rod under the accident conditions is analyzed.
• Enthalpy formulation with explicit finite difference discretization is adopted.
• Temperature histories and phase change interface positions are determined.
• The heat transfer coefficient and heat generation rate are two predominant factors.
• Classical lumped model fails to give accurate results for high heat transfer coefficient.

Melting of nuclear fuel rods is a crucial issue that must be addressed when conducting simulation analyses of severe accidents in nuclear reactors. The present work aims to develop a mathematical model and carry out numerical simulation of one-dimensional heat conduction with phase change in a nuclear fuel rod including two regions: uranium dioxide fuel pellet and zirconium alloy cladding. The temperature-dependent specific heats, thermal conductivities and densities are considered in the present investigation. The enthalpy formulation of the melting process in the nuclear rod during full reactor power operation is solved by using a finite-difference method. The thermal analysis is divided into four different phases: (1) transient phase before the cladding melting, (2) cladding melting phase, (3) transient phase before the fuel melting and (4) fuel melting phase. Then, the effect of heat transfer coefficient between coolant and fuel rod on the temperature histories of the fuel pellet and the cladding is investigated. Finally, the melting process of a nuclear rod with decay reactor power after shutdown is also simulated.