Analysis of the influence of pellet-to-cladding gap on the transient heat transfer in nuclear fuel rods via the integral transform technique

This work analyzes the transient conduction heat transfer with variable source term in a nuclear reactor fuel rod. Three models are presented to determine the transient temperature distribution across the fuel rod. The results obtained by each modeling are compared in order to evaluate the influence of the gap, as well as the cladding on the thermal problem. The main contribution of this paper is the third modeling, where a lumped analysis that encompasses the cladding material is performed, while considering convective heat transfer in the gap region. For this modeling, the integral transform technique is employed to solve the problem and the resulting ordinary differential equations system is evaluated numerically. Special attention is given to variations in the critical time as a function of Biot number, since this is an important engineering parameter to infer the beginning of the cladding melt down process. The results showed that the gap does not have a major influence in the cladding temperature profile; however, it has great influence on fuel radial temperature distribution.