Numerical modeling of fuel rod transient response under out of pile test conditions

As per revised Emergency core cooling system (ECCS) acceptance criteria, a precise prediction of fuel rod behavior is essential for realistic safety analysis of nuclear reactor. In this context, a one-dimensional code name 'TRAFR' (Transient Response Analysis of Fuel Rod) is developed to simulate the thermo-mechanical behavior of Zircaloy-4 cladding under transient conditions. The transient simulations for inert and oxidizing atmosphere were performed under out of pile test conditions and the predicted burst strains were in good agreement with the experiments conducted in past. Under inert atmosphere, the cladding rupture was delayed and burst strain was higher in all the phases due to the absence of oxidation kinetics. In the oxidizing atmosphere, the burst strain was considerably small at high temperature in mix phase (alpha + beta) and beta-phase due to increment in cladding strength and reduction in ductility. For the same internal pressure and clad surface boundary conditions, the temperature of failure was higher for oxidizing atmosphere due to heat generation by exothermic reactions at the surface of the cladding. The clad surface temperature rise rate decreased before the burst with increment in gap width between pellet and cladding owing to decrement in gap conductance. The code under-predicted the burst strain in the mix phase (alpha + beta) and beta-phase by 'Baker-Just' model. The reason for such deviation was abrupt parabolic oxide growth prediction with higher exothermic heat generation and subsequent faster reduction in cladding thickness which made 'Baker-Just' model approach more conservative than 'Cathcart-Pawel' model.