NUMERICAL SIMULATION OF THERMO-MECHANICAL COUPLED BEHAVIOR OF CLAD TUBE UNDER INTERMEDIATE BREAK LOCA
As per revised 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 thermo-mechanical behavior of Zircaloy-4 cladding under transient conditions like Loss of Coolant Accident (LOCA). The transient simulations for inert and oxidizing atmosphere were performed and the results are presented in this paper. The effect of oxidation on burst strain was significant at high temperature in mix phase (α+β) and β-phase. Under oxidizing atmosphere the cladding became brittle due to diffusion of oxygen and resulted into early failure of cladding. The temperature of failure was higher for oxidizing atmosphere due to heat generation by exothermic reactions at the surface of cladding. The gap conductance decreased with ballooning and slop of clad surface temperature rise rate declined just before burst. During transient, the Rayleigh number remained below 109 and the average convective heat transfer coefficient over cladding increased gradually from 3 W/m2K to 7 W/m2K. The radiative heat transfer coefficient due to heat transfer between clad surfaces to enclosure varied gradually from 18 W/m2K to 80 W/m2K. Hence radiation was the major mode of heat dissipation from fuel rod to surrounding under transient conditions. The predicted burst strain for inert and oxidizing atmosphere were in good agreement with previous experimental researches.