Study on cladding oxidation model improvement in SCDAP/RELAP5

At present, the parabolic rate correlation oxidation model is used in SCDAP/RELAP5 code for simulating cladding oxidation during reflooding in the early phase of severe accident, but it can not simulate oxidation well when the cladding temperature is high or the steam flowrate of cladding surface is small. Besides, this model does not analyze the oxygen distribution in the cladding and can not describe the failure of cladding suitably. In this study, those differences and relationships between parabolic rate model and diffusion model are analyzed, and the diffusion oxidation model is employed in SCDAP/RELAP5. Then, the effect of those two models on reflooding in the early phase of severe accident is investigated. It is shown that the diffusion model can simulate the reflooding phenomena better in the early phase of severe accident, and the parabolic rate model is a simplified form of diffusion model based on some specified assumptions.