Development of fuel performance code FEMAXI-6 and analysis of mechanical loading on cladding during power ramp for high burn-up fuel rod

A fuel performance code FEMAXI-6 has been developed for the analysis of LWR fuel rod behaviors in normal operation and transient (not accident) conditions. The code uses FEM for mechanical analysis, and has incorporated thermal and mechanical models of phenomena anticipated in high burn-up fuel rods, such as fuel thermal conductivity degradation and pellet-clad bonding. In the present study, PCMI induced by swelling in a high burn-up BWR type fuel rod has been analyzed by the FEMAXI-6 code. During a power ramp for the high burn-up fuel, instantaneous pellet swelling can significantly exceed the level that is predicted by a 'steady-rate' swelling model, causing a large circumferential strain in cladding. This phenomenon has been simulated by a new swelling model to take into account the fission gas bubble growth, and we found that the new model can give satisfactory predictions on cladding diametral expansion in comparison with measurements in test rod. The bubble growth model assumes an equilibrium between bubble size and external pressure on the bubble, and simultaneous solution is obtained with both bubble size determination and diffusion equation of fission gas atoms. In addition, a pellet-clad bonding model which assumes firm mechanical coupling between pellet outer surface and cladding inner surface predicts an elevated tensile stress in the axial direction of cladding during ramp, indicating the generation of bi-axial stress state in the cladding. These analyses by the FEMAXI-6 code enable us to predict the magnitude of mechanical loading on cladding during transient and also serve for failure investigation. Clearly, prediction by code calculation depends on the creep and stress-strain properties of highly irradiated cladding. (author)