Analysis on split failure of cladding of high burnup BWR rods in reactivity-initiated accident conditions by RANNS code

摘要

A computer code RANNS was developed to analyze fuel rod behaviors in the reactivity-initiated accident (RIA) conditions. RANNS performs thermal and finite-element mechanical calculation for a single rod in axis-symmetric geometry, where fuel pellet consists of 36 equal-volume ring elements and cladding metallic wall consists of eight equal-thickness ring elements and one outer oxide element. The code can calculate temperature profile inside the rod, contact pressure generated by pellet-clad mechanical interaction (PCMI), stress-strain distribution and their interactions elaborately. An experimental analysis by RANNS begins with pre-test conditions of irradiated rod which are given by the fuel performance code FEMAXI-6. In the present study, analysis was performed on the simulated RIA experiments in the "nuclear safety research reactor" (NSRR), FK-10 and FK-12, with high burnup BWR rods in a cold-start up condition, and stress-strain evolution in the PCMI process was calculated extensively. In the analysis, the pellet-clad bonding was assumed both in the heat conduction and in mechanical restraint. The calculated hoop strain increase was compared with the measured strain gauge data, and satisfactory agreement was obtained. Simulation calculations with broader power pulses anticipated in RIA of commercial BWR were carried out and the resulted cladding hoop stress was compared with the failure stress estimated by comparison of analysis with experimental data.