Application of a two-dimensional ballooning model to out-pile and in-pile simulation experiments

The FRETA-B code which has thermal and mechanical models for the fuel behavior during a postulated Loss of Coolant Accident was applied to out-pile tube ballooning experiments and an in-pile integral simulation experiment MT-1 in the NRU reactor. From the analysis of the out-pile experiments, the ballooning model based on the thin-shell model together with an empirical rupture criterion was found effective in predicting the cladding deformation kinetics and the average strain left after rod rupture. The mechanical analysis of the MT-1 experiment was based on the calculation of the temperature history and its transverse distribution under reflooding which were consistent with the limited experimental data. The analysis indicated that the restriction of flow channel blockage to 70% in MT-1 was a result of an azimuthal cladding temperature difference of up to 60 K due mainly to eccentric relocation of pellet fragments. (orig.)