Mechanistic model for the fragmentation of the high-burnup structure during LOCA

A model was developed to account for the fragmentation of the high-burnup structure in fuel pellets during a loss-of-coolant accident. The basic assumptions of the model are that the pores have reached the dislocation punching overpressure during base irradiation and that the increasing overpressure due to rising temperature causes fuel fracturing. The distribution of the pore sizes is taken into account, not only the mean value. The model predicts the existence of a minimum pore size in normal operation, the decrease of threshold burnup for fuel fragmentation when the last-cycle power rating of the rod is high and a reduced fragmentation when a strong PCMI restraint is present during the transient. It can reproduce experimental results in terms of temperature range for fragmentation.