A robust thermal model to investigate radial propagation of core damage due to total instantaneous blockage in SFR fuel subassembly

Abstract A robust enthalpy based thermal model has been developed to investigate the sequence of various phenomena that take place during total instantaneous blockage (TIB) in a fuel subassembly (SA) of a sodium cooled fast reactor. The (liquid–solid) interface movement in the multiphase and multi-material heat transfer is predicted by the Voller’s algorithm employing a dynamic node deletion procedure for realistic simulation of density difference between immiscible fuel and steel. Employing this model, the scenario of TIB in the fuel SA of Indian Prototype Fast Breeder Reactor (PFBR) has been analyzed. The focus of the investigation has been, (i) detection of the event by online monitoring of sodium outlet temperature from the neighboring subassembly and (ii) determination of number of subassemblies that are likely to get damaged severely before reactor shut down, which is an important parameter to define thermal load on core-catcher. Detailed parametric studies have been performed to bring out the effects of various parameters that influence inter-subassembly heat transfer and damage propagation. The parameters include, subassembly power (and hence reactor power), hexcan wall thickness, thermal conductivity of hexcan material and thermocouple time constant. It is found that during nominal power condition of the reactor, thermal damage propagates to only one row of SA suggesting that the thermal load on core-catcher during the event of TIB in one fuel SA can be that corresponding to decay power of seven SA. Damage of the blocked SA is completed within ∼20 s. Reactor SCRAM from neighboring SA thermocouple takes place at 55 s after the TIB and the residual thickness of hexcan wall at the time of reactor SCRAM is 55%. At low power conditions of the reactor (and hence at low SA power ratings) detection capability improves and the residual thickness of neighboring hexcan during reactor SCRAM increases. Thicker hexcan does not enhance detection. Increase in thermal conductivity of hexcan material enhances early detection of the event and improves the residual thickness. Reduction in time constant of thermocouple does not significantly help in early detection of TIB event. The results predicted for PFBR fuel SA are found to agree qualitatively with that reported for other reactors.