Integral Calculations of Severe Accident Scenarios in PWRs and BWRs

This report presents the activity performed in the framework of LP1 (Sviluppo Competenze Scientifiche nel Campo della Sicurezza Nucleare), Objective B (Safety Assessment e Valutazioni D'impatto) task B2 (Valutazioni di Rischio e Scenari Incidentali) Topic 3 (Calcolo Integrale di Scenari Incidentali) of the PAR 2015, ADP ENEA-MiSE. Three different PWR accidents scenarios has been studied by using MELCOR code: a Loss of Feed Water (LFW) transient and two Large Break Loss of Coolant Accident (LBLOCA). To lead these three accidents to a Beyond Design Basis Accident (BDBA), they will be considered as unmitigated. The characterization of the thermal hydraulic behaviour, the in-vessel phenomena, the core degradation and corium behaviour in the lower head are here analyzed. In relation to the Boiling Water Reactor (BWR) activity a nodalization of the Fukushima Daiichi Unit 1 capable to reproduce the accident occurred have been developed. The Fukushima Daichii Unit 1 severe accident (consequence of the March 11, 2011 heartquake and tsunami) was also analyzed and compared with the plant data available with the aim to evaluate code accuracy for potential analogous analysis ragarding BWRs next to italian borders.