Assessment of PWR cores behavior and estimate of confidence level got with the use of mechanistic and / or integral codes calculations of accident sequences with reference to TMI-2 reactor

This report presents the activity performed in the frame of LP1, Objective B (Valutazione del comportamento di noccioli PWR e del livello di confidenza dei risultati ottenuti con l’uso di codici meccanicistici e/o integrali mediante il calcolo di sequenze incidentali in reattori PWR con riferimento al reattore della centrale di TMI-2), Task B1.2 of PAR2014, ADP ENEA-MSE. In this study the integral codes ASTEC and MELCOR have been applied for the calculation of a reference severe accident sequence (SBLOCA) and some core reflooding scenarios on the TMI-2 plant (PWR of 900 MWe). Similar core degradation parameters/criteria have been used with the two codes to try to reduce the differences observed in previous analyses (PAR2013). Furthermore, sensitivity studies on important and uncertain core degradation parameters/criteria have been performed to investigate their influence on code results. Core reflooding scenarios starting from different core degradation conditions have highlighted the ability of emergency core cooling systems to stop the core degradation and prevent vessel failure in the late phase of a severe accident.