Updated Peach Bottom Model for MELCOR 1.8.6: Description and Comparisons

A MELCOR 1.8.5 model of the Peach Bottom Unit 2 or 3 has been updated for MELCOR 1.8.6. Primarily, this update involved modification of the lower head modeling. Three additional updates were also performed. First, a finer nodalization of the containment wet well was employed. Second, the pressure differential used by the logic controlling the safety relief valve actuation was modified. Finally, an additional stochastic failure mechanism for the safety relief valves was added. Simulation results from models with and without the modifications were compared. All the analysis was performed by comparing key figures of merit from simulations of a long-term station blackout scenario. This report describes the model changes and the results of the comparisons.