In current practice, success criteria analyses for Probabilistic Safety Assessments (PSAs) primarily use thermal-hydraulic simulation (transient analysis) codes. In dynamic event tree (DET) simulations, a stochastic model is coupled to such codes. The stochastic model allows the variability of system failures (number of trains, timing) and of operator responses (response strategies, timing of actions) to be considered. Consequently, DET simulations provide the means to examine the combined influence of such variabilities on success criteria. This paper presents initial results from DET analyses performed for Medium Loss of Coolant Accident (MLOCA) scenarios in a Pressurized Water Reactor (PWR). The analyses focus in particular on the interaction of break size, number of high pressure safety injection trains, and the timing and rate of primary cooldown and depressurization over the secondary, in terms of their impacts on sequence success.