1. A method for calculating Probabilistic Risk Assessment (PRA) of a complex super system through the use of an event tree (ET) having one or more individual paths and whose event nodes are top events of Static Fault Trees (SFT) and Dynamic Fault Trees (DFT), said method comprising: calculating the probability of various paths of said event tree (ET) by calculating the probability of various event nodes of the individual paths of said event tree (ET).