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); wherein said event tree (ET) is a binary tree; said event nodes comprise: internal nodes, which represent mitigating or aggravating events in said complex super system, and leaf nodes, which represent failure of said complex super system, non-failure of said complex super system, or non-occurring conditions in said complex super system; a Static Fault Tree (SFT) is a structured graph of an SFT sub system of events of said complex super system; said Static Fault Trees (SFT) top events represent a failure in a component corresponding to an event of said complex super system, and said Static Fault Trees (SFT) comprise one or more SFT child events, wherein said SFT child events are sub events of said complex super system and wherein said one or more SFT child events are connected by ordinary Boolean AND, OR and/or N/K gates; a Dynamic Fault Tree (DFT) is a structured graph of a DFT sub system of events of said complex super system, said Dynamic Fault Trees (DFT) top events represent a failure in a component corresponding to an event of said complex super system, and said Dynamic Fault Trees (DFT) comprise one or more DFT child events, wherein said DFT child events are sub events of said complex super system and wherein said one or more DFT child events are connected by gates comprising: Functional Dependency Gates (FDEP gates), wherein said FDEP gates model situations where one component's correct operation is dependent upon the correct operation of some other component, Spare Gates (SP gates), wherein said SP gates model cold, warm and hot pooled spares, and Priority AND (PAND) gates wherein said PAND gates model ordered ANDing of events; calculation of the probability of one or more pivot nodes of said complex super system that are top events of SFTs which have no sub events of said complex super system in common comprises inclusion/exclusion probability analysis or Binary Decision Diagram (BDD) analysis; and calculation of the probability of two or more pivot nodes of said complex super system that are top events of SFTs which have shared sub events of said complex super system comprises ANDING said SFT pivot nodes along the path and applying inclusion/exclusion probability analysis or BDD analysis.