A method of integrating traditional thermal-hydraulic (TH) analysis with probabilistic assessment (PA) (called the TH-PA method) has been developed. This method allows for an exhaustive search through a set of individually developed but subsequently linked logic models to screen and identify accident scenarios. The logic models consist of a probabilistic risk assessment (PRA) used for probabilistic screening purpose and an ensemble of integrated behavior logic diagrams (IBLDs). The PRA model represents the functional/logical relationships of the components and accident scenarios, the same way as is modeled in the conventional PRAs. The IBLDs hierarchically represent system interactions/dependencies due to TH phenomena and human actions. This hierarchy also shows causal factors and consequences of plant states, and identifies induced system failures. The TH-PA method relies on two types of scenario screening: probabilistic screening (PA screening) and TH screening. The PA screening eliminates scenarios with low frequencies (e.g., <10-10/reactor-yr). The traditional frequency-based screening method used in the PRAs has been adopted for PA screening. The TH screening eliminates scenarios that do not expect to result in core uncovery. For the TH screening, a simple accident trajectory approach has been devised. A trajectory represents the collapsed liquid volume fraction in the reactor primary system as a function of primary pressure. The trajectories are based on simple mass and energy conservation equations (if the TH-PA method is applied to a system where mechanical energy transfer is important, momentum conservation should also be considered). The roles of each plant system are then identified by indicating whether the system is a "source" or a "sink" for mass and energy at a given time during accident progression. Based on an input set that represents the plant system failures and the stage of the transient, the accident trajectory is developed. The accident trajectory allows for the evaluation of safety significance of scenarios. The trajectory also determines whether the core becomes uncovered, should the input conditions (i.e., conditions described by the input set) remain unchanged.