Abstract The Integrated Reliability and Risk Analysis System Personal Computer Version 2·0 computer code was used in a reliability analysis of the INEL Site Power System. This code was developed at the Idaho National Engineering Laboratory (INEL) as a state-of-the-art microcomputer-based probabilistic risk assessment (PRA) model development and analysis tool for the US Nuclear Regulatory Commission. IRRAS provided an efficient means for the development of a detailed and accurate fault tree model to: (a) identify the dominant causes of power outages on the site; (b) compare the performance of the system using both site-specific data and generic data; and (c) perform a sensitivity study to determine the effects of equipment unavailability caused by the ongoing preventative maintenance (PM) program. Although the final fault tree model exceeded the quantification capabilities of IRRAS, necessitating the use of the mainframe code SETS, IRRAS proved to be a valuable tool for both development of the fault tree model and preparation of the different inputs required to perform the analysis. The evaluation was performed to gain the understanding necessary to determine whether the system, under INEL Power Management control, required improvement of its reliability. This paper provides a detailed discussion of the methodology used for development of the fault tree model and evaluation of the reliability of the INEL Site Power System.