QRAS – the quantitative risk assessment system

This paper presents an overview of QRAS, the Quantitative Risk Assessment System. QRAS is a PC-based software tool for conducting Probabilistic Risk Assessments (PRAs), which was developed to address risk analysis needs held by NASA. QRAS is, however, applicable in a wide range of industries. The philosophy behind the development of QRAS is to bridge communication and skill gaps between managers, engineers, and risk analysts by using representations of the risk model and analysis results that are easy to comprehend by each of those groups. For that purpose, event sequence diagrams (ESD) are being used as a replacement for event trees (ET) to model scenarios, and the quantification of events is possible through a set of quantification models familiar to engineers. An automated common cause failure (CCF) modeling tool further aids the risk modeling. QRAS applies BDD-based algorithms for the accurate and efficient computation of risk results. The paper presents QRAS' modeling and analysis capabilities. The performance of the underlying BDD algorithm is assessed and compared to that of another PRA software tool, using a case study extracted from the International Space Station PRA.