| Probabilistic Risk Assessment (PRA) is a methodology consisting of techniques to assess the probability of failure or success of a system. In many modern technological systems, especially safety critical systems such as space systems, nuclear power plants, medical devices, defense systems, etc, PRA has been proven to be a systematic, logical, and comprehensive methodology for risk assessment, for the purpose of increasing safety in design, operation and upgrade, and for reducing the costs in design, manufacturing, assembly and operation.; Software plays an increasing role in modern safety critical systems. A significant number of failures can be attributed to software failures such as the well-known Therac-25 radiation overdose accidents, the Mars Climate Orbiter, Mariner I Venus Probe and Ariane 5 accidents. Unfortunately current PRA practice ignores the contributions of software due to a lack of understanding of the software failure phenomena. The objective of our research is to develop a methodology to account for the impact of software on system failure that can be used in the classical PRA analysis process.; To develop the methodology, a systematic integration approach is studied and defined. Next, a taxonomy of software-related failure modes is established and validated. The software representation in fault trees and event trees is defined. A test-based approach for modeling and quantifying the software contribution is presented. A Case study is provided to validate the framework.; This study is the first systematic effort to integrate software risk contributions into PRA. |
