Human factors issues in qualitative and quantitative safety analyses

Humans are a critical and integral part of any operational system, be it a nuclear reactor, a facility for assembly or disassembling hazardous components, or a transportation network. In our concern over the safety of these systems, we often focus our attention on the hardware engineering components of such systems. However, experience has repeatedly demonstrated that it is often the human component that is the primary determinant of overall system safety. Both the nuclear reactor accidents at Chernobyl and Three Mile Island and shipping disasters such as the Exxon Valdez and the Herald of Free Enterprise accidents are attributable to human error. Concern over human contributions to system safety prompts us to include reviews of human factors issues in our safety analyses. In the conduct of Probabilistic Risk Assessments (PRAs), human factors issues are addressed using a quantitative method called Human Reliability Analysis (HRA). HRAs typically begin with the identification of potential sources of human error in accident sequences of interest. Human error analysis often employs plant and/or procedures walk-downs in which the analyst considers the ``goodness`` of procedures, training, and human-machine interfaces concerning their potential contribution to human error. Interviews with expert task performers may also be conducted. In the application of HRA, once candidate sources of human error have been identified, error probabilities are developed.