Multiple Risk Modeling
Introduction
Probabilistic risk assessment (PRA) is a systematic and comprehensive methodology for assessing the risk associated with a complex engineering entity (such as a commercial airliner or a nuclear power plant).
Risk in a PRA is defined as a feasible detrimental outcome of an activity or action. In a PRA, the risk is characterized by two figures:.
-
- the magnitude (severity) of the possible adverse consequences, and.
-
- the possibility (probability) of the occurrence of each consequence.
The consequences are expressed numerically (for example, the number of people who would potentially be harmed or killed) and the possibilities of occurrence expressed as probabilities or frequencies (for example: the number of occurrences per unit of time). Total risk is the expected loss: the sum of the products of the consequences multiplied by their probabilities.
The spectrum of risks across event classes are also of interest, and are usually controlled in licensing processes – it would be of interest but rare if high consequence events were found to dominate the total risk, particularly as this risk assessment is very sensitive to assumptions (e.g. How rare is a high consequence event?).
Probabilistic risk assessment usually answers three basic questions:
-
- What can happen wrongly with the technological entity studied?, o What are the initiators or initiating events (undesirable initial events) that lead to adverse consequences?
-
- What and how severe are the potential detriments or adverse consequences to which the technological entity may eventually be subjected as a result of the occurrence of the initiator?
-
- How likely is it that these undesirable consequences will occur? Or what are their probabilities or frequencies?
Two common methods of answering this last question are Event Tree Analysis and Fault Tree Analysis - for an explanation of these, see safety engineering.
In addition to the two methods mentioned above, PRA studies require special but often very important analysis tools such as human reliability analysis (HRA) and common-cause-failure analysis (CCF). The HRA has to do with methods to model human error while the CCF has to do with the methods to evaluate the effect of intersystem and intrasystem dependencies that tend to cause simultaneous failures and thus significantly increase the total risk.