Risk-Based Engineering by Prabhakar V. Varde & Michael G. Pecht

Author:Prabhakar V. Varde & Michael G. Pecht
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

9.2 Treatment of Uncertainty: A Historical Perspective

Most of the cases evaluated as part of safety assessment employ deterministic models and methods. However, if we look at the assumptions, boundary conditions, factors of safety, data, and models of a traditional safety analysis, it can be argued that many of the elements are probabilistic. These elements or variables have qualitative notions for bounding situations and often provide comparative or relative aspects of two or more prepositions. To understand this point further, let us review the traditional safety analysis report(s) and take a fresh look at the broader aspect of this methodology. The major feature of the traditional safety analysis approach is based on the maximum credible accident, and for nuclear plants, these scenarios are mainly loss of coolant accidents (LOCAs) and loss of regulation accidents. It was assumed that the plant design should consider LOCAs and other scenarios, such as station blackout, to demonstrate that the plant is safe enough in terms of regulatory acceptance criteria. However, now the safety community considers a list of postulated initiating events ranging from anticipated, design basis, beyond design basis, and severe accident event conditions.

A reference to a typical safety analysis report or, to be precise, a deterministic analysis report will make it clear that there is an element of probability in a qualitative manner. These insights can include statements such as: (a) the possibility of two-out-of-three train failures is very low, (b) the possibility of a particular scenario involving multiple failures is very unlikely or low, or (c) the series of assumptions that certain conditions are assumed to be not probable or possible, such as failure of three components/systems in series particularly at a single instant. These assumptions or qualitative insights form the boundary conditions for the safety evaluation and show that uncertainty characterization employing qualitative probabilistic aspects is part of the deterministic methods. These aspects are qualitative in nature.

Keeping in view the above, and considerations of factors of safety in the design as part of the deterministic methods, brings out the fact that characterization of uncertainty is integral to safety. Integrated risk-based engineering (IRBE) seeks to (a) be more rational-based and not prescriptive, (b) remove over conservatism, (c) be holistic, (d) allow realistic safety margins, and (e) provide a framework for dynamic aspects of the accident scenario. Therefore, the role of uncertainty evaluation becomes integral to the IRBE approach. Concerning uncertainty characterization, IRBE requires that random phenomena are addressed through aleatory uncertainty and the model and data-related uncertainty are represented as epistemic uncertainty. There are a host of issues in risk analysis where handling of the uncertainty is more important than quantification of uncertainty. The complex issues related to human behavior through the subjective treatment of cognition, consciousness, and conscience are summed up as ethical modeling to provide a holistic approach. These rather qualitative aspects of uncertainty or cognitive uncertainty need to be addressed by having safety provisions in the plant. For application of the integrated risk-based framework, it is vital to address these

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