Developing probabilistic safety performance margins for unknown and underappreciated risks

Abstract

Probabilistic safety requirements currently formulated or proposed for space systems, nuclear reactor systems, nuclear weapon systems, and other types of systems that have a low-probability potential for high-consequence accidents depend on showing that the probability of such accidents is below a specified safety threshold or goal. Verification of compliance depends heavily upon synthetic modeling techniques such as PRA. To determine whether or not a system meets its probabilistic requirements, it is necessary to consider whether there are significant risks that are not fully considered in the PRA either because they are not known at the time or because their importance is not fully understood. The ultimate objective is to establish a reasonable margin to account for the difference between known risks and actual risks in attempting to validate compliance with a probabilistic safety threshold or goal. In this paper, we examine data accumulated over the past 60 years from the space program, from nuclear reactor experience, from aircraft systems, and from human reliability experience to formulate guidelines for estimating probabilistic margins to account for risks that are initially unknown or underappreciated. The formulation includes a review of the safety literature to identify the principal causes of such risks.

Design margins in industrial practice

“…Some industry sectors, such as in construction, add safety factors to the requirements, to deal with uncertainties arising during the building process. Safety margins and safety factors are critical in the certification and licensing of products (Benjamin, Dezfuli & Everett 2016). Tolerances can become critical as design margins in optimized and detailed designs, where so called stack-up effects can cause multiplication.…”

On the meaning of accuracy and precision in a risk analysis context

“…A large number of similar examples exist, where the terms 'accuracy' and 'precision' are used but are not explained. For more examples, see Gurcanli et al, 31 Mohammadfam and Zarei, 32 Benjamin et al, 33 Tian et al, 34 Al-Anbaria et al 35 and Ahmed et al 36 All these leave it somewhat open to the reader as to how to interpret the terms and what is the analyst's or authors' intended meaning. It is then likely that two readers, both skilled within the risk analysis discipline, may place a different meaning on the terms and draw different conclusions.…”