Applying risk models to formulation of safety cases

Importance measures have been useful in the process of extracting insights from risk analyses. Importance measures have also been suggested for use in component classification. However, there are difficulties of interpretation associated with component classification based on importance measures. This paper briefly reviews an alternative method, "Top Event Prevention Analysis," and, based on a simple example, compares its key characteristics to those of conventional applications of importance measures. The methods are compared with respect to the task of formulating a safety case for a complex and potentially hazardous facility, in which component classification plays an important role. A key subtask is identification of a collection of design elements that is necessary and sufficient to achieve the desired level of protection of the public, the workers, and the environment. At the design stage, identifying this set helps to determine what elements to include in the final design. Separately, a similar selection process could be used in order to justify limiting the scope of regulatory oversight to a subset of design elements, on which a safety case is to be based. This step could be taken during initial review of a design, or later as part of an effort to justify relief from regulatory requirements that are burdensome but provide little actual risk reduction.