On the proper formulation of safety goals and assessment of safety margins for rare and high-consequence hazards

The issue of ‘uncertainty’ is addressed in the special context of assessing and managing risks from rare, high-consequence hazards. It is suggested that, rather than the usual ‘formal treatments’ on how to combine expert opinions that diverge widely, such ‘uncertainty’ must be approached in each case as a research question that encompasses frame of assessment, approach methodology, risk management, and safety goals, with the aim of obtaining resolution in a clear, consistent, and complete manner. This, together with some basic considerations on ‘defense-in-depth,’ and certain practical aspects of communications and synergism needed for resolution (of such uncertainties), leads us to the Risk Oriented Accident Analysis Methodology (ROAAM). The purpose of this paper is to explain these views, to follow them through to the definition of the methodology and its implementation, and to indicate some of the insights gained through the several practical applications available so far.