The assessment of major accidents caused by external events

The analysis of accident databases evidences that several of the major accidents recorded were caused by “external events” as domino effects from adjacent plants, or natural events as earthquakes or floods. More recently, high public concerns were raised among the possibility of external attacks to industrial facilities aimed to trigger severe accidental scenarios. In this framework, the assessment of equipment vulnerability to external impacts is a key issue for a correct identification of all accidental scenarios and a “global” quantitative risk assessment. To this aim, we have assumed that the impact should be classified and identified by means of only two criteria: i) the probability of occurrence; ii) the intensity of impact, as characterised by a single degree of freedom, for any accidental scenario. In the paper, this second point only has been developed. More specifically, radiation, overpressure and fragment impact were considered as variables to describe and classify respectively fires, vapour cloud explosions and catastrophic equipment failure when domino effects are of concern. On the other hand, peak ground acceleration was considered for the evaluation of equipment fragility with respect to seismic events. A slightly different approach was developed for external acts of interference, as terrorist attacks. In this case, models for the vulnerability of equipment and of the available layers of protection were introduced to understand the potential effects of the credible modes of external attack identified in SVA procedures.