Risk quantification within the framework of risk studies

The purpose of the hazard study is to report on the operator's examination of the characterization, analysis, assessment, prevention and reduction of the risks associated with a facility or group of facilities located in a defined industrial, natural and human environment, which is as technologically feasible as it is economically acceptable, whether their causes are intrinsic to the substances or materials used, linked to the processes implemented in the facility, to the management of the establishment or due to the proximity of other risks of internal or external origin to the facility.

Article L. 512-1 of the French Environment Code specifies that "this study gives rise to a risk analysis which takes into account the probability of occurrence, the kinetics and the severity of accidents, according to a methodology which it specifies". As a reminder, this risk analysis is essential for facilities subject to authorization with servitudes.

To assess a risk, it is necessary to be able to estimate the two components of the probability/severity pair. Severity is usually estimated by modeling phenomena. This is the case, for example, for the atmospheric dispersion of toxic substances, fires and explosions. Today, estimating the probability of occurrence of industrial risks requires recourse to methodologies that have been used for many years in other fields, such as nuclear power and aeronautics.

In France, before the 2000s and in contrast to Anglo-Saxon countries, the approach to risk was deterministic, based mainly on expert opinion to estimate the probability of accident occurrence. The accident in Toulouse on September 21, 2001 marked a turning point for the French authorities, who are now advocating the use of probabilistic approaches to risk assessment in hazard studies.

To date, it is possible to conclude that the application of the probabilistic approach to risk analysis has had the effect of greatly complicating hazard studies, and of drifting dangerously towards the production of analyses that are difficult to understand for the uninitiated. This article therefore sets out to explain the various methods that can be used in a hazard analysis.