Predicting the consequences of a NaTech event: occupational short-term inhalation risk supported by advanced pollutant dispersion modelling.

Abstract

In this work, a methodology for the preventive assessment of the risk associated with the accidental inhalation of toxic substances at an industrial site is presented. The methodology is based on a NaTech (Natural-Hazard Triggered Technological Accidents) sequence modelling: event and site characterisation; simulation of the accidental release and pollutant dispersion; and calculation of short-term risk, by averaging concentrations and comparing them with the reference values proposed by the main occupational exposure organizations worldwide. The proposed model is applied to a case study of a chemical company in central Italy. A hypothetical vessel failure leads to a pool release, evaporation, and dispersion of tetrahydrofuran. A pool evaporation model is applied, and the Lagrangian particle model Parallel Micro-Swift Spray (PMSS) is used for dispersion modelling. The resulting concentration fields show that the pollutant magnitude and distribution can vary depending on wind speed and direction and atmospheric conditions. Concentrations decrease rapidly with distance from the source, both horizontally and vertically. The maximum modelled concentration of 596 mg m-3 is recorded at the emission source. Inhalation hazard quotient (HQ) is calculated against the threshold limit value-Short Term Exposure Limit (TLV-STEL = 295 mg m-3). HQ exceedances are reported at the emission source. No exceeding is reported away from the source. The uncertainty on the calculated risk arises from considerations on modelling choices, threshold limit values, and the correction method for short-term concentration averaging. For the proposed general methodology, the presented model can be applied with relatively limited calculation resources and practical applicability.