The consequences of accidents involving hazardous materials

Considerable quantities of toxic industrial chemicals (TICs) are produced and stored in the chemical and process industries, and transported by rail, road or sea. The consequences of potential chemical accidents can be estimated by a dispersion model developed at the Finnish Meteorological Institute, called ESCAPE.

Accidental releases of TICs may potentially cause significant hazard to both human health and environment, in some cases even far away from the release point. Such releases may be caused, for example, due to a process failure, accident or terrorist attack. Reliable methods are therefore needed for estimating the release rates and the atmospheric dispersion of TICs. The methods can be used, for example, in the emergency contingency planning, in preparing for conceivable accidents, and in the safety analyses of industrial installations.

The ESCAPE model can be used for analysing the consequences of accidents

Consequences of chemical accidents can be estimated by a modelling system that contains discharge models, source term models, dispersion models and impact analysis models, developed at the Finnish Meteorological Institute. The dispersion model ESCAPE (Expert System for Consequence Analysis and Preparing for Emergencies) is a validated assessment tool for the consequence analysis of accidents involving hazardous materials. The mathematical structure and evaluation of the model has been described by Kukkonen et al. (2017).

The model can be used for evaluating releases, the formation of source terms, atmospheric dispersion and consequences of hazardous materials. The model is applicable to evaluate both continuous and instantaneous ground-level releases of toxic and flammable gases into the atmosphere.

Toxic and flammable chemicals are often stored either in liquid form under pressure at near ambient temperature, or liquefied by cooling the chemical to or below its boiling point. The model can be utilized in release cases, which are a consequence of a sudden rupture (catastrophic failure) of a container or the rupture of a pipe or container wall, either from the liquefied or gaseous state of a refrigerated or pressurized container. An aerosol or gaseous plume or puff will be formed, and in certain conditions, if the release scenario results in the formation of a liquid pool, its evaporation may produce either a gas puff or plume. A potential predicted concentration distribution near ground level has been illustrated in the example figure.

The predicted ground level concentration of chlorine for a leak from a pipe rupture, due to an assumed railway container accident in downtown Helsinki, Finland (map © National Land Survey of Finland). The location of the accident is shown with a red star on the map. The result was computed using the ESCAPE model.

An operational version of the ESCAPE model can be used by the rescue authorities

We have also developed an operational version of the ESCAPE model, which is based on web-browser technology. This operational model application has been developed for the needs of the Finnish emergency authorities, including the rescue services. The model is used via a graphical user interface. The aim is to provide estimates for emergency response personnel of the nature and spatial extent of hazards associated with chemical spills.

The software has been designed to utilise in real-time the meteorological data produced by numerical weather prediction (NWP) models. Automatic utilization of, for instance, NWP-model products simplifies the input data, which a user has to provide, and this will result in more reliable and accurate predictions. The user needs to specify only input data regarding the characteristics of the release and the environment (the type of release and its associated details, the location and time of the accident, and the contaminant).

Limitations of the model

The model is not designed for evaluating the dispersion of buoyant plumes originating from, for example, major fires. Another model (BUOYANT) developed at the FMI can be used to estimate the dispersion of pollutants emitted from warehouse and wild-land fires. The ESCAPE model is also limited in its ability to account for the detailed effects associated with complex terrain, buildings and other obstacles. Further, the detailed description of time-varying releases is outside of the scope of the model.

Model evaluation and validation

The ESCAPE model has been scientifically evaluated, for example, within the EU-project SMEDIS (Scientific Model Evaluation of Dense Gas Dispersion Models; Carissimo et al., 2001). Comparison of model predictions against measured experimental data from six field campaigns indicates a fairly good or excellent agreement between predictions and measurements.

The predictions of the ESCAPE and 16 other internationally widely-used dense gas dispersion models have recently been compared against a series of major field experiments in 2015 and 2016, conducted in the United States. The trials are called ‘Jack Rabbit II’ field experiments. The ESCAPE model performed excellently, both with respect to the measured data and compared to the other models (Mazzola et al., 2021).

Literature

Carissimo, B., Jagger, S.F., Daish, N.C., Halford, A., Selmer-Olsen, S., Riikonen, K., Perroux, J.M., Würtz, J., Bartzis, J., Duijm, N.J., Ham, K., Schatzmann, M. and Hall, D.-R., 2001. The SMEDIS database and validation exercise. Int. J. Environ. Pollut. 16, pp. 614-629, doi:10.1504/IJEP.2001.000654.

Kukkonen, J., Nikmo, J. and Riikonen, K., 2017. An improved version of the consequence analysis model for chemical emergencies, ESCAPE. Atmos. Environ. 150, pp. 198-209, doi:10.1016/j.atmosenv.2016.11.050.

Kukkonen, J. 1990. Modelling source terms for the atmospheric dispersion of hazardous substances, Commentationes Physico-Mathematicae 115, Dissertationes No. 34, The Finnish Society of Sciences and Letters, Helsinki, 111 p. + app.

Mazzola, T., Hanna, S., Chang, J., Bradley, S., Meris, R., Simpson, S., Miner, S., Gant, S., Weil, J., Harper, M., Nikmo, J., Kukkonen, J., Lacome, J.-M., Nibart, M., Björnham, O., Khajehnajafi, S., Habib, K., Armand, P., Bauer, T., Fabbri, L., Spicer, T. and Ek, N., 2021. Results of comparisons of the predictions of 17 dense gas dispersion models with observations from the Jack Rabbit II chlorine field experiment. Atmos. Environ. 244, 117887, doi:10.1016/j.atmosenv.2020.117887.

Nikmo, J., Kukkonen, J. and Riikonen, K., 2002. A model for evaluating physico-chemical substance properties required by consequence analysis models. J. Hazard. Mater. A91, pp. 43-61, doi:10.1016/S0304-3894(01)00379-X.

Riikonen, K., Nikmo, J. and Kukkonen, J., 1999. The extension of a consequence analysis model to include liquid pool vaporisation. Finnish Meteorological Institute, Publications on Air Quality 29. Helsinki, 22 p.

Riikonen, K., Nikmo, J. and Kukkonen, J., 2002. The extension of a consequence analysis modelling system to allow for continuous vapour release, gas cloud explosion and plume rise. Finnish Meteorological Institute, Publications on Air Quality 32. Helsinki, 40 p.

Webber, D.M., Jones, S.J., Tickle, G.A. and Wren, T., 1992a. A model of a dispersing dense gas cloud, and the computer implementation D*R*I*F*T: I. Near-instantaneous releases. AEA Report SRD/HSE R586, 89 p.

Webber, D.M., Jones, S.J., Tickle, G.A. and Wren, T., 1992b. A model of a dispersing dense gas cloud, and the computer implementation D*R*I*F*T: II. Steady continuous releases. AEA Report SRD/HSE R587, 101 p.