Comparison of results from some chemical dispersion models and hazard prediction and assessment tools

Dispersion models and hazard prediction and assessment software tools are used to assess consequences from dispersion of hazardous materials, such as toxic chemicals. Such tools can be employed during and after an event to support crisis and consequence management, or to assess hypothetical scenarios for emergency preparedness planning, training and exercises. The software HPAC, ARGOS, DEGADIS, NBC-Analysis, ERGO and “Farlig gods” are compared by performing calculations of the dispersions and predicted hazard areas for the three scenarios: rupture of a tank containing 20 tonnes of pressurized liquefied chlorine, rupture of a tank containing 10 tonnes of pressurized liquefied ammonia, and finally an attack with soman by bomber aircrafts. For the first two scenarios, several meteorological conditions are considered. The motivations of this study are: to investigate the required input parameters and exemplify possible output of the various tools; to outline the assumptions and limitations of the programs; and to discuss the user friendliness and the pre-required user knowledge and competence. The main conclusions are:  The box model for heavy gases (DEGADIS) and the Gaussian dispersion models which are included in two decision support systems (HPAC and ARGOS) give large variations in calculated plume prediction patterns for the three scenarios. Not all programs are suited for all scenarios. Hence, decisions based solely on the use of one of these programs can lead to serious misjudgements. It is important to have several models available and to know which model to employ for a given scenario. FFI will continue to test and evaluate these and possible other models for other scenarios.  A fairly high user competence level is required for HPAC, ARGOS and DEGADIS.  When HPAC or ARGOS is used in an operation or in a crisis situation, a real-time connection to a meteorological service is highly desirable.  The “Farlig gods” program and ERGO are both simple to use, even for inexperienced users. They only give areas where protective actions should be considered. NBC-Analysis gives no safety distances based on quantitative hazard levels, but produces an area which is considered unsafe to enter.  The inter-comparison of results in this study is purely based on a relative comparison since the true dispersion patterns for selected scenarios are not known. There is an urgent need for additional experimental data in order to have data sets for model validation and improvements. FFI is currently pursuing this goal through international collaborations and project initiatives.