Sensitivity analysis of atmospheric dispersion modeling in emergency situation

Overview & Synopsis

Each day, accidents involving hazardous materials are managed by Emergency Services. In order to bring the best adapted operational answer, it is necessary to determine with accuracy the concentrations of the gas to which people will be exposed. The expert in charge of modeling is facing a major difficulty: few (or no) information. Therefore, for a given situation, the generation of different effect distances is possible. A sensitivity analysis was carried out in order to determine the most influential parameters on the estimation of safety distances. The goal is to identify the parameters for which it is necessary to pay a very detailed attention. The first step of the study was to define the system to be analyzed. The second step consisted in working out a complete model of evaluation of safety distances in emergency situation. This was done by combining an existing dispersion model (SLAB) with a specifically developed application for the calculation of input parameters. The input data are operational data which can be collected from the accident site and the output data are safety distances. The third step consisted in selecting two test cases representative of an accident situation involving hazardous chemicals (leak on a wagon of ammonia and a wagon of propane). After that, it was characterizing the range of possible values of the whole input parameters. A fourth step consisted in applying two methods of sensitivity analysis: a screening method (MORRIS's method) and a local sensitivity analysis. Finally, the application of these methods, on the same case study, made it possible to highlight the points of convergence of the methods and their advantages. Finally, the parameters for which it is useless / useful to initiate considerable efforts to recover a reliable value were highlighted.