The complexity of industrial systems, the use of dangerous chemical substances have dreadfully increased the potential accident destruction especially in the field of chemical industry. The technical analysis f accident scenarios are essential not only in learning lessons from unfortunate events in the chemical industry but also in preventing the occurrence of such events in the future. In this paper we propose an approach for analyzing the scenarios of an accident related to a chemical process, we consider the case of a chemical reactor for the application of this approach. This approach is based on four steps, risk analysis, study of top event, accident scenarios, and study of system evolution.