In this paper the basis of the energy flow/barrier analysis are used to define an accident as impact of a hazardous agent to a target, through failure of control and protective barriers. This definition is enhanced for comprising the serial and parallel barriers and distinguishing the energy barriers from target barriers. The barrier attributes are defined and are used to create a qualitative scenario-building model. The probability and severity of various accidents are estimated, by considering barriers reliability and efficiency. This approach is used to develop and simulate the accident scenarios and calculating their consequences. This model can be used for analyzing the risk and estimating the barriers importance, in complex systems
