AbstractImportance categorization and hazard identification methods have been proposed for a fusion experimental reactor. A parameter, the system index, is introduced in the categorization method. The relative importance of systems with safety functions can be classified by the largeness of the system index and whether or not the system acts as a boundary for radioactive materials. This categorization can be used as the basic principle in determining structure design assessment, seismic design criteria etc. For the hazard identification the system time energy matrix is proposed, where the time and spatial distributions of hazard energies are used. This approach is formulated more systematically than an ad-hoc identification of hazard events and it is useful to select design basis events which are employed in the assessment of safety designs.