5.40. Two main methods have been used for constructing scenarios. The method described, for example, in the ISAM project [26] may be described as a ‘bottom- up’ method and is based on screening of features, events and processes. When using this method, a comprehensive list of features, events and processes should be developed as a starting point. This may involve the use of generic lists of features, events and processes (internationally agreed lists, regulations, etc.) and the determining of site specific and system specific features, events and processes. This is followed by a screening process to exclude features, events and processes from further consideration that would have either a very small impact on the disposal system or a very low probability of occurrence. For the relevant features, events and processes, a thorough examination of interactions between them and their combination in suitable scenarios is performed. The process used for development of scenarios should be fully documented and justified. Criteria for screening features, events and processes may include rules relating to regulations and/or to the probability of occurrence or consequences of events and processes.