4.4.1 Sequences and scenarios
When specific events in a specific plant are discussed, specific accident sequences are defined. A collection of accident sequences can have some features in common for instance, type of plant, type of accident initiator, type of containment structure. Such collections of sequences are referred to as accident scenarios. Because of similarities between scenarios, they are useful in this Handbook, for e.g. source term predictions. Some scenario features strongly affect the source term. In Table 4.1 are mapped out those scenario features at Ignalina NPP that most strongly affect source terms and consequently should be recognized when planning source term studies or making source term predictions.
4.4.2 Scenario map
The scenario map, Table 4.1, recognizes the plant states, core states and containment states as main features which are further divided into sub-features according to the table. The severity of an accident can in retrospect be compared with the INES scale [8] which recognizes seven classes of accidents from the mildest to the most severe ones. The INES scale is suggested to be used here in this Handbook also as a prognostic tool to differentiate scenarios according to the size of their release potential.