After the accidents in TMI-2 and Chernobyl, the study of phenomena, reactor system, and containment behaviour for sequences with core melts became a major research area. Compared to phenomena under DBA conditions, the phenomena that are present during accident sequences with core melt are very complex and, evidently, much less understood. The main phenomena influencing containment behaviour are

— the release, mixing and burning of hydrogen,

— the release of volatile fission products and its decrease due to condensation and sedimentation processes,

— the transport of solid or liquid fuel (melt) into the containment and its interaction with concrete and steel structures and with the atmosphere or water.

While loads on the containment wall or structures resulting from DBAs stay within design limits and are well below loads threatening the containment integrity, this is not the case for loads resulting from severe accidents. For these the containment integrity may be threatened or even lost.

A useful overview about severe accidents can be taken from [6]:

— Although severe accidents do not belong to design basis accidents, the knowledge about the loads resulting from these sequences are needed for probabilistic safety assessments and also for the design of advanced LWRs which might take core melt sequences into account in the design process;

— The modelling of phenomena related to severe accidents should be as realistic as possible to avoid unbalanced designs or unrealistic predictions of loads on the containment resulting in unrealistic predictions of doses outside the plant;

— The modelling of phenomena related to severe accidents can only be done with relatively large uncertainty bands due to the limited experimental basis but also due to the uncertainty in predicting reliably the accident progression;

— For the calculation of phenomena and system behaviour the major lumped parameter codes as CONTAIN, MELCOR and COCOSYS in addition to many codes for special purposes(e. g. melt/concrete interaction) are used. For the assessments which need a very detailed simulation of geometrical structures (e. g. hydrogen accumulation, deflagration to detonation-transition and burns) CFD-codes are used with increasing frequency. Nevertheless, validation and model improvements are still needed.