Simulation of Late Phase Phenomena with the System Code AC2

Simulation codes like the AC 2 system code package developed by German Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) gGmbH are used in international reactor safety research to investigate hypothetical severe incidents and accidents in nuclear power plants. Valuable impetuses for the development of prevention and mitigation measures for such events can be derived from the analyses. This article presents selected work by PSS on the development of simulation models for the late phase of nuclear accidents. Topical issues as the formation and thermodynamic interactions of debris beds in the lower plenum of the reactor pressure vessel (RPV), the release of fission products as well as the interaction of the molten core material with concrete in the reactor cavity in case of RPV failure are discussed. The analyses carried out provide important findings regarding the validation of the developed models. In addition, future development needs are derived for the consideration of further relevant phenomena. • Development need in AC 2 for modelling of phenomena in core melt accidents addressed. • New model enables simulation of thermodynamic behaviour in debris beds. • More detailed representation of fission product release processes from molten pools. • Significant reduction of user influence regarding the simulation of MCCI. • Development work based on the implementation of established correlations.