A simplified core catcher model for the containment code AC2/COCOSYS

Abstract

In Generation 3+ NPPs such as EPR or WWER-1200 the ex-vessel core catcher presents a key component of the severe accident mitigation concept. A simplified core catcher model in the containment code COCOSYS (part of AC2 code package) is developed to consider core melt retention and cooling phenomena in core catchers for containment simulations. The model enables the characterization of the corium in the core catcher including processes in the corium conditioning phase (melting of sacrificial material in the core catcher, oxidation of metallic components, possible layer flip) and the long-term cooling phase (evolution of long-term corium pool bulk temperature and changes in physical state). The core catcher model is based on the existing molten corium / concrete interaction (MCCI) model in COCOSYS with additions for melt retention within water-cooled structures and input data can be adapted to simulate various core catcher design features of e. g. NPPs with EPR or WWER.The current state of the model, including basic assumptions, calculation methods, simplifications, and two application examples are outlined. The recalculation of the water-cooled basemat test OECD WCB-1 shows that the experimental data regarding e. g. the ablation of the sacrificial concrete on top of the water-cooled basemat, corium pool temperature and heat fluxes at water-cooled interfaces can be adequately reproduced by the COCOSYS simulation, when using empirical data for the heat transfer, including the MCCI phase. These empirical heat transfer data agree well with findings obtained from recent analytical work on MCCI tests. However, the model does not predict high local bottom plate temperatures as observed in the WCB-1 test due to a restriction of the model to quasi-steady state processes. The second calculation example demonstrates the current modelling features when applied to an axisymmetric geometry of a reactor-scale core catcher. With view to reactor-scale, the model is ready for a further verification, like e.g. comparison with other analytical core catcher approaches.