Validation of MCCI models implemented in ASTEC MEDICIS on OECD CCI-2 and CCI-3 experiments and further consideration on reactor cases

Within a severe accident in a light water reactor a loss of coolant can result in core melting and vessel failure. Afterwards, molten core material may discharge into the containment cavity and interact with the concrete basemat. Due to concrete erosion gases are released, which lead to exothermic oxidation reactions with the metals in the corium and to formation of combustible mixtures. In this work the MEDICIS module of the Accident Source Term Evaluation Code (ASTEC) is validated on experiments of the OECD CCI programme. The primary focus is set on the CCI-2 experiment with limestone common sand (LCS) concrete, in which nearly homogenous erosion appeared, and the CCI-3 experiment with siliceous concrete, in which increased lateral erosion occurred. These experiments enable the analysis of heat transfer depending on the axial and radial orientation from the interior of the melt to the surrounding surfaces and the impact of top flooding. For the simulation of both tests, two existing models in MEDICIS are used and analysed. Results of simulations show a good agreement of ablation behaviour, layer temperature and energy balance with experimental results. Furthermore the issue of a quasi-steady state in the energy balance for the long term appeared. Finally the basic data are scaled up to a generic reactor scenario, which shows that this quasi-steady state similarly occurred. (author)