The development of the MCCI code COSACO specifically addresses the ex-vessel MCCI phase of the core melt retention concept of the EPR. The general philosophy behind COSACO is a rigerous representation of thermochemical phenomena related to the MCCI. In particular, the code incorporates a real solution database to predict the simultaneous formation of solid and liquid phases as well as chemical reactions for a significant number melt constituents. This offers a great flexibility in terms of application to MCCIs involving reactor materials and to tests conducted with simulant melts. The approach to model heat transfer in oxidic melt pools is based on the phase segregation hypothesis. Besides a brief description of the principal models incorporated in COSACO, this paper highlights specific thermochemical effects that arose as part of post-test calculations of the tests MACE M3b and MACE M4 with this new code version. Particular attention is drawn to the effect of melt ejections on the pool temperature as well as to the evolution of solid volumetric fraction and of melt front progression during the MCCI. Finally, the application to a representative EPR specific sequence indicates that the principal objectives of the MCCI in the reactor pit can be safely fulfilled.

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