Capabilities of MC3D to investigate the coolability of corium debris beds

A nuclear severe accident progression may lead to the formation of a corium debris bed either in the vessel lower head (in-vessel debris bed) or in the vessel pit (ex-vessel debris bed). For safety analyses it is essential to know if a debris bed is coolable or not, i.e. whether a given water mass flow rate poured into the debris bed - either from its top or from its bottom - will be sufficient to evacuate the residual heat and stop the accident progression. The IRSN code, mostly used for fuel-coolant interaction studies, has been modified with the addition of new friction laws for diphasic flows in porous media. The validation of the code in the case of debris coolability concerns the friction in isothermal configuration in cold and hot situations, the evaluation of critical heat flux and the bottom and top reflooding of debris beds. The results obtained with MC3D are in good agreement with the experimental data and are estimated satisfactory regarding to the nuclear safety issues.