In case of a severe accident in a light water reactor, core melt can be released from the reactor pressure vessel and dislocate to the reactor cavity where it attacks the concrete structures. In order to avoid possible containment failure due to molten corium concrete interaction, the molten corium is to be retained and cooled. Core-catcher concepts considering water-injection via the bottom into the melt layer can lead to rapid quenching and solidification of the melt layer, forming a highly porous structure. The COMET-PC concept relies on porous concrete layers to distribute the water below the melt layer. This paper presents investigations on hydraulics of prototypical porous concretes that have been being used for the experimental verification of the COMET-PC core-catcher system. Pressure losses within these concretes were measured for various water flow rates to determine permeability and passability of the porous concretes. Measurement results were applied in simulations of COMET-PC experiments and reactor application with the COCOMO3D code. The simulation results show that using these concretes in large reactor cavity would not provide sufficiently homogeneous cooling of the entire corium layer unless additional water distribution systems are installed.