Design Considerations for a Core-Catcher Concept Based on Bottom-Flooding through Porous Concrete

Abstract : In case of a severe accident in a light water reactor, core melt can be released from the reactor pressure vessel and dislocate to 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. A design variant of the comet concept relies on porous concrete layers to distribute the water below the melt layer. This paper presents a methodology that was developed to determine major design parameters for a bottom cooling concept with porous concrete structure considering a scale dimensioning of a generic pressurized water reactor of Gen II. Design parameters were systematically varied in order to calculate the optimum concrete configuration in view of porosity and permeability. Using the COCOMO3D code (COrium COolability MOdel, 3Dimensional), which is being developed at IKE, simulations were carried out both in 2D and 3D for symmetrical and non-symmetrical water inlet flow configurations. The results show that sufficient water flow rates can be reached in order to cool the molten corium, which is promising for the applicability of this concept as an accident mitigation device for existing and new reactors.