Mitigation of a core meltdown scenario by means of a core catcher located inside the reactor pressure vessel

This paper describes an original solution of core catcher to managing the in vessel retention of the Corium in the accidental event of the core meltdown. The solution envisaged intends to verify the possibility of managing the accidental event within the pressure vessel, ensuring that the CORIUM is confined and cooled. The core catcher, elaborated at the DIMNP, is made of a ceramic pebbel bed (Alumina Al2O3) contained in a metallic or Ceramic Matrix Composite (CMC) structure. The paper illustrates a theoretical model to simulate the thermal-mechanical behaviour of the pebble beds under extremely high loads, developed by the authors. This model has been used to design the core catcher and to determine the effective conductivity and the effective stiffness of the pebble bed. These values have been used in order to implement a numerical model of the core catcher. The results of the thermal and mechanical coupled simulation have permitted to determine the maximum time that the core catcher could resist and the mechanical resistance of the core catcher in the case of RPV external or internal cooling .The preliminary analyses performed have emphasised the good performance of pebble bed core catcher in order to mitigate the envisaged severe accident.