Experimental investigations on the coolability of stratified debris beds consisting of prototypical particles

In case of a severe accident, the reactor core can melt due to an insufficient removal of the emerging decay heat. If melt gets in contact with residual water, a particle debris bed can be formed in the lower plenum of the reactor pressure vessel (RPV). It is of great importance to remove the decay heat from the debris bed to avoid any damage to the RPV. The DEBRIS test facility at IKE was established to investigate the coolability limits of such debris beds. The cylindrical bed (150 mm inner diameter, 640 mm height) is heated by an inductive heating system, which acts as a volumetric heat source. The heat input is increased in small increments until dryout is reached. To investigate the thermal debris bed behavior the test section is equipped with roughly 60 thermocouples and 8 differential pressure transducers. In this paper, the results of systematic boiling and dryout experiments are presented. In contrast to previous IKE experiments, the focus is on debris beds consisting of more complex particles with well-defined geometries such as cylinders and screws to investigate the influence of the particle shape on the bed's coolability. The experiments were carried out under variation of pressure (0.1 - 0.5 MPa), inflow conditions (top-flooding and combined top- and bottom-flooding), bed inventory and azimuthal stratification. By a stratification of the bed inventory an inhomogeneous permeability in the bed can be achieved, which can lead to multidimensional flow conditions and cooling effects. The experiments show that higher permeability of the bed and higher system pressure significantly increase the bed's coolability. Additionally, the measured pressure gradients are compared with several friction models in order to validate their applicability. The validation of these friction models plays a key role in the development of IKE's simulation code MEWA, which is implemented in the German system code ATHLET-CD. (author)