Experimental results on the coolability of a debris bed with down comer configurations

Abstract

Abstract In case of a severe accident, a debris bed is formed from a mixture of molten core and the residual water in the lower plenum of the reactor pressure vessel. The presence of decay heat in a debris bed poses a critical threat to the reactor pressure vessel. To avoid any damage to the reactor pressure vessel, the removal of decay heat from the debris bed is of great importance. In order to investigate experimentally the long-term coolability of debris beds, the non-nuclear test facility “DEBRIS” has been established at IKE. Experimental investigations of coolability limits for such a debris bed, based on two different bed configurations and at various thermo-hydraulic conditions, are carried out at IKE. This paper presents the experimental results for multidimensional cooling effects on boiling and dryout tests with different bed configurations and different system pressures. Two different down comer configurations (cylindrical and perforated cylindrical tubes with inner diameters of 10 mm each) are used to investigate the multidimensional cooling effects. The down comer is concentrically installed inside the debris bed which is contained in a cylindrical crucible with an inner diameter of 125 mm. Different bed configurations, e.g. polydispersed particle bed with spherical particles 2, 3 and 6 mm in diameters and irregular particles of equivalent diameters 2–10 mm, have been used with a bed height of 640 mm. A layer of ceramic balls of diameter 4 mm with 40% porosity is used to make up a water pool at the bottom of the bed. The bottom inflow via the down comer tube as well as the lateral inflow of water through the perforated down comer tube into the bed improves the coolability of the debris bed, and therefore an increase of the dryout heat flux can be observed. Experimental results also show that the system pressure has no significant effect on the characteristics of pressure gradients inside the bed, whereas with increasing system pressure the coolability limits are increased.