EXPERIMENTAL INVESTIGATIONS ON THE COOLABILITY OF DEBRIS BEDS UNDER VARIATION OF INFLOW CONDITIONS

摘要

In case of a severe accident, continuous unavailability of cooling water to the reactor core may result in over heating of the fuel elements and the loss of core integrity. Under such conditions, the molten core can relocate to the lower plenum of the reactor pressure vessel (RPV) and form a structure of heated particles of different sizes and shapes (debris) due to the possible presence of residual water. To avoid a remelting and in the further course of the accident a damage to the RPV it is of great importance to establish long-term coolability. If long-term coolability cannot be established a failure of the RPV and uncontrolled melt release would be the consequence. In this case for certain reactor types again a debris bed can be formed in the reactor cavity. The DEBRIS test facility at IKE was established to investigate the coolability limits of such debris beds. The cylindrical bed (150 mm diameter, 640 mm height) is heated by an inductive heating system, which acts as a volumetric heat source. To investigate the thermohydraulic debris bed behavior the test section is equipped with 55 thermocouples and 8 differential pressure transducers. The maximum removable heat flux (dryout heat flux, DHF) from a debris bed can be determined by increasing the heat input in small increments. This paper presents mainly the results of systematic boiling and dryout experiments at IKE. In contrast to IKE experiments in the past the focus is on debris beds consisting of non-spherical particles with well-defined geometries such as cylinders and screws to investigate the influence of the particle shape on the bed's coolability. For the current set of experiments, the cooling behavior of two kinds of particles was investigated under variation of system pressure (0.1, 0.3 and 0.5 MPa) and inflow condition. The debris bed was flooded either from top or simultaneously from top and from bottom by a natural circulation loop. Additionally, several downcomer configurations were investigated. The experimental results show that a downcomer can significantly improve the bed's coolability since the penetration of water into the bed is not limited by the counter-current flow limitation (CCFL).