SODIUM EXPERIMENTS OF BUOYANCY-DRIVEN PENETRATION FLOW INTO LOW-POWER SUBASSEMBLIES IN A SODIUM-COOLED FAST REACTOR DURING NATURAL CIRCULATION DECAY HEAT REMOVAL

Natural circulation plays a significant role in the decay heat removal function of a sodium-cooled reactor. A recent design of the Japan Sodium-Cooled Fast Reactor (JSFR) fully uses natural circulation for a decay heat removal system (DHRS). A dipped heat exchanger (DHX) is immersed in the reactor upper plenum as the DHRS. The DHX provides cold sodium in the upper plenum during the decay heat removal operation. This cold sodium covers the top of the core under the low-flow-rate conditions of natural circulation. Several water experiments of natural circulation in fast reactors revealed that the cold fluid in the reactor upper plenum might partially and temporally penetrate into the low power core channels, e.g., the radial blanket fuel subassemblies. Sodium experiments were carried out to find the onset conditions and the penetration depth of such partial reverse flow driven by buoyancy force. A blanket subassembly and the upper plenum were modeled in the test section including the axial upper neutron shielding of the subassembly. The experimental parameters were the temperature difference between the hot upward flow in the channel and the cold fluid in the upper plenum and the flow velocity in the channel. The onset conditions of the penetration flow were correlated with Gr and Re numbers as well as with basic water experiments. The observed penetration depths were limited to the upper axial neutron shielding of the subassembly.