The test rig was built to investigate the heat transfer process to the containment pool, including natural circulation flow establishment, along the inclined bottom of the catcher, the bottom to vertical corner and the vertical section of the catcher. In the design of the test rig, geometrical similarity with the original core catcher of BWR 90+ was preserved as much as possible. The elevations of different components of the rig were maintained the same as in the original core catcher.
The coolant flow channel represents a 20 cm wide 'slice' of the original cooling loop of the core catcher. In the original core catcher, the coolant flow channel broadens as the cooling water flows from the core catcher bottom to the vertical section. The ratio of heat transfer area in the bottom section to the area in the vertical section is larger in the test rig than in the original core catcher. The aim of the test rig design was to try to get as realistic flow conditions as possible in the area where the maximum heat flux is expected i.e. in the area near the corner of the core catcher plate.
The heat from the molten corium is simulated with 311 cartridge heaters, mounted inside the core catcher plate. The cartridge heaters are 20 cm long, have diameter of 10 mm and the maximum power of 2 kW each. The maximum power density in the inclined bottom of the core catcher plate in the test rig is 550 kW/m². In the vertical section, the maximum heat flux is about 2000 kW/m². The cover of the cartridges is made of inconel 600, insulation material inside the cartridges is magnesium oxide and the actual resistor is made of chrome-nickel steel.
The pressure suppression pool of the test rig is smaller than the corresponding part of the real core catcher cooling circuit. It is simulated with a water pool with 60 cm inside diameter and 10 meters high. In addition, the volume in the lower drywell pool has been made smaller to make the installation of the test rig to the laboratory room easier. The pressure suppression pool is connected to the lower drywell pool with DN200 pipe what is smaller than in the real core catcher cooling circuit. The lower drywell pool is separated from the flow channel below the core catcher plate with a flow distributor. The water enters the core catcher bottom through a 52 cm x 20 cm opening. The flow channel below the core catcher is 20 cm wide and has a height of 42 cm at the inlet and 32 cm at the outlet. The size of the vertical channel is 30 cm x 20 cm. The steam vent pipe is made of DN350 pipe.
The design principle for the differential pressure measurements of the test rig was to provide the test rig with instrumentation, which makes it possible to determine the pressure loss distribution around the cooling loop. In the same way, thermocouples were installed around the test loop to detect the coolant temperature distribution. The thermocouple measurement positions in the copper block were selected to measure the temperature profile in the heated copper block and to detect the location of the possible dryout. The thermocouples near the core catcher plate surface were put in constant distance between each others to ensure the measurements cover the whole heated area.
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