During a severe accident, the reactor core may melt and be relocated to the lower plenum to form a hemispherical pool. If there is no effective cooling mechanism, the core debris may heat up and the molten pool run into natural convection. Natural convection heat transfer was examined in SIGMA RP (Simulant Internal Gravitated Material Apparatus Rectangular Pool). The SIGMA RP apparatus comprises a rectangular test section, heat exchanger, cartridge heaters, cooling jackets, thermocouples and a data acquisition system. The internal heater heating method was used to simulate uniform heat source which is related to the modified Rayleigh number Ra′. The test procedure started with water, the working fluid, filling in the test section. There were two boundary conditions: one dealt with both walls being cooled isothermally, while the other had to with only the upper wall being cooled isothermally. The heat exchanger was utilized to maintain the isothermal boundary condition. Four side walls were surrounded by the insulating material to minimize heat loss. Tests were carried out at 1011 < Ra′ < 1013. The SIGMA RP tests with an appropriate cartridge heater arrangement showed excellent uniform heat generation in the pool. The steady state was defined such that the temperature fluctuation stayed within ±0.2 K over a time period of 5,000 s. The conductive heat transfer was dominant below the critical Rayleigh number Rac, whereas the convective heat transfer picked up above Ra′c. In the top and bottom boundary cooling condition, the upward Nusselt number Nuup was greater than the downward Nusselt number Nudn. In particular, the discrepancy between Nuup and Nudn widened with Ra′. The Nuup to Nudn ratio was varied from 7.75 to 16.77 given 1.45×1012 < Ra′ < 9.59×1013. On the other hand, Nuup was increased in absence of downward heat transfer for the case of top cooling. The current rectangular pool testing will be extended to include circular and spherical pools.

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