Natural convection thermo fluid dynamics in a volumetrically heated rectangular pool

During a severe accident the nuclear reactor core may melt and, further, relocate to the lower plenum in the reactor vessel. Should there be no effective cooling mechanism, the core debris may heat up on account of the volumetric decay heat generation to form a molten pool in natural convection. The Simulant Internal Gravitated Material Apparatus Rectangular Pool (SIGMA RP) tests were performed to study high modified Rayleigh number (Ra′) natural convection in a rectangular pool. The test apparatus was 500 mm long, 500 mm high and 160 mm wide. Forty thin cartridge heaters, with a sheath diameter of 4.2 mm and a length of 600 mm, were used to simulate internal heating in the pool. They were uniformly distributed in the rectangular pool so as to supply a maximum of 2.5 kW power to the pool. Ra′ based on the power input was varied from 109 to 1014. The working fluid Prandtl number (Pr) ranged from 4 to 8 for water, and 0.7 for air. Particular attention was paid to the Pr influence on natural convection heat transfer in the pool. The relation between the Nusselt number (Nu) and Ra′ was determined for different boundary conditions in the rectangular pool. In one case the upper wall was cooled isothermally, while the lower wall was kept adiabatic. In the other case both the upper and lower walls were cooled isothermally. The experimental results indicated that both the upward and downward natural convection heat transfer rates were affected by Pr. The resultant engineering correlations were applied to simulation of a spectrum of loss-of-coolant accidents to determine the reactor vessel failure time.