Abstract SIGMA CP (Simulant Internal Gravitated Material Apparatus Circular Pool) tests were performed to study natural convection characterized by a high modified Rayleigh number (Ra′) in a semicircular pool. The test apparatus had a 500mm diameter, 250mm height and 100mm width. Two thin cartridge heaters, which had a sheath diameter of 6mm and a length of 2000mm, were used to simulate internal heating in the pool. They were uniformly distributed in the semicircular pool to supply a maximum heating power of 3kW to the pool. The Ra′ was varied from 106 to 1011 based on the power input. The working fluid Prandtl number (Pr) ranged between 4 and 8 for water and was 0.7 for air. Particular attention was paid to the influence of Pr on the natural convection heat transfer in the pool. A number of relations between the Nusselt number (Nu) and Ra′ were determined for the boundary conditions in the semicircular pool. The boundary condition is that the upper and lower walls are cooled isothermally. The experimental results indicated that both the upward and downward natural convection heat transfer rates were affected by Pr. The Rayleigh numbers were determined up to 7×1011 from this test. The resultant engineering correlations can be applied to simulations of a spectrum of loss-of-coolant accidents to determine the reactor vessel failure time.