Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident in nuclear power plants. The natural convection heat transfer involving internal heat generation is generally represented by the modified Rayleigh number, Ra', which quantifies the internal heat source and hence the strength of the buoyancy force. The SIGMA RP (Simulant Internal Gravitated Material Apparatus Rectangular Pool) tests are concerned with high Ra' turbulent natural convection in a rectangular pool. The internal heating was realized by cable-type heaters in the test to simulate uniform heat generation. Ra' was found to be in the range of 1×109 - 1×1012. Experiments were conducted using air with the Prandtl number, Pr, of about 0.7 as the working fluid. The test results confirmed feasibility of the direct heating method to simulate uniform volumetric heat generation in the pool.