Copra experiments on natural convection heat transfer with high Rayleigh numbers

Daring a severe accident in light water reactors, the melt of the core may relocate into the tower head of the reactor pressure vessel Natural convection plays an important rofe in determining the thermal toad from the debris pool, which is directly relevant to the problem of retention of molten corium inside the lower plenum. This paper presents the description of COPRA (COnum Pool Research Apparatus) experiments and results on natural convection heat transfer m an internally heated melt pooL The test apparatus is a two-dimensional 1/4 circuhr slice structure with an amer radius of 2.2 m to simulate the lower plenum of reactor vessel at 1:1 scale for the Chinese advanced PWR. A non-eutectic binary mixture of 20mo1%NaNO₃-80mol%KNO₃ compositions is selected as the simulant material. The melt is preheated to about 350C r a custom-designed heating furnace before being transferred to the facility. 20 electrical heating rods, each with a diameter of 16 mm but different lengths according to their locations, arc uniformly distributed in the vessel to simulate the homogenous internal decay heat. The top surface of the pool can be maintained insulated with an adiabatic lid The outside of the curved waO s enclosed with a regulated external cooling path to keep the boundary temperature nearly isothermal. Due to the full scale geometry, the Rayleigh numbers with in the pool could reach up to 10¹⁶, matching those in the prototypical stuation for PWR. The results have been compared with the correlations from other experiments.