COPRA experiments on natural convection heat transfer in a volumetrically heated slice pool with high Rayleigh numbers

Large scale COPRA experiments were performed to study the natural convection heat transfer in corium pools inside the reactor pressure vessel lower plenum during severe accidents. The test facility is designed to simulate the lower plenum of reactor vessel at 1:1 scale for the Chinese advanced PWR, which is a two-dimensional 1/4 circular slice structure with an inner radius of 2.2 m. A non-eutectic binary mixture of 20 mol%NaNO3-80 mol%KNO3 compositions was selected as the simulant material. Twenty custom-designed heating rods were located in the facility to provide homogenous internal heating. Due to the full scale geometry, The Rayleigh numbers within the corium pools could reach to 1.188 × 1015-1.784 × 1016, matching those in the prototypical situation during severe accident. The pool temperature field and heat flux distribution, as well as the crust thickness distribution from tests with different pool heights and heating powers were obtained and compared in the experiments. Influences of the relocation position and upper cooling on the heat transfer characteristics were also studied in the experiments. Relations of pool temperature and heat flux distribution, as well as Nudn-Ra′ relation were developed. Comparison with previous experiments showed that the downward heat transfer Nudn prediction from COPRA experiments were in good agreement with SIMECO and LIVE salt experimental results.