Experimental study of a natural circulation loop and RELAP5-3D analysis

Abstract

Passive safety systems, which are being largely adopted in advanced nuclear reactors to enhance the safety of the plant, are often based on natural circulation for the decay heat removal and are characterized by the presence of non-condensable gases and pools with stagnant liquid at atmospheric pressure. At the Energy Department of Politecnico di Torino (Italy), the test facility PROPHET has been designed and operated with water in both single-phase and two-phase flow. It consists of a bayonet heat exchanger electrically heated at constant power, a condenser immersed in a pool containing a fixed initial mass of water and the connecting pipes. The effect of loop filling ratio and condenser shell-side level on the system behaviour are here studied. The experimental transients have been analysed in terms of phenomenological windows. Heat losses and the heat sink influence both the time behaviour and the final values of pressure and temperatures in the facility; the filling ratio determines the kind of natural circulation (single-phase or two-phase) and strongly affects the phenomena occurring during the experiments. The experimental results are also compared with the predictions of a RELAP5-3D model. Discrepancies between calculated and experimental values are estimated during the transients in order to evaluate the code prediction capabilities.