Natural circulation tests in HANARO

HANARO, 30MWTH research reactor, was designed to remove the decay heat by the natural circulation cooling after reactor shutdown. HANARO has two natural circulatin cooling modes ; the loop an the pool natural circulation. Natural circulation tests have been carried out upto 6 % full power. In this paper, recent test results conducted at 5% and 6% full power are presented. It was confirmed that both the loop and the pool natural circulation were simultaneously established regardless of the secondary cooling system conditions. But the heat exchanger becomes main heat sink in case of the secondary pump-on condition and the reactor pool water does in case of the secondary pump-off condition. It was observed under the secondary pump-on condition that the recirculation flow found in the previous low power tests did not appear at two outlet nozzles and small amount of bypass flow existed. The flap valves opened a little under the primary pump-off and the secondary pump-on condition and opened more under the secondary pump-off condition. In conclusion, the natural circulation tests verified the flap valve performance and that the decay heat in HANARO core is sure to be passively removed by the natural circulation