Experiment investigation on pool boiling of the modelled spent fuel tube bundle under partial dry-out conditions

Under normal conditions, the decay heat of the spent fuel can be removed by the forced-circulation cooling system of the spent fuel pool. In the case of a system failure, the water level in the pool will be gradually decreased owing to an evaporation of the water, and the spent fuel bundles may be exposed to air, i.e., a dry out. Since the Fukushima accident, researchers have been paying significant attention to the safety of the spent fuel under such dry-out conditions. To study the boiling behaviour of the spent fuel bundle under a partial dry-out condition, an experimental investigation has been carried out. A newly constructed 9 × 9 heated tube bundle with the same geometry as the prototype spent fuel assembly in an AP1000 was used as the test section. Two-phase flow patterns were recorded. The heat transfer coefficient of the rods in different locations was obtained. A temperature fluctuation was observed at the beginning of the dry-out process mainly owing to the reduced heat transfer effectiveness of the steam. Although the maximum wall temperature (about 570 °C) exceeded the safety criterion when 50% of the tube bundle at the top end was exposed to air, the bundle maintained its integrity.