Abstract
It is essential to understand the thermal hydraulics of natural circulation in a pool type fast reactor which uses the passive direct heat exchanger (DHX) for decay heat removal. A transparent slab water model has been developed at China Institute of Atomic Energy, which allows the application of Particle Image Velocimetry (PIV) for the visualization measurement of flow characterization. An experiment was carried out on this water model to research the mechanism of natural circulation of the primary circuit and the process of the heat removal based on the DHX which is installed in the hot pool. This will help to understand the temperature and flow patterns of coolant in the reactor during decay heat removal condition. Transient temperature change of coolant in the core and hot pool in the experiment indicated that the DHX is useful, not only for the establishment of inter-wrapper flow but also for the maintaining of heat sink of natural circulation through the core during decay heat removal. The coolant of hot pool absorb the heat brought from the core and the temperature difference between hot pool and cold pool will disappear at steady state for losing of heat sink of intermediate heat exchanger. For the consistent cooling of DHX, cold coolant can stratify in lower part of the hot pool and a temperature fluctuation has been observed at the top of core which may be a challenge to the structure above the core. The visualization measurement of flow characterization shows that, there was a downward flow in the hot pool when the hot coolant from core flows upwards to the hot pool. This downward flow can penetrate to periphery of core and meet the hot coolant from the core outlet. The experimental results can be a foundation for the realization of the mechanism of natural circulation of pool type fast reactor.
