Effect of grid spacers on reflood heat transfer in PWR-LOCA

Experiments were performed in order to clarify the effect of the grid spacers on reflood heat transfer in PWR-LOCA. The flow pattern, the thermal responses and the water accumulation near the grid spacer were investigated by shifting the grid spacer at the midplane of the simulated core. The heat transfer coefficient before the quenching was about 20 to 50% higher just above the grid spacer than just below the grid spacer. The decrease of the droplet diameter due to the grid spacer was observed in the droplet dispersed flow regime. In the slug flow regime, the grid spacer was rewetted early in the reflood transient and the increased water accumulation near the grid spacer was observed. Hence, the heat transfer enhancement due to the grid spacer is mainly attributed to the increased interfacial surface area of droplets in the dispersed flow and also to the increased film boiling heat transfer in the slug flow. The thermo-hydraulic behavior near the grid spacer was well predicted with the model developed based on the present experiment. (author)