Reflood heat transfer in the FLECHT-SEASET 163-rod bundle with flow blockage and bypass

A series of two-phase forced and gravity reflood experiments have been conducted in the NRC/EPRI/ Westinghouse FLECHT/SEASET 163-Rod Blocked Bundle Program. The objective of these experiments was to examine the local heat transfer effects in a large blocked rod bundle with ample flow bypass. The flow blockage shape was hydroformed into a long, nonconcentric shape and distributed noncoplanar on two small ''islands'' of heater rods in the center of the bundle. Rod and vapor temperature measurements were made throughout the bundle with special emphasis in and downstream of the blockage region. The rod heat transfer downstream of the blockage was found to be greater for rods in a blocked bundle than for similar rods in an unblocked bundle. The heat transfer improvement decreased both with time after flood initiation and as the distance increased downstream of the blockage. The improvement in the heat transfer is attributed primarily to the breakup of the water droplets entrained in the steam flow. The smaller droplets subsequently evaporate and rapidly desuperheat the steam, which then improves the heat transfer between the rods and the steam in and downstream of the blockage zone