The effects of different spacer grid designs on heat transfer during the reflooding period of a pressurized water reactor loss-of-coolant accident (LOCA) were explored using a 3 × 3-rod bundle with full-length indirectly electrically heated, cosine axial power-shaped heater rods. Three different spacer grid configurations were studied: spacer grids without mixing vanes, mixing vane spacer grids, and mixing vane spacer grids together with intermediate flow mixers.

The test results indicate that the influence of spacer grids with mixing vanes on the reflood heat transfer is complex and beneficial at least for the most severe LOCA conditions. The mixing vane spacer grids together with the intermediate flow mixers significantly improved the reflood heat transfer for the whole range of considered test conditions, compared with spacer grids without mixing promoters. The use of mixing vane spacer grids, instead of spacer grids without mixing promoters, induced a significant decrease in the maximum cladding temperature at low flooding rates, especially at a high power generation rate. At high flooding rates the mixing vanes degraded slightly the post-critical-heat-flux heat transfer conditions.