The effects of flow blockage on bottom-cooling heat transfer effectiveness were studied in the PWR-FLECHT program. Blockages of 50 and 75% of the flow area of a 5 × 5 array in the center of a 7 × 7 rod bundle were studied with 12-ft-long heater rods with an axial cosine heat generation distribution of 1.66 peak-to-average and decay-heat simulation. The test section consists of 42 heated rods (fuel rods) and 7 non-heated tubes (control rod thimbles). Flow blockage is effected by a -in.-thick flat plate mounted at the peak heat generation location (6-ft elevation) with the heater rod thermocouples being located ∼1 in. downstream from the flow blockage plate. The results of a flooding rate of 6 in./sec showed the maximum temperature rise in the heater rod was the same for 0, 50, and 75% flow blockage. Lower flooding rates of 4- and 2-in./sec showed higher temperature rises in the heater rod for the 0% flow blockage case than for the 75% flow blockage case. These flow blockage tests demonstrate that bottom-flooding heat transfer effectiveness is not impaired with the flow blockage configurations tested. Further tests are planned at flow area blockages of 90%.