An experimental investigation was performed to compare the axial velocity profiles occurring downstream of the inlet nozzle region of nuclear PWR fuel assemblies for two bottom nozzle designs. Axial velocities were measured in a 3.763:1 over-scale air test section simulating a 6×6 rod array of the inlet nozzle region and first grid span. The measurements were taken in multiple planes at 23 positions to map the velocity in both rod gaps and subchannels. Pressure drop measurements were also taken at selected axial elevations. The two bottom nozzle designs were tested to study how each nozzle impacts the dissipation of the lower support flow hole jet velocity downstream of the bottom nozzle. The nozzle designs included a standard round-hole flow plate and a slotted flow plate. The velocity profile results with these bottom nozzle flow plate designs are of particular interest since these bottom nozzle designs had previously been used in full scale out-of-pile water flow tests with contrasting results on wear at the first grid elevation. There were no rods with fretting wear in the test with the standard round hole flow plate and ten rods with fretting wear with varying depth in the test with the slotted flow plate. The full scale flow tests were performed at reactor operating temperatures (620°F), pressures (2250 psia) and at the same test flow rate. The grid cell settings were unchanged for the two tests. Therefore, the only test variable that differed for the full scale flow tests was the bottom nozzle design. The axial velocity profile investigation provided evidence that the standard round-hole flow plate dissipated the lower support jet velocity much better than the slotted plate design. In addition, based on the air test results, the slotted plate design had the largest axial velocity gradients at the same location as the observed rod wear. Based upon this investigation, it is concluded that excessive rod vibration in this region can occur due to high axial jet velocities and steep axial velocity gradients generated from the flow holes in the lower support plate. The excessive rod vibration can lead to fuel rod wear and fuel failure.

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