Flow Induced Vibration Testing of a PWR Fuel Assembly

All recent qualification tests (since 1990) of various fuel designs that have been performed in TF-2 were conducted in the main test section because of its added stability (vibrations of less than 0.02 Gs-RMS or 0.004 mm-RMS) when investigating fuel assembly vibration phenomena. Special, heavy wall single or dual bundle shrouds are used for these tests. Flow from the loop passes from the pump through several flow straightener sections, into the shroud and test fuel assembly, and finally through a calibrated, internal flow meter section before it exits the test vessel and is returned to the test loop. Flow Sweep Test A single fuel assembly flow sweep test was performed on an early version of a 17xl7XL-fuel assembly. Figure 1 shows such a PWR fuel assembly with mixing vanes. Figure 2 shows the layout of the test shroud. The same fuel assembly prototype had been tested previously in the FATS test loop located at the Stem Laboratories, Hamilton, Canada. The TF-2 test was set up to verify the flow induced vibration behavior that had been observed during the Stem tests. Because of concerns with the observed level of vibration, the fuel assembly was subsequently redesigned and proven to provide acceptable vibration response. Some of these later test results are included in this publication. The flow sweep test was performed at two temperatures 100°C and 150°C (212°F and 300°F) using both incremental (22 m3/h or 75 gpm) and continuous flow sweep methods. Flow was varied from 50% to 120% (or bundle liftoff) of best estimate reactor flow. Test Instrumentation The fuel assembly motion was monitored with a complement of eight (8) high temperature proximity transducers, placed at three grid elevations to allow investigation of assembly motions at multiple assembly vibration modes. A total of eleven (11) bi-axial micro-miniature accelerometers were used to monitor fuel rod vibrations. The test shroud and test vessel were instrmnented with a complement of (ten) 10 accelerometers along with a dynamic pressure transducer, placed just below the fuel assembly entrance flow region. The accelerometers were located at the same elevations of the fuel assembly proximity probes to assure absence of any external sources to fuel assembly vibration. See Figure 3. Data Acquisition Transient and steady state vibration data are acquired via a 64-channel capacity data acquisition system. Each data channel is sampled at 100 and at 1000 samples per second in order to provide the desired resolutions in both the frequency and the time domains. The test signals are viewed and analyzed real time on monitors and spectrum analyzers. The CENP Darwin computer program is used for final processing of results in terms of PSD, Phase, Coherence, Time History, and Transfer Function plots, as well as of Waterfall graphs. Special spreadsheet templates are used to display time synchronized orbital motions and time history collages. T E S T R E S U L T S The objective of the flow sweep test is to determine if there is an assembly resonance over the entire range of flows at reactor operating temperatures and pressures. Proximity displacement data are processed both on-line and off-line to track grid locations with respect to shroud walls. These signals are processed to provide time histories and orbital motion displays for visualization of general motion behavior, and to express vibration amplitudes in terms of RMS and RSS values for each monitoring elevation. These instruments are also used to demonstrate that there is no contact between the test fuel assembly and the shroud. Figure 4 shows a typical motion collage for a flow rate with nearly deterministic fuel assembly response. The orbital motions are graphed both AC-coupled to emphasize the motion characteristics, and also in relationship to available in-shroud clearance ranges, in order to demonstrate no contact with the test shroud. Of interest is the observation of strong diagonal (comer to comer) motions at single, discrete frequencies, rather than at two closely coupled frequencies (which might be expected due to slightly different stiffnesses in two horizontal directions). The RSS response amplitude graph of Figure 5 shows the variation of vibration levels with flow at three grid elevations. It is interesting to note that the inter-grid amplitude ratios of the first and third peak, as well as of the second and fourth peak, are constant, which suggests that there are only two vibration modes associated with the four peaks and thus existence of "mode switching". The Waterfall graph of Figure 6 provides a more dramatic display of this characteristic vibration response, as recorded over a one-hour flow sweep cycle with continuous flow increase. With increase in flow the response frequency increases first and then changes back and forth. The associated frequencies are quite high, 21 and 26 Hertz, and are attributed to seventh and eight order resonances. Spectral graphs for each proximity sensor are created to permit identification of significant response frequencies and modal fuel assembly response behavior. The fuel rod biaxial accelerometer instnnnentation is used to identify fuel rod vibration behavior as compared to fuel assembly vibrations. The test shroud and the test vessel are instrumented with accelerometers at three elevations each to identify the contributions from test facility induced vibrations. Review of the fuel rod vibration data revealed no vibrations that could be attributed to fuel rod resonances. Rather they complimented the results obtained from the proximity probes and provided additional mid-span amplitude results, suited for developing more accurate experimental mode shapes. This confirms that the flow-induced vibrations are due to fuel assembly vibration rather than fuel rod vibration. The RSS responses of the four (4) fuel rods are included in Figure 8. The fuel rod waterfall plots were quite sinfilar to the fuel assembly waterfall plots of Figure 6. Evaluation of the test shroud and test vessel accelerometers demonstrated on the first hand that the overall loop vibration and test hardware induced vibrations were quite small and negligible. Also identified were various secondary response peaks in the fuel assembly PSD data attributable to external vibrations rather than to fuel assembly vibration. While this observation is quite consistent with several other programs, the observation of virtually deterministic fuel assembly response behavior was not. Typically, fuel assemblies display stochastic responses and occasionally some low frequency deterministic responses that are attributed to test vessel motions. Comparison of the data recorded for the two test temperatures did not reveal any differences. This observation, however, can not be considered conclusive since testing for other fuel assembly designs seems to suggest threshold temperatures below which vibrations are small. These thresholds also vary, although all observed thresholds have been below 350°F (177°C). See Figure 7 for the amplitude response of one particular test assembly with temperature. It is hypothesized that the variability of damping, as affected by different designs, fluid temperature and "condition of the test assembly" affect the onset of these instability vibration modes. Therefore, precautions are taken and high-temperature monitoring instrumentation is used to assure that there is no fuel assembly to shroud contact over the entire test range, which otherwise may have a tendency to suppress vibrations. I N T E R P R E T A T I O N O F R E S U L T S Strong flow induced vibrations associated with high fuel assembly modes have been reported by other vendor's for some of their fuel assembly designs (Reference 1). For Westinghouse Atom fuel designs, the susceptibility of the 17xl 7XL fuel assembly with original grid designs to flow induced vibrations was first observed in the Stern tests and later verified in TF-2 as reported herein. The following hypothesis for potential excitation mechanisms is provided: Lacking the presence of any prominent external, deterministic forcing mechanism, the suggestion is made that Axial Flow Velocity, Span Length, Grid Orientation, coupled with the Modal Fuel Assembly Deflection Shapes are pertinent parameters that control the onset of the observed self-excited vibrations. The onset of instability is furthor associated with fuel assembly damping and can be eliminated by certain design changes that are beyond the topic of this article. Figure 9 plots the multiples of 1, 1.5 to 4 times the reciprocal values of the time periods it takes for the fluid to flow axially from grid to grid versus test section flow. The figure also shows two horizontal lines, one at 21.8 and one at 25.5 Hertz that represent respective fuel assembly Mode Nos. 7 and 8. Review of this mapping approach suggests that forcing frequencies equal to both 2.5 and 3.5 times the frequency associated with inter-grid axial flow velocity control the onset of the observed vibration mode. Figure 8 shows the modal deflection shapes as confirmed by test measurements. The illustration of Figure 10 provides a plausible model for how this self-excited vibration mechanism may manifest itself. The sinusoidal vibration amplitudes of Grid Nos. 3, 4, 5 and 6 (at the respective grid elevations) are shown in red along the abscissa of time. The corresponding modal vibration deflection shape inside the test shroud is implied oll the right hand side of Figure 10. It is assumed that each grid, as it reverses direction (near a shroud wall), produces dynamic fluctuations (shown as pulses in the blue traces). It should be noted here that the motion actually occurs in ,a diagonal direction, thus is two-dimensional. The direction of these pulses reverses when the grids are located at opposite sides of the shroud, which creates the suggested alternating pattern in the blue pressure pulse traces shown above each of the four grids. With time th