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MSL Acoustic Source Load Reduction By Amir Shahkarami, Exelon Nuclear. Background: From the beginning of plant operations, Dresden and Quad Cities main steam line vibration levels were different. The Quad Cities units had higher vibration levels than the Dresden units. The main steam line vibration started to become a problem at Quad Cities by 1977, when a unit 2 main steam electromatic relief valve (ERV) failed to operate during surveillance. In March 1978, the site performed main steam line (MSL) testing to determine the root cause. The site concluded high frequency pressure pulsations from the valve standpipe were causing the valve disc piston rings to wear a groove into the valve disc guides and thereby locking the valve in the closed position. The solution was to improve the valve components so that the higher vibration levels could be withstood. The extended power uprate (EPU) vibration evaluation performed included Nuclear Energy Institute’s Top Industry Practice (TIP) Award’s highlight the nuclear industry’s most innovative techniques and ideas. They promote the sharing of innovation and best practices, and consequently improve the commerical prospects and competitive position of the industry as a whole. This innovation was a 2008 NEI Process Award Winner. The team members who participated included: Amir Shahkarami, Senior Vice President Engineering and Technical Services, Exelon Nuclear; Roman Gesior, Director of Asset Management and Engineering Programs, Exelon Nuclear; Guy DeBoo, Senior Staff Engineer, Exelon Nuclear;Kevin Ramsden, Senior Staff Engineer, Exelon Nuclear, Alan Bilinan, President, Continuum Dynamics, Inc.; and Tom Beringer, Sargent and Lundy. the MSLs, ERVs and main steam safety valves (MSSV), but not the steam dryer. It was believed that main steam line acoustic oscillations would dissipate rapidly once inside the larger reactor steam dome cavity, thereby having minimal structural impact on the steam dryer. After Quad Cities reached EPU power in March 2002, steam path component vibration induced problems began to increase in both number and severity. The Dresden units had relatively few problems due to steam path vibrations after EPU implementation. This led to further efforts to understand the cause and corrective actions to prevent recurrence. One of the first steps in understanding the root cause was to carefully determine the Dresden and Quad Cities MSL as built dimensions. The detailed as built dimensions were studied to identify MSL differences that were potentially resulting in structural performance differences. The review found very few significant differences actually existed. One notable difference was the Dresden ERV and MSSV standpipe internal diameter was over one inch smaller than the Quad Cities standpipe diameter. The Dresden standpipe internal diameter is 4.625 inches compared with the Quad Cities 5.761 inch standpipe internal diameter. The next important step in understanding the issue was to comprehensively instrument and monitor both Dresden and Quad Cities MSLs. Strain gages were installed and monitored at different power levels during plant maneuvers. This step leads to the interesting observation that the high frequency response at Quad Cities started to significantly increase just prior to original licensed thermal power and continued to increase through EPU power levels. In contrast, the MSLs at Dresden showed a smaller spike in high frequency response at lower thermal power levels that dissipated quickly before reaching the original licensed thermal operating limits. At full EPU, the Dresden units showed very little high frequency response on the MSLs. Amir Shahkarami Shahkarami is the Senior Vice President of Engineering & Technical services at Exelon Nuclear. He is responsible for fuel, engineering, project management, license renewal, industry organizations, innovation, and the international exchange program. Shahkarami joined Exelon Nuclear in 2002 as Engineering Director of the Dresden Nuclear Station. He received his Bachelor and Master of Engineering degrees from Tulane University, MBA from Mississippi College, and completed PhD studies in nuclear engineering at Louisiana State University. He is a registered Professional Engineer and has a Senior Reactor Operator Certifi cate. The preliminary conclusion at this point was the difference in standpipe dimensions resulted in different acoustic pressure oscillation scenarios for the two stations. This is similar to different size organ pipes producing different sounds. As is normal with complex problems, independent reviews were performed to challenge the conclusions of dryer and main steam line loading sources. From these challenges, it was determined that additional in-plant testing was necessary to prove the hypothesis before development of appropriate mitigation devices. To accomplish this, Exelon, Exelon vendors and an independent team of experts developed a start-up test plan with the steam path instrumented. Based on domestic and international benchmarking this was the most extensive BWR instrumented steam path start up test plan ever performed. The Quad Cities unit 2 steam dryer was instrumented with the following sensors: nine strain gages, six accelerometers and twenty-seven pressure transducers. In addition, the existing high-speed pressure transducers on the reactor water level reference leg instruments were maintained in place from previous testing. 38 www.nuclearplantjournal.com Nuclear Plant Journal, May-June 2009
Exelon, Continuum Dynamics, Inc. and Structural Integrity Associates identified MSL locations for 56 strain gages and 33 accelerometers. The instruments were strategically placed on the MSL as input to the acoustic circuit analysis for steam dryer loading and MSL vibration evaluations. In May 2005, Quad Cities unit 2 ramped up in power based on the startup test plan and collected data from the sensors on the steam path at the designated power levels. Data acquisition equipment was obtained to collect steam dome, steam dryer and main steam line strain gage, pressure transducer and accelerometer data simultaneously. When a power level was reached, the data were collected and analyzed based on pre-established go / no-go criteria until 2832 MWt; which was the highest MWt at EPU that could be reached with the environmental factors for that time of year. The data from the instrumented MSLs and steam dryer revealed the pressure oscillations at approximately 157 Hz that dramatically increased in amplitude above the original licensed thermal power. These high frequency pressure oscillations peaked on the dryer surface directly in front of the main steam line nozzles. This testing showed that the ERV and MSSV standpipes at Quad Cities were a probable cause of the increased loading on not only the dryer, but also on the entire steam path. In December 2005, an electrical ground was found present on the Quad Cities unit 2 3D ERV. The subsequent inspection revealed significant internal damage to the solenoid actuator. Additional inspections of the remaining ERVs for both Quad Cities units show significant wear and loose parts on the solenoid actuators. The root cause once again pointed to high frequency vibration damage. These valves are safety related components and are required to operate in the event of MSL over pressurization. The probable root cause identification and the additional ERV damage escalated the need for a MSL load reduction device. The starting point for this project was to assemble a team of industry experts to brainstorm possible mitigation devices. The ideas for mitigation ranged from making Quad Cities ERV and MSSV standpipes the same diameter as Dresden to changing the height of the standpipes as well as several other combinations of geometry changes. To ensure the various ideas were tested appropriately, the team determined a scale model test program that would be benchmarked against the in-vessel and steam path data collected at Quad Cities was required. To perform this successfully, the appropriate Reynolds Number had to be applied correctly throughout the test rig, otherwise the results would not reflect actual in-plant data. This presented a challenge since to achieve the appropriate Reynolds numbers, the scale model test had to be at elevated pressures, making the test rig at full scale very large. Continuum Dynamics, Inc. (CDI) was contracted to develop a pressurized test rig and perform the benchmark to Quad Cities in-plant data at subscale to avoid this problem. With this approach CDI was able to correctly duplicate the high frequency response from the Quad Cities main steam line actual in-plant data with the scale model test rig. This confirmed the probable root cause to be from the relief valve standpipes. Based on domestic and international benchmarking, this is an industry first, to take in-plant data from an instrumented steam path and successfully benchmark a scale model test rig against the data. With the benchmark completed, hundreds of tests were performed on numerous mitigation concepts, in addition to optimizing the acoustic side branch concept. In the end the team selected an acoustic mitigation device that branches off the valve standpipe side. This option was chosen for the following reasons: • The acoustic side branch (ASB) addition increases the effective length (L) of the standpipe, decreasing the acoustic standing wave frequency (f). • The vortex shedding frequency remains unchanged at the same power level, but the acoustic and vortex shedding frequencies are no longer coupled, so that resonance does not occur. • The acoustic frequency decrease lowers the velocity at which vortex shedding will excite the acoustic standing wave (i.e. the acoustic (Continued on page 40) Nuclear Plant Journal, May-June 2009 www.nuclearplantjournal.com 39
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