Impact of Flow Induced Vibration Acoustic Loads on the Design of the Laguna Verde Unit 2 Steam Dryer David R. Forsyth, Leslie F. Wellstein, Robert C. Theuret, Yan Han, Charles Rajakumar Westinghouse Electric Company LLC Cranberry Township, PA 16066 forsytdr@westinghouse.com;wellslf@westinghouse.com;theurerc@westinghouse.com; han1y@westinghouse.com; rajakuc@westinghouse.com César Amador Cabrera, Wendy Sosa Flores CFE Laguna Verde Central Nucleoelectrica Laguna Verde Km. 42.5 Carretera Cardel-Nautla Laguna Verde, Ver. 91680 Mexico cesar.amador@cfe.gob.mx; wendy.sosa@cfe.gob.mx Summary Industry experience with Boiling Water Reactors (BWRs) has shown that increasing the steam flow through the main steam lines (MSLs) to implement an extended power uprate (EPU) may lead to amplified acoustic loads on the steam dryer, which may negatively affect the structural integrity of the component. The source of these acoustic loads has been found to be acoustic resonance of the side branches on the MSLs, specifically, coupling of the vortex shedding frequency and natural acoustic frequency of safety relief valves (SRVs). The resonance that results from this coupling can contribute significant acoustic energy into the MSL system, which may propagate upstream into the reactor pressure vessel steam dome and drive structural vibration of steam dryer components. This can lead to high-cycle fatigue issues. Lock-in between the vortex shedding frequency and SRV natural frequency, as well as the ability for acoustic energy to propagate into the MSL system, are a function of many things, including the plant operating conditions, geometry of the MSL/SRV junction, and placement of SRVs with respect to each other on the MSLs. Comisión Federal de Electricidad and Westinghouse designed, fabricated, and installed acoustic side branches (ASBs) on the MSLs which effectively act in the system as an energy absorber, where the acoustic standing wave generated in the side-branch is absorbed and dissipated inside the ASB. These ASBs have been very successful in reducing the amount of acoustic energy which propagates into the steam dome. In addition, modifications to the Laguna Verde Nuclear Power Plant (LVNPP) Unit 2 steam dryer have been completed to reduce the stress levels in critical locations in the dryer. The objective of this paper is to describe the acoustic side branch concept and the design iterative processes that were undertaken at Laguna Verde Unit 2 to achieve a steam dryer design that meets the guidelines of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code Section III, Subsection NG high-cycle fatigue criteria and to ensure the safe operation of the steam dryer due to acoustic loads as part of the extended power uprate (EPU) from 2027 MWt to 2317 MWt yielding an increase in power of 14.3 percent. 1/9 Mexico Nuclear Society Conference Paper Rev. 2015
David R. Forsyth, et al, Impact Flow Induced Vibration Acoustic Loads on Design LV 2 Steam Dryer 1. INTRODUCTION Operating experience in Boiling Water Reactors (BWRs) has shown that steam flow through the main steam lines (MSLs) can cause acoustic pressure fluctuations due to the coincidence of vortex shedding frequencies and natural acoustic modes of safety relief valves (SRVs). The resonance that results from this coupling can contribute significant acoustic energy into the MSL system, which may propagate upstream into the reactor pressure vessel steam dome and drive structural vibration of steam dryer components. This can lead to high-cycle fatigue issues. Lock-in between the vortex shedding frequency and SRV natural frequency, as well as the ability for acoustic energy to propagate into the MSL system, are a function of many things, including the plant operating conditions, geometry of the MSL/SRV junction, and placement of SRVs with respect to each other on the MSLs. Evaluations have been performed to determine the acoustic loads due to possible resonance of any standpipes on the MSLs at Laguna Verde Unit 2. These evaluations involved multiple acoustic and structural analyses, scale model testing, and the collection of plant-specific data. Several computer codes which are both commercially-available and special-purpose codes developed in conjunction with the prediction and evaluation of acoustic loads were used in the dryer evaluations. The results of these evaluations indicated that the steam flow conditions at Laguna Verde Unit 2 are such that the original as-built plant will operate in a region susceptible to acoustic resonance of the SRVs. As a result, Comisión Federal de Electricidad (CFE) and Westinghouse designed, fabricated, and installed acoustic side branches (ASBs) on the MSLs and completed modifications to the critical stress locations on the steam dryer to ensure that the structural integrity of the steam dryer at EPU conditions is maintained. 2. DESIGN CONSIDERATIONS Reactor internals components, including the steam dryer, must be designed to meet the requirements of the applicable General Design Criteria, commensurate with their safety function. Although some internals components, such as the steam dryers, perform no safety function, they must retain their structural integrity to avoid generation of loose parts that may adversely impact the capability of other systems, structures, and components to perform their safety function. In addition to the installation of acoustic side branches at the MSL/SRV locations, to alter the dynamic steam flow effects in the MSLs and in the reactor pressure vessel steam dome, a number of steam dryer design changes were incorporated. The major steam dryer design changes were: a) steam dryer lifting rod bracket modifications, b) steam dryer drain channel weld reinforcement, and c) lower ring guide rod bracket weld reinforcement. These steam dryer modifications were implemented during the Laguna Verde Unit 2 2014 outage. The steam dryer locations that required modifications are shown in Figure 1. In accordance with the intent of BWRVIP-181-A (Reference 1), the steam dryer modifications were considered a Category B repair. The guidelines from BWRVIP-84 (Reference 2) were utilized for the steam dryer modification material specification, fabrication, and implementation. 2/9 Mexico Nuclear Society Conference Paper Rev. 2015
a b c Figure 1. Overall Laguna Verde Unit 2 steam dryer modifications installed a) lifting rod bracket modifications b) drain channel weld reinforcement c) lower ring guide rod bracket weld reinforcement 3/9 Mexico Nuclear Society Conference Paper Rev. 2015
David R. Forsyth, et al, Impact Flow Induced Vibration Acoustic Loads on Design LV 2 Steam Dryer 3. MSL/SRV AND STEAM DRYER MODIFICATIONS 3.1 Incorporation of Acoustic Side Branches Since a significant amount of acoustic energy in the MSL system was propagating upstream into the reactor pressure vessel steam dome and impinging on the outer hood of the steam dryer, one of the first design considerations was to install a series of acoustic side branches at selected MSL/SRV locations. The addition of a side branch to the standpipe, schematically shown in Figure 2, effectively shifts the resonant frequency of the standpipe, therefore eliminating the acoustic standing wave generated by vortex shedding expected at EPU flow conditions. In addition, the ASB is filled with mesh screens to provide a damping effect of the acoustic energy input to the system. Designing the type and quantity of mesh screens for the specific application, along with the proper dimensions of the ASB, allows the design of a customized ASB which is capable of a narrow-band suppression of the acoustic resonance frequency of interest. The effect of the ASB on the pressure response of an SRV is shown in Figure 3. It is important to note that the incorporation of the acoustic side branches on the SRVs does not impact the response or safety functions of the SRV. In addition, the ASBs do not impact the pressure drop and flow from the reactor to the inlet of the SRV. The pressure boundary piping of the ASBs was designed to the original code of construction, and the pressure piping and fittings material is equivalent to the existing main steam line material. Evaluations were performed to ensure that the structural integrity and pressure boundary integrity design function of the main steam line was not adversely affected. The incorporation of the acoustic side branches did require a minor change to the Laguna Verde Unit 2 in-service inspection program. Safety Relief Valve Acoustic Side Branch Main Steam Line Figure 2. Schematic of an acoustic side branch design 4/9 Mexico Nuclear Society Conference Paper Rev. 2015
Pressure response WITHOUT ASB Pressure response WITH ASB INSTALLED Figure 3. Effectiveness of an acoustic side branch design 3.2 Steam Dryer Lifting Rod Bracket Modification Each of the four (4) dryer lifting rods (see Figure 4) is supported by four (4) lifting rod brackets. The upper two (2) lifting rod brackets have a reinforcing bracing hardware welded above and below the brackets. The bottom two (2) brackets did not require additional bracing hardware, but their welds were required to be reinforced. Figure 4 shows the locations of the brackets on a lifting rod. The four (4) brackets are secured to the dryer bank end plate by fillet welds. The installed configuration of the lifting rod bracket hardware and reinforced welds (highlighted in blue) are presented in Figure 5. 5/9 Mexico Nuclear Society Conference Paper Rev. 2015
David R. Forsyth, et al, Impact Flow Induced Vibration Acoustic Loads on Design LV 2 Steam Dryer Figure 4. Lifting bar bracket reinforcing hardware and weld buildup locations Figure 5. Steam dryer lifting rod bracket modifications installed and highlighted 6/9 Mexico Nuclear Society Conference Paper Rev. 2015
3.3 Lower Ring Guide Rod Bracket Weld Reinforcement The Laguna Verde Unit 2 steam dryer has two (2) guide rod brackets for the vessel guide rods. Each guide bracket has two (2) gusseted vertical plates welded to the lower dryer ring. Each plate has two (2) vertical fillet welds that run the length of the plate on either side. The existing vertical welds have reinforcing fillet welds along their full lengths, making a total of eight (8) vertical weld modifications (4 per guide bracket). These reinforcing fillet welds doubled the size of the original fillet welds. Figure 6 shows these vertical welds (indicated in blue) on one of the guide rod brackets. Figure 6. Guide rod bracket weld buildup location 7/9 Mexico Nuclear Society Conference Paper Rev. 2015
David R. Forsyth, et al, Impact Flow Induced Vibration Acoustic Loads on Design LV 2 Steam Dryer 3.4 Steam Dryer Drain Channel Weld Reinforcement The Laguna Verde Unit 2 steam dryer has four (4) drain channels, each secured to the dryer skirt by two (2) vertical welds. Although some of the drain channel/skirt components are below the water line, a substantial portion of the drain channel experiences acoustic-related loadings that are transferred downward. During the design/analysis process, significant stress levels were calculated in the drain channel regions. To reduce these stress levels, the bottom portions of these welds were replaced with a stronger weld, for a total of eight (8) welds that were modified. The bottom few inches of these vertical welds were removed and replaced with full penetration groove welds. Additionally, these welds were further reinforced with fillet welds along the inside and outside of the drain channels. Figure 7 shows these weld buildup locations on the drain channels on one half of the dryer. Figure 7. Drain channel weld buildup locations on one side 8/9 Mexico Nuclear Society Conference Paper Rev. 2015
4. CONCLUSIONS Comisión Federal de Electricidad (CFE) and Westinghouse have successfully performed modifications to the Laguna Verde Nuclear Power Plant Unit 2 steam dryer and to the main steam line/safety relief valves locations to mitigate the impact of acoustic resonances of the side branches on the main steam line and their effect on the structural integrity of the steam dryer. The acoustic resonance of the side branches on the main steam lines (MSLs) was due to the coupling of the vortex shedding frequency, and natural acoustic frequency of safety relief valves (SRVs) which have been proactively mitigated by the installation of acoustic side branches on selected SRVs in the MSLs. In addition, modifications were made to the steam dryer at critical stress locations to reduce the stresses at those locations due to acoustic loads. With these changes, the steam dryer design meets the guidelines of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) code, Section III, Subsection NG high-cycle fatigue criteria for extended power uprate (EPU) conditions, ensuring the structural integrity of the steam dryer for a 14.3 percent power increase from 2027 MWt to 2317 MWt. REFERENCES 1. BWRVIP-181-A, BWR Vessel and Internals Project, Steam Dryer Repair Design Criteria, Final Report July, 2010. 2. BWRVIP-84, Rev. 2, BWR Vessel and Internals Project, Guidelines for Selection and Use of Materials for Repairs to BWR Internal Components, Final Report September 2012. 9/9 Mexico Nuclear Society Conference Paper Rev. 2015