PVP PVP EFFECTS OF WELD OVERLAYS ON LEAK-BEFORE BREAK MARGINS

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1 Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference PVP2011 July 17-21, 2011, Baltimore, Maryland, USA Proceedings of PVP ASME Pressure Vessels and Piping Division Conference July 17-21, 2011, Baltimore, USA EFFECTS OF WELD OVERLAYS ON LEAK-BEFORE BREAK MARGINS PVP PVP G. Angah Miessi Structural Integrity Associates, Inc. San Jose, CA Peter C. Riccardella Structural Integrity Associates, Inc. Centennial, CO Peihua Jing Structural Integrity Associates, Inc. San Jose, CA ABSTRACT Weld overlays have been used to remedy intergranular stress corrosion cracking (IGSCC) in boiling water reactors (BWRs) since the 1980s. Overlays have also been applied in the last few years in pressurized water reactors (PWRs) where primary water stress corrosion cracking (PWSCC) has developed. The weld overlay provides a structural reinforcement with SCC resistant material and favorable residual stresses at the ID of the overlaid component. Leak-beforebreak (LBB) had been applied to several piping systems in PWRs prior to recognizing the PWSCC susceptibility of Alloy 82/182 welds. The application of the weld overlay changes the geometric configuration of the component and as such, the original LBB evaluation is updated to reflect the new configuration at the susceptible weld. This paper describes a generic leak-before-break (LBB) analysis program which demonstrates that the application of weld overlays always improves LBB margins, relative to un-overlaid, PWSCC susceptible welds when all the other parameters or variables of the analyses (loads, geometry, operating conditions, analysis method, etc ) are kept equal. Analyses are performed using LBB methodology previously approved by the US NRC for weld overlaid components. The analyses are performed for a range of nozzle sizes (from 6 to 34 ) spanning the nominal pipe sizes to which LBB has been commonly applied, using associated representative loads and operating conditions. The analyses are performed for both overlaid and un-overlaid configurations of the same nozzles, and using both fatigue and PWSCC crack morphologies in the leakage rate calculations and the LBB margins are compared to show the benefit of the weld overlays. LEAK-BEFORE-BREAK INTRODUCTION The purpose of a leak-before-break (LBB) evaluation is to demonstrate through deterministic fracture mechanics analyses that through-wall flaws in high energy piping systems will result in leaks that can be detected by the plant leak detection system before the flaws grow to critical through-wall flaw sizes that can result in double-ended guillotine break. The demonstration of LBB for a particular high-energy piping system permits the removal of the massive protective devices such as pipe whip restraints on that system which are required by USNRC regulation [1] to protect against the dynamic effects of a postulated double-ended guillotine break. In addition, other dy- 1 Copyright 2011 by ASME

2 namic effects such as those due to jet impingement shields and reactor internal loadings need not be included as a condition for design once LBB has been demonstrated. General technical guidance for LBB evaluation is provided in Section 5 of NUREG-1061, Vol. 3 [1] and SRP [2]. In particular, Section 5.2 of NU- REG-1061, Vol. 3 provides a detailed step-by-step approach for performing LBB evaluations. A summary of the key technical requirements is provided below: - The limitations imposed in NUREG-1061, Vol. 3 on the use of LBB for high-energy piping is addressed for the particular system under consideration. LBB is not considered applicable to systems if operating experience indicates particular susceptibility to failure from the effects of various corrosion mechanisms (e.g., intergranular stress corrosion cracking, IGSCC, or flow assisted corrosion, FAC), water hammer, or low and high cycle fatigue. NUREG-1061, Vol. 3 and SRP require a margin of 2 on flaw size which means that the critical flaw size must be equal or greater than twice the leakage flaw size, with the leakage flaw size defined has the size of the through-wall flaw that would lead to leakage 10 times greater than the plant minimum leakage detection capability during normal operation. - Crack growth evaluation of sub-critical flaws is performed to show that they will not grow to the critical flaw size between inspections. LBB for the piping system is demonstrated if adequate margin exists between the leakage flaw size and the critical flaw size and if there is adequate inspection interval to supplement the LBB evaluation. WELD OVERLAY LBB METHODOLOGY One of the limitations imposed by the NRC in SRP [2] and NUREG-1061, Vol.3 [1] is that locations on piping systems that are susceptible to corrosion mechanisms such as PWSCC do not qualify for application of LBB. However, in a Revision 1 of SRP 3.6.3, it is stated that nonconforming piping that has been treated by two mitigation methods may qualify for LBB if the piping contains no flaws larger than those permitted by ASME Code, Section XI without repair. The weld overlay provides an additional structural reinforcement with SCC resistant material and favorable residual stresses at the ID of the weld overlaid component. LBB evaluations involve critical flaw size calculations as well as leakage rate calculations. The methodologies for the critical flaw size and leakage calculations are summarized in the following sections. 1. Critical Flaw Size Determination Standard Review Plan (SRP) [2] provides methods for determining critical flaw size using the net section collapse approach. The original methodology, as described in Reference 1, is based on a single material cylinder. In the case of an overlaid piping location, the original weld is repaired by applying a weld overlay using a different material compared to the original weld material. Hence, a revised methodology is needed to consider both materials such that the intent of SRP can be met. Deardorff et al. proposed a method to determine the critical through-wall flaw size for circumferential cracked pipe with weld overlays [3]. The schematic of this configuration is shown in Figure 1. The methodology is based on the net section collapse solution, but has some additional considerations due to the weld overlay: The effects of two materials are considered. The limit load tensile force and bending moment can be evaluated for the circumferential cracked pipe with weld overlays. The analytical model allows for the arbitrary definition of the circumferential through-wall crack length for the weld overlay and both circumferential crack length and depth (in the radial direction 2 Copyright 2011 by ASME

3 from the inside wall) for the base material as illustrated in Figure 2). The evaluation method allows for a reduction in the fracture toughness of the materials to be considered. The Z-factor load multiplier can be applied to the specific material to account for the reduced material fracture toughness (e.g. for thermally aged material). 2. Leakage Rate Calculation The determination of the leakage rate is performed using the EPRI program, PICEP [4]. The flow rate equations in PICEP are based on Henry s homogeneous non-equilibrium critical flow model [5]. The program accounts for non-equilibrium flashing mass transfer between liquid and vapor phases, fluid friction due to surface roughness and convergent flow paths. To determine material properties for the leakage evaluations of overlaid locations with PICEP, composite material properties will be used based upon the relative thicknesses of the base material and the weld overlay. For example, the yield stress, can be calculated as S y Composite t base S t y base base t t WOL WOL S This applies to modulus of elasticity, yield strength, and Ramberg-Osgood parameters. An important parameter in the leak rate calculations is the crack morphology which is characterized by the crack surface roughness, number of turns along the flow path through the pipe thickness, etc... Primary Water Stress Corrosion Cracking (PWSCC) morphology has been shown to be more restrictive than fatigue cracking morphology which was used in all LBB evaluations prior to the discovery of the PWSCC susceptibility of Alloy 600 base metal and its associated Alloy 82/182 weld metals [6]. Thus, in LBB evaluations of weld overlaid dissimilar metal y WOL weld (DMW) locations, PWSCC crack morphology has been considered for calculating the leak rates. In such cases, the adverse effects of PWSCC crack morphology are considered for leakage through the affected materials and, for the non-susceptible materials such as the Alloy 52M weld overlay metal, the crack morphology for fatigue cracking is used. Below are the PWSCC crack morphology parameters as described in Reference 6: μ L = in where, μ G = in N 90 = L G /t = L/t = 1.243, μ L μ G N 90 L G /t L/t = Local roughness, inches = Global roughness, inches = Number of 90 degree turns per inch = Global flow path length to thickness ratio = Global plus local flow path length to thickness ratio For non-susceptible materials (i.e., Alloy 52M weld metal for the overlay, CASS, carbon steel, etc.), the crack morphology for fatigue cracking is used. Below are the fatigue crack morphology parameters as described in Reference 7: μ L = in μ G N 90 = 0 = in L G /t = L/t = Copyright 2011 by ASME

4 Figure 1 Weld Overlay Configuration Similarly, the required piping interface loads at the weld locations under consideration are obtained from plant design reports and presented in Table 1. Per the requirements of SRP 3.6.3, normal operating (NOP) load combinations consisting of internal pressure and normal operating forces and moments (deadweight + thermal expansion) are applied for the leakage evaluations, while the load combinations for critical flaw size evaluations consist of the normal operation loads and safe-shutdown earthquake (SSE) loads. The internal pressure and temperature at normal operating conditions for each of the selected locations are also listed in Table 1. Elastic and elastic-plastic material properties (modulus of elasticity, yield strength, and Ramberg- Osgood parameters) for the Alloy 82/182 and Alloy 52 weld metals at the normal operating temperature for each of the selected locations are used in the LBB analyses. TABLE 1: LBB EVALUATION DESIGN INPUT Figure 2 Pipe Cross Section of Through-wall Circumferential Crack with Applied Overlay LBB ANALYSIS INPUTS Four typical nozzle/safe-end locations where weld overlays have been applied to mitigate against PWSCC of the dissimilar metal welds are selected for this analysis program. The locations are selected to represent the range of nominal pipe sizes in US PWR plants that have been qualified for LBB application. The selected locations are: o 34 Reactor Coolant Pump (RCP) Suction nozzle o 14 Surge Nozzle o 12 Safety Injection Nozzle o 6 Safety Relief Nozzle The characteristic dimensions of the selected weld locations for LBB evaluation are listed in Table 1. The dimensions which include the weld overlay thickness are obtained from actual LBB applications from different plants. Input Dimensions (in) Critical Flaw Size Loads Leakage Loads RCP Suction Nozzle Surge Nozzle Safety Safety Injection Relief Nozzle Nozzle Do_pipe Di_pipe t_pipe t_overlay F Axial (kips) M Total (in-kips) M Primary (in-kips) F Axial (kips) M NOP (in-kips) NOP Temp ( o F) Press(psia) Nomenclature: Do_pipe = Outer diameter of base material Di_pipe = Inner diameter of base material t_pipe = Thickness of base material 4 Copyright 2011 by ASME

5 t_overlay = Thickness of weld overlay F Axial = Axial force M Total = Deadweight (DW) + Safe-Shutdown Earthquake (SSE) +Thermal Expansion M Primary = DW+SSE NOP = Normal operating condition RESULTS AND DISCUSSION The LBB analyses are performed for both overlaid and un-overlaid configurations of each of the four selected nozzle locations to which LBB has been commonly applied, with associated representative geometries, loads and operating conditions as shown in Table 1. The calculated critical flaw sizes for a range of nozzle sizes (from 6 to 34 ) are shown in Table 2. As expected, in each case, the additional thickness provided by weld overlay makes the pipe more resistant to net section collapse, thus resulting in a larger critical flaw size compared to the unoverlaid pipe. The increase in critical flaw size varies from 21% at the RCP suction nozzle to as much as 52% at the surge nozzle. Using the calculated critical flaw sizes, two sets of leakage rate analyses are conducted which show the impact of the weld overlay and crack morphology on LBB margins. The first set of leakage analyses are performed using PWSCC crack morphology for nonoverlaid and combined PWSCC/fatigue morphology for overlaid configurations. The leak rates results for cracks equal to half of the critical flaws at each of the weld locations are shown in Table 2. The leakage comparison between overlaid and un-overlaid configurations indicates that, in all the cases studied, pipes with weld overlays produce higher leakage than the original pipes when all the other aspects of the analyses (loads, geometry, operating conditions, analysis method, etc ) are kept equal. The increase in leakage rate due to the weld overlay varies from 8% at the RCP suction nozzle to as much as 64% at the surge nozzle. Since the higher leakage means more safety margin for leakage detection, the overlaid locations are yielding LBB margins than un-overlaid pipe. The second set of analyses are performed using fatigue crack morphology only for both the un-overlaid pipe and pipe with weld overlay in order to determine the effects of crack morphology on the leakage. The leak rates through cracks of lengths equal to half of the critical flaw sizes are again calculated and listed in Table 2. The results show that, with all the other aspects of the analyses kept equal, pipes with weld overlays produce slightly more (~ 2%) leakage than un-overlaid pipes using fatigue morphology, except for the case of the surge nozzle which shows an increase in leakage rate of approximately 47%. TABLE 2: LBB EVALUATION RESULTS RCP Safety Safety Suction Surge Injection Relief Analysis Cases Nozzle Nozzle Nozzle Nozzle Critical Flaw Sizes (in) No Overlay With Overlay Leakage Rates (gpm) PWSCC Considered No Overlay (PWSCC) With Overlay (PWSCC+Fatigue) Leakage Rates (gpm) No PWSCC No Overlay (Fatigue) With Overlay (Fatigue) Note: Leakage rate calculations were performed for crack lengths equal to ½ the critical crack sizes. An additional comparison is made between the two sets of analyses to show the effect of the crack morphology on leakage. Comparing the "No Overlay (PWSCC)" case of the first set and the "No Overlay (Fatigue)" of the second set, it is can be seen that 5 Copyright 2011 by ASME

6 compared to the fatigue cracking morphology, the PWSCC morphology has a strong adverse effect on leakage, reducing the leak rates, and thus the associated LBB margins, by about a factor of 5. It is also noteworthy that, for all nozzle sizes and crack morphologies, comparing the no overlay to with overlay cases, overlays always increase the leakage rates at 1/2 the critical crack size (and thus the LBB margins). CONCLUSION LBB analyses have been performed for a set of nozzle locations with nominal pipe sizes ranging from 6" to 34", using representative pipe and weld overlay dimensions, loads and operating conditions. Critical flaw sizes and leakage rates for both overlaid and unoverlaid configurations of each of the nozzles were determined with consideration of PWSCC and/or fatigue cracking morphology. Comparing the results of the analyses for the unoverlaid and overlaid configurations, it was shown that the weld overlay provided higher LBB margins when either fatigue or PWSCC crack morphologies were considered in the leakage rate calculations. In addition, the results of the analyses confirm that applying PWSCC morphology rather than fatigue cracking morphology to LBB evaluations can significantly reduce the LBB margins. This analysis program can be expanded to add more nominal pipe sizes and increase the number of representative weld locations analyzed in order to further strengthen the conclusion that weld overlays always improve LBB margins. REFERENCES 1. NUREG-1061, Volume 3, Report of the U.S. Nuclear Regulatory Commission Piping Review Committee, prepared by the Piping Review Committee, NRC, April U.S. Nuclear Regulatory Commission, Standard Review Plan 3.6.3, "Leak-before-Break Evaluation Procedures," Revision 1, March, Deardorff, A, et.al., "Net Section Plastic Collapse Analysis of Two-Layered Materials and Application to Weld Overlay Design," PVP2006- ICPVT , ASME PVP Conference D. M. Norris and B. Chexal, PICEP: Pipe Crack Evaluation Program (Revision 1), EPRI NP SR, Revision 1, December P.E. Henry, The two-phase Critical Discharge of Initially Saturated or Subcooled Liquid, Nuclear Science and Engineering, Vol. 41, Rudland, D., Wolterman, R., Wilkowski, G., Tregoning, R., Impact of PWSCC and Current Leak Detection on Leak-Before-Break, Proceedings of Conference on Vessel Head Penetration, Inspection, Cracking, and Repairs, Sponsored by USNRC, Marriot Washingtonian Center, Gaithersburg, MD, September 29 to October 2, D. Abdollahian and B. Chexal, Calculation of Leak Rates Through Cracks in Pipes and Tubes, EPRI NP-3395, Electric Power Research Institute, Palo Alto, CA, December Copyright 2011 by ASME