Advanced FEA Modeling of PWSCC Crack Growth in PWR Dissimilar Metal Piping Butt Welds and Application to the Industry Inspection and Mitigation Program

2008 ◽  
Author(s):  
G. White ◽  
J. Broussard ◽  
J. Collin ◽  
M. Klug ◽  
C. Harrington ◽  
...  
Keyword(s):  
Stress Corrosion ◽  
Crack Growth ◽  
Circumferential Stress ◽  
Welding Residual Stress ◽  
Sensitivity Matrix ◽  
Element Analysis ◽  
Pressurized Water ◽  
Stability Calculation ◽  
Butt Welds ◽  
Dissimilar Metal

In late summer 2005, the U.S. pressurized water reactor (PWR) fleet imposed mandatory inspection requirements upon itself to address the challenge posed by primary water stress corrosion cracking (PWSCC) in PWR reactor coolant system (RCS) dissimilar metal (DM) piping butt welds. Under this program, the highest temperature, and thus most susceptible, locations have been addressed first. The set of highest temperature locations comprises the DM piping butt welds on the pressurizer. Within three years of promulgating the requirements, all pressurizer locations will have been inspected and nearly 90% of these locations will have been mitigated. In October 2006, several indications of circumferential flaws were reported in the pressurizer nozzles at Wolf Creek. These indications raised questions about the need to accelerate refueling outages or take mid-cycle outages at other plants. In order to address these concerns, an industry effort was undertaken to evaluate the viability of detection of leakage from a through-wall flaw in an operating plant to preclude the potential for rupture of pressurizer nozzle DM welds given the potential concern about growing circumferential stress corrosion cracks. Previous calculations of growth of PWSCC in Alloy 600 wrought materials and Alloy 82/182 weld metal materials have assumed an idealized crack shape, typically a semi-ellipse characterized by a length-to-depth aspect ratio. A key aspect of the industry effort involved developing an advanced finite-element analysis (FEA) methodology for predicting crack growth when loading conditions do not lead to a semi-elliptical flaw shape. The work also investigated an extensive crack growth sensitivity matrix to cover geometry, load, and fabrication factors, as well as the uncertainty in key modeling parameters including the effect of multiple flaw initiation sites in a single weld. Other key activities included detailed welding residual stress simulations covering the subject welds, development of a conservative crack stability calculation methodology, development of a leak rate calculation procedure using existing software tools (EPRI PICEP and NRC SQUIRT), and verification and validation studies. This paper will describe the study undertaken to model growth of circumferential weld cracks and its application to a group of nine PWRs with regard to implementation of the industry inspection and mitigation program [1]. The paper will also explore implementation progress of the industry program as the three-year mark approaches, as well as industry actions to support completion of baseline DM weld examinations.

Effect of Uneven Crack Front on Crack Tip Mechanics and the Implication to Stress Corrosion Crack Growth

2009 ◽  
Author(s):  
He Xue ◽  
Zhanpeng Lu ◽  
Hiroyoshi Murakami ◽  
Tetsuo Shoji
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Stress Corrosion ◽  
Crack Growth ◽  
Crack Front ◽  
Pressurized Water Reactor ◽  
Element Analysis ◽  
Pressurized Water ◽  
Water Reactor ◽  
Nickel Base

Uneven crack fronts have been observed in laboratory stress corrosion cracking tests. For example, cracking fronts of nickel-base alloys tested in simulated boiling water reactor (BWR) and pressurized water reactor (PWR) environments could exhibit uneven crack front. Analyzing the effect of an uneven crack front on further crack growth is important for quantification of crack growth. Finite-Element analysis shows that the local KI distribution can be significantly affected by the shape and size of the uneven crack front. Stress intensity factor at the locally extended crack front can be significantly reduced. Since generally there is a nonlinear CGR versus KI relationship, it is expected that crack growth rate at the locally extended crack front can be significantly different from those in the neighboring areas. There could be several patterns for the growth of an uneven crack front. For example, once initiated, the crack growth rate in areas other than the locally protruded front would become higher and then the whole crack front would tend to become uniform. On the other hand, if the crack growth in other areas is still low, there is a possibility that the crack growth rate at the front tip would slow down.

Through-Thickness Welding Residual Stress Profile in Dissimilar Metal Nozzle Butt Weld

2010 ◽  
Author(s):  
Tae-Kwang Song ◽  
Ji-Soo Kim ◽  
Chang-Young Oh ◽  
Hong-Yeol Bae ◽  
Jun-Young Jeon ◽  
...  
Keyword(s):  
Residual Stress ◽  
Welding Residual Stress ◽  
Stress Profile ◽  
Pressurized Water ◽  
Butt Weld ◽  
Butt Welds ◽  
Residual Stress Profile ◽  
Dissimilar Metal ◽  
Stress Profiles ◽  
Water Reactors

This paper provides the through-thickness welding residual stress profile in dissimilar metal nozzle butt welds of pressurized water reactors. For systematic investigations of the effects of geometric variables, i.e. the thickness and the radius of the nozzle and the length of the safe end, on welding residual stresses, idealized shape of nozzle is proposed and elastic-plastic thermo-mechanical finite element analyses are conducted. Through-wall welding residual stress profiles for dissimilar metal nozzle butt welds are proposed, which take a modified form of existing welding residual stress profiles developed for austenitic pipe butt weld in R6 code.

Crack Growth Evaluation of Remnant Cracks Underneath an Excavate and Weld Repair

2017 ◽  
Author(s):  
Francis H. Ku ◽  
Steven L. McCracken
Keyword(s):  
Weld Metal ◽  
Stress Corrosion ◽  
Crack Growth ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
Weld Repair ◽  
Element Analysis ◽  
Pressurized Water ◽  
Multiple Materials ◽  
Growth Evaluation

This paper presents a finite element analysis (FEA) based approach to perform crack growth evaluation of remnant cracks in a mockup with a partial arc excavate and weld repair (EWR). The partial arc EWR is a mitigation option to address stress corrosion cracking (SCC) in nuclear power plant piping systems. The mockup is a dissimilar metal weld (DMW) consisting of an SA-508 Class 3 low alloy steel forging buttered with Alloy 182 welded to a Type 316L stainless steel plate with Alloy 82/182 weld metal. This material configuration represents a typical DMW of original construction in a pressurized water reactor (PWR). To create a representative partial arc EWR application, the outer half of the DMW is excavated and repaired with Alloy 52M weld metal. The crack growth evaluation process presented herein represents an advanced method to evaluate the Alloy 82/182 remnant crack growth as required by ASME Code Case N-847 for implementing a partial arc EWR, which is currently being considered via letter ballot at ASME BPV Standards Committee XI. After the repair, any crack that remains in the Alloy 82/182 remnant and underneath the EWR needs to be evaluated for stress corrosion cracking (SCC) to assess its potential to grow beyond the EWR coverage area. Conventional fracture mechanics approach may not be suitable to evaluate such a remnant crack because of its close proximity to multiple materials of different mechanical properties and unconventional crack shape. In the crack growth evaluation, a crack that is reminiscent of a circumferential crack in a pipe, and a crack that is reminiscent of a laminar crack in a pipe are evaluated to predict the time for each of them to grow beyond the partial arc EWR coverage arc length. It is expected that the approach, analysis steps, calculation procedures presented in this paper will be applicable to analyzing a pipe geometry using realistic residual stresses and operating stresses for an EWR.

Preemptive Weld Overlays to Mitigate PWSCC Concerns in PWR Piping System Dissimilar Metal Butt Welds

2005 ◽  
Author(s):  
Peter C. Riccardella ◽  
Marcos L. Herrera ◽  
Arthur F. Deardorff ◽  
Shu S. Tang ◽  
Anthony J. Giannuzzi
Keyword(s):  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
Piping System ◽  
Primary System ◽  
Pressurized Water ◽  
Butt Welds ◽  
Dissimilar Metal ◽  
Structural Reinforcement ◽  
Weld Overlays

Primary water stress corrosion cracking (PWSCC) continues to be a concern in nickel-based alloys (Alloy 600 and the associated weld metals, Alloys 82 and 182) in pressurized water reactors (PWRs). It has caused cracking and leakage in a number of components, including steam generator tubes, vessel head penetrations, and most recently, the dissimilar metal butt welds (DMWs) commonly used to connect vessel nozzles to PWR primary system piping. Weld overlays (WOLs) have been used extensively in the past twenty years to repair nuclear plant piping that has been found to be cracked or leaking due to stress corrosion cracking [1]. This paper summarizes the advantages of and technical justification for applying preemptive weld overlays (PWOLs) before cracking or leakage is observed, to mitigate PWSCC in Alloy 82/182 butt welds. PWOL design is governed by a number of considerations. The PWOL must supply sufficient thickness of resistant material (Alloy 52 weld metal) to provide new structural reinforcement of the original pipe weld sufficient to sustain design basis loads within ASME Code margins. Structural reinforcement calculations are presented demonstrating the achievement of this capability in accordance with ASME Section XI rules for evaluation of flaws in austenitic piping. The PWOL must supply sufficient thickness to effectively reverse the highly tensile residual stresses from the original DMW, including the potential detrimental effects of an in-process repair weld. Residual stress evaluations using elastic-plastic finite element models are presented that demonstrate the achievement of this objective for several typical nozzle geometries. Finally, analyses are presented to demonstrate that a dissimilar metal weld, with PWOL applied, meets the Nuclear Regulatory Commission (NRC) criteria for leak-before-break (LBB).

Crack Growth Analysis in Alloy 82/182 Butt Weld by Using the Finite Element Alternating Method

2009 ◽  
Author(s):  
Maan-Won Kim ◽  
Young-Jong Kim ◽  
Byoung Chul Kim
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Growth Analysis ◽  
Welding Residual Stress ◽  
Analysis Model ◽  
Element Analysis ◽  
Pressurized Water ◽  
Butt Weld ◽  
Butt Welds ◽  
Alternating Method

Primary water stress corrosion cracking (PWSCC) of Alloy 82/182 butt welds has been a concern for pressurized water reactor (PWR) plants worldwide for the past decade. A lot of works have been performed to calculate exact welding residual stresses and PWSCC growth rate for Alloy 82/182 butt welds. The PWSCC growth analysis of Alloy 82/182 butt welds has been performed by using the Raju-Newman type solutions for the crack tip stress intensity factors (SIFs). In this study, a finite element alternating method (FEAM) was used to calculate the SIFs and crack propagation in Alloy 82/182 butt weld. The FEAM is consisted of two solution parts: an exact theoretical SIF solution for a crack embedded in infinite body and a simple finite element analysis model with coarse mesh for finite body. In this study, the theoretical SIFs were derived by using a dislocation density function. Finite element (FE) analysis was performed to obtain the welding residual stress distribution for a nozzle with Alloy 82/182 butt weld and PWSCC growth analysis was performed by using the FEAM with PWSCC growth model described in MRP reports under welding residual stress along the nozzle thickness.

NRC/EPRI Welding Residual Stress Validation Program: Phase III Details and Findings

2011 ◽  
Author(s):  
Lee F. Fredette ◽  
Matthew Kerr ◽  
Howard J. Rathbun ◽  
John E. Broussard
Keyword(s):  
Residual Stress ◽  
Stainless Steel ◽  
Residual Stresses ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Plant Material ◽  
Welding Residual Stress ◽  
Phase Iii ◽  
Pressurized Water ◽  
Dissimilar Metal

The US Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) are working cooperatively under a memorandum of understanding to validate welding residual stress predictions in pressurized water reactor primary cooling loop components containing dissimilar metal (DM) welds. These stresses are of interest as DM welds in pressurized water reactors are susceptible to primary water stress corrosion cracking (PWSCC) and tensile weld residual stresses are one of the primary drivers of this stress corrosion cracking mechanism. The NRC/EPRI welding residual stress (WRS) program currently consists of four phases, with each phase increasing in complexity from lab size specimens to component mock-ups and ex-plant material. This paper discusses Phase III of the WRS characterization program, comparing measured and predicted weld residual stresses profiles through the dissimilar metal weld region of pressurizer safety and relief nozzles removed from a cancelled plant in the United States. The DM weld had already been completed on all of the plant nozzles before use in the mock-up program. One of the nozzles was completed with the application of the stainless steel safe-end weld to a section of stainless steel pipe. Measurements were taken on the nozzles with and without the welded pipe section. Several independent finite element analysis predictions were made of the stress state in the DM weld. This paper compares the predicted stresses to those found by through-thickness measurement techniques (Deep Hole Drilling and Contour Method). Comparisons of analysis results with experimental data will allow the NRC staff to develop unbiased measures of uncertainties in weld residual stress predictions with the goal of developing assurances that the analysis predictions are defensible through the blind validation provided using well controlled mock-ups and ex-plant material in this program.

Further Welding Residual Stress and Flaw Tolerance Assessment of Dissimilar Metal Welds With Alloy 52 Inlays

2010 ◽  
Author(s):  
D. Rudland ◽  
F. Brust ◽  
D. J. Shim ◽  
T. Zhang
Keyword(s):  
Residual Stress ◽  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Safety Concern ◽  
Mitigation Strategies ◽  
Welding Residual Stress ◽  
Residual Stress Field ◽  
Pressurized Water ◽  
Dissimilar Metal

Primary water stress corrosion cracking (PWSCC) in nickel-based dissimilar metal (DM) welds (specifically Alloy 82/182 welds) in pressurized water reactors (PWRs) can cause a safety concern due to the high crack growth rate and irregular shaped flaws. Since many of these welds reside in primary piping systems that have been approved for Leak-Before-Break (LBB), the domestic commercial nuclear power industry has proposed a number of mitigation strategies for dealing with the issue and assuring LBB is still applicable. Some of these methods include Mechanical Stress Improvement Process (MSIP), Full and Optimized Structural Weld Overlay (FSWOL, OWOL), and Inlay and Onlay cladding. The industry claims that these methods provide either a reduction in the inner diameter residual stress field (MSIP and WOL), and/or apply a non-susceptible corrosion resistant barrier to stop or retard PWSCC crack growth to form a through-wall leak path (WOL, Inlay, Onlay). At last years PVP conference, a companion paper was published that described the initial welding residual stress and flaw evaluation analyses to investigate the effectiveness of inlay welds as a mitigative technique. The results from that effort suggested that the time to leakage with an inlayed weld is highly affected by the depth of the inlay and the crack growth rate within the inlay. In this ongoing effort, further welding residual stress analyses are presented that investigate the effects of the inlay depth and a variety of weld repair options before the standard 3mm deep inlay. In addition, further crack growth analyses, assuming idealized crack shapes, were conducted to investigate the effects of weld residual stress, crack growth rate, global bending stress, and flaw size and orientation. The results of these analyses aid in determining appropriate inspection intervals for dissimilar metal welds with this mitigation technique.

NRC/EPRI Residual Stress Validation Program Phase I: Experimental Specimen Modeling and Measurement

2011 ◽  
Author(s):  
J. Broussard ◽  
P. Crooker
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Phase 1 ◽  
Measurement Techniques ◽  
Corrosion Cracking ◽  
Welding Residual Stress ◽  
Hole Drilling ◽  
Pressurized Water

The US Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) are working cooperatively under a memorandum of understanding to validate welding residual stress predictions in pressurized water reactor primary cooling loop components containing dissimilar metal welds. These stresses are of interest as DM welds in pressurized water reactors are susceptible to primary water stress corrosion cracking (PWSCC) and tensile weld residual stresses are one of the primary drivers of this stress corrosion cracking mechanism. The NRC/EPRI weld residual stress (WRS) program currently consists of four phases, with each phase increasing in complexity from lab size specimens to component mock-ups and ex-plant material. This paper describes the Phase 1 program, which comprised an initial period of learning and research for both FEA methods and measurement techniques using simple welded specimens. The Phase 1 specimens include a number of plate and cylinder geometries, each designed to provide a controlled configuration for maximum repeatability of measurements and modeling. A spectrum of surface and through-wall residual stress measurement techniques have been explored using the Phase 1 specimens, including incremental hole drilling, ring-core, and x-ray diffraction for surface stresses and neutron diffraction, deep-hole drilling, and contour method for through-wall stresses. The measured residual stresses are compared to the predicted stress results from a number of researchers employing a variety of modeling techniques. Comparisons between the various measurement techniques and among the modeling results have allowed for greater insight into the impact of various parameters on predicted versus measured residual stress. This paper will also discuss the technical challenges and lessons learned as part of the DM weld materials residual stress measurements.

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