Uncertainty Sampling of Weld Residual Stress Fields in Probabilistic Analysis: Part II — Examples

2016 ◽  
Author(s):  
Robert E. Kurth ◽  
Cédric J. Sallaberry ◽  
Frederick W. Brust ◽  
Elizabeth A. Kurth ◽  
Michael L. Benson ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Residual Stresses ◽  
Assessment Tool ◽  
Lessons Learned ◽  
Piping System ◽  
Primary System ◽  
Cracking Behavior ◽  
System Pressure ◽  
Piping Systems ◽  
Low Probability

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. US NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, conducted a multi-year project that focused on the development of a viable method and approach to address the effects of PWSCC in primary piping systems approved for LBB. This project, called eXtremely Low Probability of Rupture (xLPR) [1], defined the requirements necessary for a modular-based probabilistic fracture mechanics assessment tool to directly assess compliance with the regulations. Using the lessons learned from the pilot study, the production version of this code, designated as Version 2.0, focused on those primary piping systems previously approved for LBB. In this version the appropriate fracture mechanics-based models are employed to model the physical cracking behavior and a variety of computational options are provided to characterize, categorize and propagate problem uncertainties. One of the most influential uncertainty on risk in the xLPR code is the one associated with weld residual stresses (WRS). WRS plays a key role in both crack initiation and crack growth. PWSCC is mainly driven by tensile stresses, whose major contributors are the tensile weld residual stresses that develop during fabrication of the piping system. Handling the uncertainty involved with WRS within a probabilistic framework is quite challenging. A companion paper presents the selected approach to represent uncertainty within the framework of the xLPR code while respecting a set of requirements in term of smoothness of profile, efficiency of (potential) importance sampling and (for axial WRS) equilibrium. This paper illustrate with examples the implementation of the described methods into xLPR v2.0.

Uncertainty Sampling of Weld Residual Stress Fields in Probabilistic Analysis: Part I — Theory

2016 ◽  
Author(s):  
Robert E. Kurth ◽  
Cédric J. Sallaberry ◽  
Frederick W. Brust ◽  
Elizabeth A. Kurth ◽  
Michael L. Benson ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Residual Stresses ◽  
Assessment Tool ◽  
Lessons Learned ◽  
Piping System ◽  
Primary System ◽  
Cracking Behavior ◽  
System Pressure ◽  
Piping Systems ◽  
Low Probability

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. US NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, conducted a multi-year project that focused on the development of a viable method and approach to address the effects of PWSCC in primary piping systems approved for LBB. This project, called eXtremely Low Probability of Rupture (xLPR), defined the requirements necessary for a modular-based probabilistic fracture mechanics assessment tool to directly assess compliance with the regulations [1]. Using the lessons learned from the pilot study [2] the production version of this code, designated as Version 2.0, focused on those primary piping systems previously approved for LBB [3]. In this version the appropriate fracture mechanics-based models are employed to model the physical cracking behavior and a variety of computational options are provided to characterize, categorize and propagate problem uncertainties. One of the most influential sources of uncertainty on risk in the xLPR code is the one associated with weld residual stresses (WRS). WRS plays a key role in both crack initiation and crack growth. PWSCC is mainly driven by tensile stresses, whose major contributors are the tensile weld residual stresses that develop during fabrication of the piping system. Handling the uncertainty involved with WRS within a probabilistic framework is quite challenging. This paper presents the selected approach to represent uncertainty within the framework of the xLPR code while respecting a set of requirements in term of smoothness of profile, efficiency of (potential) importance sampling and (for axial WRS) equilibrium. The current WRS sampling scheme employs correlation in order to smooth the shape of the WRS fields through the thickness of a dissimilar metal weld. This method presents an enrichment of the Cholesky decomposition on the correlation matrix, in order to satisfy the other two requirements.

Development of the Extremely Low Probability of Rupture (xLPR) Version 2.0 Code

2015 ◽  
Author(s):  
D. Rudland ◽  
C. Harrington ◽  
R. Dingreville
Keyword(s):  
Pilot Study ◽  
Fracture Mechanics ◽  
Lessons Learned ◽  
Primary System ◽  
Cracking Behavior ◽  
Code Structure ◽  
System Pressure ◽  
Piping Systems ◽  
Version 2.0 ◽  
Low Probability

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. US NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, conducted a multi-year project that focused on the development of a viable method and approach to address the effects of PWSCC in primary piping systems approved for LBB. This project, called eXtremely Low Probability of Rupture (xLPR), defined the requirements necessary for a modular-based probabilistic fracture mechanics assessment tool to directly assess compliance with the regulations. As reported in previous technical papers, the first version of the xLPR code was developed as part of a pilot study, which leveraged existing fracture mechanics based models and software coupled to both a commercial and open source code framework to determine the framework and architecture requirements appropriate for building a modular-based code with this complexity. Using the lessons learned from the pilot study, the production version of this code, designated as Version 2.0, focuses on those primary piping systems previously approved for LBB. In this version, the appropriate fracture mechanics-based models are employed to model the physical cracking behavior and a variety of computational options are provided to characterize, categorize and propagate problem uncertainties. This paper examines the xLPR Version 2.0 model by presenting a brief overview of the xLPR scope, the code structure, computational framework and fracture mechanics-based models. As a demonstration of the xLPR Version 2.0 capabilities, an example is presented that focuses on PWSCC in large-bore piping systems. This example exercises some functionalities of the xLPR code to demonstrate its application to assess compliance with 10CFR50 Appendix A, GDC-4. This paper concludes with a brief discussion on the path forward and plans for the control and maintenance of the xLPR code.

Extremely Low Probability of Rupture (xLPR) Version 1.0 Code: Pilot Study Problem Results

2011 ◽  
Author(s):  
D. Rudland ◽  
C. Harrington
Keyword(s):  
Pilot Study ◽  
Assessment Tool ◽  
Lessons Learned ◽  
Nuclear Industry ◽  
Piping System ◽  
Primary System ◽  
Degradation Mechanisms ◽  
System Pressure ◽  
Piping Systems ◽  
Low Probability

Nuclear Regulatory Commission (NRC) Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to demonstrate compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. Along with a series of existing qualitative steps to assure safety in LBB-approved lines experiencing PWSCC, NRC staff, working cooperatively with the nuclear industry through a memorandum of understanding with the Electric Power Research Institute, is developing a new, modular based, comprehensive piping system assessment methodology to directly demonstrate compliance with the regulations. This project, called xLPR (eXtremely Low Probability of Rupture), will model the effects and uncertainties of relevant active degradation mechanisms and the associated mitigation activities. The resulting analytical tool will be comprehensive, vetted with respect to the technical bases of models and inputs, flexible enough to permit analysis of a variety of in-service situations and adaptable such as to accommodate evolving and improving knowledge. A multi-year project has begun that will first focus on the development of a viable method and approach to address the effects of PWSCC as well as define the requirements necessary for a modular-based assessment tool. To meet this goal, the first version of this code has been developed as part of a pilot study, which leverages existing fracture mechanics based models and software coupled to both a commercial and an open source code framework to determine the framework and architecture requirements appropriate for building a modular-based code with this complexity. The pilot study focused on PWSCC in pressurizer surge nozzles, and is meant to demonstrate the feasibility of this code and approach and not to determine the absolute values of the probability of rupture. Later development phases will broaden the scope of xLPR to appropriate primary piping systems in pressurized and boiling water reactors (PWR and BWR), using an incremental approach that incorporates the design requirements and lessons learned from previous iterations. This paper specifically examines the xLPR Version 1.0 model, the methods and approach used to couple the deterministic modules within a probabilistic software framework, and the results from the pilot study. A comparison of the results specific to the surge nozzle sample problem is presented. This paper concludes with lessons learned from the pilot study.

scholarly journals Development of Computational Framework and Architecture for Extremely Low Probability of Rupture (XLPR) Code

2010 ◽  
Author(s):  
D. Rudland ◽  
P. Mattie ◽  
R. Kurth ◽  
H. Klasky ◽  
B. Bishop ◽  
...  
Keyword(s):  
Pilot Study ◽  
Nuclear Industry ◽  
Current Status ◽  
Study Phase ◽  
Piping System ◽  
Primary System ◽  
Degradation Mechanisms ◽  
System Pressure ◽  
Piping Systems ◽  
Low Probability

The Nuclear Regulatory Commission (NRC) Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to demonstrate compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB exemptions. Along with the existing qualitative steps to assuring safety in LBB lines with PWSCC, the NRC staff, working cooperatively with the nuclear industry through a memorandum of understanding, is developing a new, modular based, comprehensive piping system assessment methodology to directly demonstrate compliance with the regulations. This tool, called xLPR (eXtremely Low Probability of Rupture), would properly model the effects and uncertainties of both active degradation mechanisms and the associated mitigation activities. The tool will be comprehensive with respect to known challenges, vetted with respect to scientific adequacy of models and inputs, flexible enough to permit analysis of a variety of in-service situations and adaptable such as to accommodate evolving and improving knowledge. A multi-year project has begun that will first focus on the development of a viable method and approach to address the effects of PWSCC as well as define the requirements necessary for a modular-based assessment tool. A prototype xLPR model and pilot study case is first being conducted leveraging existing fracture mechanics models and software coupled to both a commercial and open source code framework to determine the framework and architecture requirements appropriate for building a modular-based code with this complexity. The pilot study phase is focusing on PWSCC in pressurizer surge nozzles. Later development phases will broaden the scope of xLPR to all primary piping systems in pressurized and boiling water reactors (PWR and BWR), using an incremental approach that incorporates the design requirements and lessons learned from previous iterations. This paper specifically examines the prototype xLPR model and includes the methods and approach used to couple existing models and software as modules within a probabilistic software framework. Since the pilot study is currently still ongoing, this paper provides a discussion of the current status and plans to move forward after the pilot study is complete.

Initial Development of the Extremely Low Probability of Rupture (xLPR) Version 2.0 Code

2012 ◽  
Author(s):  
D. Rudland ◽  
C. Harrington
Keyword(s):  
Pilot Study ◽  
Piping System ◽  
Primary System ◽  
Development Effort ◽  
Degradation Mechanisms ◽  
Pressurized Water ◽  
Initial Development ◽  
System Pressure ◽  
Version 2.0 ◽  
Low Probability

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. Along with a series of existing qualitative steps to assure safety in LBB-approved lines experiencing PWSCC, NRC staff, working cooperatively with the Electric Power Research Institute through a memorandum of understanding, is developing a new, modular based, comprehensive piping system assessment methodology to directly assess compliance with the regulations. This project, called eXtremely Low Probability of Rupture (xLPR), will model the effects and uncertainties of relevant active degradation mechanisms and the associated mitigation activities. The resulting analytical tool will be comprehensive with respect to known and significant materials challenges (PWSCC, etc.), vetted with respect to the technical bases of models and inputs, flexible enough to permit analysis of a variety of in-service situations and adaptable such as to accommodate evolving and improving knowledge. A multi-year project has begun that has been focused on the development of a viable method and approach to address the effects of PWSCC as well as define the requirements necessary for such a modular-based assessment tool. As reported in a previous paper, the first version of this code was developed as part of a pilot study, which leveraged existing fracture mechanics based models and software coupled to both a commercial and open source code framework to determine the framework and architecture requirements appropriate for building a modular-based code with this complexity. The pilot study focused on PWSCC in pressurizer surge nozzles, and was meant to demonstrate the feasibility of this code and approach and not to determine the absolute values of the probability of rupture. This paper examines the plans for the xLPR Version 2.0 model which will broaden the scope of xLPR to all LBB-approved primary piping in pressurized water reactors (PWR), using an incremental approach that incorporates the design requirements and lessons learned from previous iterations. After a review of the Version 1.0 final results, this paper will document the plans for Version 2.0 including the revised management structure, the technical scope, and the progress in the code development effort to date.

Benchmarking Probabilistic Code for LBB Analysis for Circumferential Cracks

2017 ◽  
Author(s):  
Robert E. Kurth ◽  
Cédric J. Sallaberry ◽  
Bruce A. Young ◽  
Paul Scott ◽  
Frederick W. Brust ◽  
...  
Keyword(s):  
Probability Of Failure ◽  
Primary System ◽  
Degradation Mechanisms ◽  
Detection Systems ◽  
System Pressure ◽  
Piping Systems ◽  
Primary Water ◽  
Low Probability ◽  
Assessment Procedures ◽  
Leak Before Break

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. There are several codes available for addressing the requirements of GDC-4. This paper addresses three of these codes: (1) xLPR 2.0; (2) PROLOCA; and (3) PROMETHEUS. Each of these codes is described and applied to a representative plant where active degradation mechanisms have been found. Conclusions about the design, results, and interpretation of the results is then provided. In all cases the probability of failure of the pipe is found to be extremely low when the crack inspections and leak detection systems are modeled.

Analysis and Testing of Freezing Phenomena in Piping Systems

2008 ◽  
Author(s):  
Kenneth M. Smith ◽  
Michael P. Van Bree ◽  
Joseph F. Grzetic
Keyword(s):  
Weak Link ◽  
Frozen Section ◽  
Cooling Water ◽  
Freezing Temperature ◽  
Elevated Pressure ◽  
Piping System ◽  
Actual System ◽  
System Pressure ◽  
Piping Systems ◽  
The Cost

From time to time installed piping systems, including wet sprinkler, potable water and cooling water systems, have failed due to freezing. The cost of such events is significant due to the need for system repair, the loss of service, and the secondary water damage that results. The presumption is often that the pipe was frozen at the point of failure. This paper describes the theory, calculations, and experiments that demonstrate that freezing of a pipe causes pressure in the system to rise between the frozen section of pipe and the blind end of the system. The piping system typically fails at a weak link in the system at a significantly elevated pressure. The location of the breech is almost always at a point in the system where the water is still liquid at the time of the breech. An additional consideration is that the high system pressure depresses the freezing temperature of the water, and a flash-freeze within the pipe occurs when the pressure suddenly drops after a piping system rupture. Armed with this knowledge, the site of the actual system freeze can often be found remote from the point of failure where remedy can be best effected to eliminate the root cause.

Lessons Learned for Nuclear Piping Integrity in New Reactors

2011 ◽  
Author(s):  
G. Wilkowski ◽  
F. Brust ◽  
P. Krishnaswamy ◽  
K. Wichman ◽  
D.-J. Shim
Keyword(s):  
Power Plant ◽  
Nuclear Power Plant ◽  
Nuclear Power ◽  
Structural Integrity ◽  
Lessons Learned ◽  
Dynamic Strain ◽  
Piping System ◽  
Aging Effects ◽  
Piping Systems ◽  
Loading Effects

From the early 1980’s to the present time, there has been a significant amount of research and development on the structural integrity of nuclear power plant piping. From those efforts, there are a number of lessons that could be applied to design and fabrication of new nuclear power plant piping systems. In this paper, the various aspects evaluated in NRC-funded efforts for understanding degraded piping were reviewed and implications on how to avoid detrimental aspects were discussed, as well as some more recent efforts. Some of these aspects include; (1) materials aspects (variability of wrought stainless steel base metal toughness with composition, dynamic strain aging effects on toughness of ferritic steels, fracture toughness in HAZ/fusion lines, material anisotropy effects on toughness, effects of static versus dynamic loading on material toughness, cyclic loading effects during seismic loading on toughness, thermal aging effects on strength and toughness), (2) designing weld sequencing to avoid SCC cracking; (3) crack morphology effects on leak-rate evaluations, (4) system effects that can significantly affect the structural integrity analysis of the piping system (secondary stresses, restraint of pressure induced bending, system displacement and rotation constraints, and margins associated from full dynamic analyses).

Development of Initial Input Parameters for a Probabilistic Pipe Rupture Assessment Code for Nuclear Reactor Coolant Pressure Boundary Leak-Before-Break Analyses

2010 ◽  
Author(s):  
Eric M. Focht ◽  
Guy DeBoo
Keyword(s):  
Pilot Study ◽  
Nuclear Reactor ◽  
Computer Code ◽  
Nuclear Regulatory Commission ◽  
Primary System ◽  
Reactor Coolant ◽  
System Pressure ◽  
Input Parameters ◽  
Low Probability ◽  
Leak Before Break

The U.S. Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) entered into a cooperative agreement to develop a modular computer code for determining the probability of rupture of reactor coolant pressure boundary piping. The goal of xLPR project (Extremely Low Probability of Rupture), as it is known, is to develop a flexible modular code that satisfies the assessment methodology in the NRC Standard Review Plan (SRP) 3.6.3 leak-before-break analysis. The NRC SRP 3.6.3 provides guidance on satisfying Title 10 of the Code of Federal Regulations Part 50 (10 CFR Part 50) Appendix A, General Design Criteria 4 that requires primary system pressure piping to exhibit an extremely low probability of rupture. The initial phase of the xLPR project is the Pilot Study where the base structure of the code will be developed and tested on a known case. The Pilot Study version of the xLPR code consists of an alpha version where proxy modules and inputs are used and a beta version where the state-of-the art modules and inputs are used. This paper will discuss the input parameters used for the alpha xLPR code.

A Probabilistic Approach to Heatup and Cooldown Operating Limits for a Nuclear Pressure Vessel

2004 ◽  
Author(s):  
Randy G. Lott ◽  
William L. Server
Keyword(s):  
Fracture Mechanics ◽  
Pressure Vessel ◽  
Probabilistic Approach ◽  
System Pressure ◽  
Operating Limits ◽  
Temperature Operating ◽  
Upper Bound Estimate ◽  
Low Probability ◽  
Material Fracture ◽  
Plant Integrity

During plant heatup and cooldown, the stresses in a nuclear pressure vessel are the membrane stress associated with system pressure and the stress caused by the thermal gradient across the vessel wall. The system pressure and the rate of temperature change (heatup or cooldown) must be controlled in a manner that protects against failure of the vessel. The current basis for setting these pressure-temperature operating limits is a deterministic fracture mechanics analysis that conservatively applies a safety of factor of two to the system pressure and assumes a large flaw corresponding to 1/4 of the thickness of the vessel. This calculation produces an upper bound estimate of the stress intensity at the flaw tip, which is compared to a lower bound estimate of the material fracture toughness to set the pressure-temperature operating limits. This methodology results in restrictive operating limits that correspond to an extremely low probability of vessel failure. The development of state-of-the art probabilistic fracture mechanics for the FAVOR Code has made it possible to determine the risk of vessel failure associated with these restrictive limits. Relaxation of these limits may be possible with negligible impact on the risk of vessel failure. Overall plant risk may be reduced by eliminating other potential challenges to plant integrity that are associated with the current restrictive operating limits.

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