Supplementary Technical Basis for ASME Section XI Code Case N-597

Efforts to develop clear and conservative methods to measure and evaluate wall thinning in nuclear piping have been underway since the late 1980’s. The Electric Power Research Institute (EPRI) carried out a successful campaign to address programmatic issues, such as locating and predicting flow-accelerated corrosion (FAC) degradation. This included developing a computer code (CHECWORKS), a users group (CHUG), and a comprehensive program guideline document for the effective prediction, identification and trending of flow-accelerated corrosion degradation. U.S. Nuclear Regulatory Commission (NRC) guidelines are provided in the NRC Inspection Manual Inspection Procedure 49001. At the same time, committees under Section XI of the ASME Boiler and Pressure Vessel Code have addressed evaluation of structural integrity of piping subjected to wall thinning. Code Case N-480 of Section XI provided acceptance criteria that focused on primary piping stresses, with evaluation based on a uniform wall thinning assumption for evaluating the minimum wall thickness of the piping. However, when applying this methodology to low pressure piping systems, Code Case N-480 was very conservative. Code Case N-597 was first published in 1998, and supercedes Code Case N-480. The current version is N-597-2. Code Case N-597-2 provides acceptance criteria and evaluation procedures for piping items, including fittings, subjected to a wall thinning mechanism, such as flow-accelerated corrosion. Code Case N-597-2 is a significant improvement over N-480, containing distinct elements to be satisfied in allowing the licensee to operate with piping degraded by wall thinning. The Code Case considers separately wall thickness requirements and piping stresses, and maintains original design intent margins. The Code Case does not provide requirements for locations of inspection, inspection frequency or method of prediction of rate of wall thinning. As described in the original technical basis document published at the 1999 ASME PVP Conference, the piping stress evaluation follows very closely the Construction Codes for piping. Five conditions related to industry use of Code Case N-597-1 have been published by the NRC in Regulatory Guide 1.147, Revision 13. A number of these issues are related to a need for additional explanation of the technical basis for the Code Case, such as the procedures for evaluation of wall thickness less than the ASME Code Design Pressure-based minimum allowable wall thickness. This presentation addresses these NRC conditions by providing additional description of the technical basis for the Code Case.

Technical Basis for Revisions to ASME Section XI Code Case N-597-2 on Requirements for Analytical Evaluation of Pipe Wall Thinning

Volume 1A: Codes and Standards ◽

10.1115/pvp2015-45099 ◽

2015 ◽

Author(s):

Douglas A. Scarth ◽

Michael Davis ◽

Phil Rush ◽

Steven X. Xu

Keyword(s):

Nuclear Regulatory Commission ◽

Pressure Vessels ◽

Acceptance Criteria ◽

Accelerated Corrosion ◽

Evaluation Procedures ◽

Technical Requirements ◽

Wall Thinning ◽

Regulatory Commission ◽

Technical Basis ◽

Code Case N-597-2 provides procedures and acceptance criteria for the evaluation of piping items subjected to wall thinning mechanisms such as flow-accelerated corrosion (FAC). The acceptance criteria ensure that margins equivalent to those of the ASME B&PV Code are maintained. Subsequent to the publication of Code Case N-597-2, the U.S. Nuclear Regulatory Commission (NRC) found the Code Case conditionally acceptable. A number of task items have been undertaken by the ASME Section XI Working Group on Pipe Flaw Evaluation (WGPFE) to address the NRC conditions. A 2006 ASME Pressure Vessels and Piping (PVP) Division conference paper was published to provide an expanded explanation of the technical basis for Code Case N-597-2. A 2009 PVP paper was published to provide results of validation of evaluation procedures and acceptance criteria in Code Case N-597-2 against experimental and historic wall thinning events. More recently, revisions to Code Case N-597-2 have been made and were proposed as N-597-3. Significant changes have been made in the proposed revised Code Case to clarify the technical requirements and address the NRC concerns over N-597-2. The technical basis for revising Code Case N-597-2 is provided in this paper.

Supplementary Technical Basis for ASME Section XI Code Case N-597-2

Volume 1: Codes and Standards ◽

10.1115/pvp2006-icpvt-11-93278 ◽

2006 ◽

Cited By ~ 2

Author(s):

Douglas A. Scarth ◽

Kunio Hasegawa ◽

Lee F. Goyette ◽

Phil Rush

Keyword(s):

Wall Thickness ◽

Nuclear Power ◽

Pressure Vessels ◽

Code Design ◽

Acceptance Criteria ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Class 1 ◽

American Society ◽

Technical Basis

Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code provides rules and requirements for maintaining pressure boundary integrity of components, piping, and equipment during the life of a nuclear power plant. Code Case N-597-2 of Section XI, Requirements for Analytical Evaluation of Pipe Wall Thinning, provides evaluation procedures and acceptance criteria to justify continued operation of Class 1, 2 and 3 piping items subjected to wall thinning by a mechanism such as flow-accelerated corrosion. The acceptance criteria ensure that margins equivalent to those of the ASME B&PV Code are maintained. The technical basis for Code Case N-597-2 was previously presented at the 1999 ASME Pressure Vessels and Piping Conference. Since then, the ASME Section XI Working Group on Pipe Flaw Evaluation has identified the need for further explanation of the technical basis for the Code Case, such as the procedures for evaluation of wall thickness less than the Construction Code Design Pressure-based minimum allowable wall thickness, tmin. This paper provides an additional description of the Code Case technical basis and validation against experimental and historic wall thinning events.

Numerical Calculation of Shear Stress Distribution on the Inner Wall Surface of CANDU Reactor Feeder Pipe Conveying Two-Phase Coolant

Volume 8: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A and B ◽

10.1115/imece2007-41374 ◽

2007 ◽

Author(s):

Jong Chull Jo ◽

Dong Gu Kang ◽

Kyung Wan Roh

Keyword(s):

Shear Stress ◽

Stress Distribution ◽

Wall Thickness ◽

Shear Stress Distribution ◽

Cfd Analysis ◽

Two Phase ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Local Areas ◽

Two-phase flow fields inside feeder pipes of a CANDU reactor have been simulated numerically using a CFD (computational fluid dynamics) code to calculate the shear stress distribution which is the most important factor to be considered in predicting the local areas of feeder pipes highly susceptible to FAC (flow-accelerated corrosion)-induced wall thinning. The CFD approach with schemes used in this study to simulate the turbulent flow situations inside the CANDU feeder pipes had been verified by showing a good agreement between the investigation results for the failed feedwater pipe at Surry Unit 2 plant in U.S. and the CFD calculation. Sensitivity studies of the three geometrical parameters such as angle of the 1st and 2nd bends, length of the 1st span between the grayloc hub and the 1st bend, and length of the 2nd span between the 1st and the 2nd bends had already been performed. In this study, the effects of void fraction of the primary coolant coming out from the exit of pressure tubes containing nuclear fuels on the fluid shear stress distribution at the inner surface of feeder pipe wall have been investigated to find out the local areas of feeder pipes conveying two-phase coolant, where are highly susceptible to FAC (flow-accelerated corrosion)-induced wall thinning. As the results of CFD analysis, it is seen that the local regions of feeder pipes of the operating CANDU reactors in Korea, on which the wall thickness measurements have been performed so far, are not coincided with the worst regions predicted by the present CFD analysis where is the connection region of straight & bend pipe near the inlet part of the bend intrados. Finally, based on the results of the present CFD analysis a guide to the selection of the weakest local positions where the measurement of wall thickness should be performed with higher priority has been provided.

Fitness for Service, Life Extension, Remediation, Repair, and Erosion/Corrosion Issues for Pressure Vessels and Components ◽

Managing Flow Accelerated Corrosion in Carbon Steel Piping in Nuclear Plants

10.1115/pvp2004-2251 ◽

2004 ◽

Cited By ~ 3

Author(s):

Z. H. Walker

Keyword(s):

Carbon Steel ◽

Steam Generator ◽

Wall Thickness ◽

Degradation Mechanism ◽

Reactor Core ◽

Operating Conditions ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Steel Piping

In 1996, Flow Accelerated Corrosion (FAC) was identified as a degradation mechanism affecting carbon steel outlet feeder pipes in CANDU® (CANadian Deuterium Uranium) reactors. The maximum rate of FAC was estimated to be <0.120 mm/year. In response, wall thickness inspection programs have been implemented to identify and measure the minimum wall thickness in outlet feeder pipes. These data are necessary to ensure fitness-for-service of the feeder pipe. These data, together with the thermalhydraulic and geometric parameters for the measured feeders, are also very useful for developing empirical wall thickness models. Such models can be used to enhance the understanding of feeder wall thinning leading to an improved capability to predict future wall thickness minima and their locations. The determined dependency of the wall-thinning rate on thermalhydraulic conditions can be used to quantify the potential benefits of maintenance activities, such as steam generator cleaning. Activities such as steam generator cleaning are generally viewed as beneficial in recovering lost thermal efficiency, thereby reducing the severity of the thermalhydraulic conditions by reducing the amount of quality (steam phase) exiting the reactor core. Finally, when wall thickness models are applied to data from different plants, there is the potential of identifying operating conditions that can lead to lower rates of wall loss. This paper addresses the aforementioned important issues associated with FAC of ASME PVP Class 1 carbon steel piping.

Volume 1: Plant Operations, Maintenance, Engineering, Modifications and Life Cycle; Component Reliability and Materials Issues; Next Generation Systems ◽

The Impact of Flow Accelerated Corrosion (FAC) on Feeder Life Cycle Management

10.1115/icone17-75897 ◽

2009 ◽

Author(s):

Mahesh D. Pandey ◽

Mikko I. Jyrkama ◽

Edward M. Lehockey

Keyword(s):

Life Cycle ◽

Wall Thickness ◽

Safety Concern ◽

Probabilistic Method ◽

Life Cycle Management ◽

Economic Losses ◽

Accelerated Corrosion ◽

Wall Thinning ◽

The Impact

Wall thinning of outlet feeder piping by flow accelerated corrosion (FAC) is a serious form of degradation affecting some CANDU® stations. The general and localized loss of wall thickness is typically highest at or near welds and changes in pipe geometry due to increased velocity or turbulence. While the process is not a high safety concern because catastrophic failure is unlikely, feeder wall thinning may result in significant economic losses as a result of forced shutdowns for repair and replacement. Accurate modelling and prediction of feeder replacements and the probability of feeder failure is not only important for continued fitness-for-service, but essential for feeder life cycle management (LCM). This paper discusses some of the key issues related to feeder FAC, and presents a probabilistic method for modelling the wall thinning process. The wall thickness loss due to FAC is modelled using a random rate model, while the probability of feeder failure is based on an empirical approach. The developed methodology allows the estimation of the remaining life of both inspected and uninspected feeder populations, while methodically accounting for the uncertainties in the problem.

Numerical Evaluation of Wall Thinning Profile in Separation and Union Pipe due to Flow Accelerated Corrosion

Volume 6A: Materials and Fabrication ◽

10.1115/pvp2014-28058 ◽

2014 ◽

Author(s):

Shun Watanabe ◽

Kimitoshi Yoneda

Keyword(s):

Wall Thickness ◽

Power Plants ◽

Pipe Wall ◽

Branch Pipe ◽

Residual Lifetime ◽

Accelerated Corrosion ◽

Geometry Factor ◽

T Tube ◽

Wall Thinning ◽

Flow Accelerated Corrosion (FAC) is a pipe wall thinning phenomenon to be monitored and managed in power plants with high priority. In Japan, its management has been conducted with conservative evaluation of thinning rate and residual lifetime of the piping based on wall thickness measurements. However, noticeable case of the wall thinning occurred at separation and union pipe. In such pipe system, it is a problem to manage a section beneath reinforcing plate of T-tube pipe and a crotch of T-joint pipe; wall thickness measurement with high accuracy is difficult to conduct in the region by using ordinary ultrasonic testing devices. In this study, numerical analysis for separation and union parts of T-tube and T-joint pipes was conducted, and wall thinning profile by FAC was evaluated by calculating mass transfer coefficient and geometry factor. Based on these results, applicable wall thinning management for T-tube and T-joint pipes was considered. In the case of union flow from main and branch pipe, the wall thinning profile of T-tube showed the tendency of increase at main pipe like semielliptical region. On the other hand, noticeable profile appeared at crotch in T-joint although it was found that geometry factor of T-joint in this flow pattern was half the value of T-tube. An alternative evaluation method to previous one might be needed for such semielliptical region in T-tube and crotch in T-joint.

Prediction of the Local Areas of CANDU Feeder Pipes Highly Susceptible to Wall Thinning Due to Flow-Accelerated Corrosion

Volume 4: Fluid-Structure Interaction ◽

10.1115/pvp2007-26507 ◽

2007 ◽

Author(s):

Dong Gu Kang ◽

Jong Chull Jo

Keyword(s):

Wall Thickness ◽

Geometrical Parameters ◽

Cfd Analysis ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Sensitivity Studies ◽

Thickness Measurements ◽

Selection Of ◽

Good Agreement

Flow fields inside feeder pipes have been simulated numerically using a CFD (computational fluid dynamics) code to calculate the shear stress distribution which is the most important factor to be considered in predicting the local regions of feeder pipes highly susceptible to FAC (flow-accelerated corrosion)-induced wall thinning. The CFD approach with schemes used in this study to simulate the flow situations inside the CANDU feeder pipes has been verified by showing a good agreement between the investigation results for the failed feedwater pipe at Surry Unit 2 plant in U.S. and the CFD calculation. Sensitivity studies of the three geometrical parameters such as angle of the 1st and 2nd bends, length of the 1st span between the grayloc hub and the 1st bend, and length of the 2nd span between the 1st and the 2nd bends have been performed. As the results of CFD analysis, it is seen that the local regions of feeder pipes of Wolsung Unit 1 in Korea, on which the wall thickness measurements have been performed so far, are not coincident with the worst regions predicted by the present CFD analysis where is the connection region of straight & bend pipe near the inlet part of the bend intrados. Finally, based on the results of the present CFD analysis a guide to the selection of the weakest local positions where the measurement of wall thickness should be performed with higher priority has been provided.

Iatan Desuperheater Pipe Failure Caused by FAC: September 28, 2007

ASME 2010 Pressure Vessels and Piping Conference: Volume 6, Parts A and B ◽

10.1115/pvp2010-26066 ◽

2010 ◽

Author(s):

Chong Chiu ◽

Lance B. Gockel

Keyword(s):

Wall Thickness ◽

Pipe Wall ◽

Preliminary Evaluation ◽

Pipe Failure ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Plant Personnel ◽

Serious Injury ◽

Flow Blockage

At approximately 11:40 AM on May 9, 2007, Iatan Unit 1 experienced a catastrophic rupture of a 4 inch superheater (SH) attemperator spray line after nearly 27 years of commercial operations. At the time of the rupture, several plant personnel were in the immediate vicinity performing maintenance on a plugged coal feeder. Plant operators immediately initiated a plant shutdown. This incident resulted in two fatalities and one serious injury. Subsequent examination of the ruptured line indicated significant pipe wall thinning had occurred, leading to the sudden failure of the pipe pressure boundary and the pipe rupture event. The preliminary evaluation of the failed pipe determined that flow accelerated corrosion (FAC) was the likely failure mechanism. To prevent this and similar events, the PII team recommends the following actions be taken to identify other potential areas which may have similar characteristics to the failed pipe: 1. Employ the EPRI method CHECKWORKS (as has been implemented) to identify the susceptible areas. 2. Supplement the EPRI model with connected flow modeling techniques to identify additional inspection areas. 3. If the measured wall thickness is less than 30% of the minimum allowable wall thickness, replace or repair the pipe immediately. 4. If the measured wall thickness is less than the minimum allowable wall thickness (as specified by the B31.1 code), but no less than 30% of the minimum allowable, perform a safety risk assessment. If the risk is determined acceptable, replace or repair the pipe at the next planned plant outage with temporary compensatory actions (such as caution tags, leak flow blockage facilities, etc.). 5. Identify and replace all throttled gate valves and replace them as soon as practical. Until these valves are replaced, utilize NDE techniques to monitor the pipe wall thinning downstream of the valves and replace pipe based on the above criteria in 3 and 4.

Techniques for Improved Reliability of Wall Thinning Estimation

Volume 6: Materials and Fabrication ◽

10.1115/pvp2006-icpvt-11-93414 ◽

2006 ◽

Author(s):

Yogendra S. Garud

Keyword(s):

The Other ◽

Remaining Life ◽

Data Filtering ◽

Large Database ◽

Accelerated Corrosion ◽

Wall Thinning ◽

Simplified Method ◽

The Impact ◽

Thinning Rate

Wall thinning in pressure retaining components, especially due to the flow-accelerated corrosion, has been a significant factor affecting the safety and unplanned system downtimes. On the other hand, overestimating the impact of possible wall thinning often leads to unnecessary or expensive inspections and replacements. The simplified or quick (short-cut) methods of analysis and prediction often lack the requisite degree of accuracy and confidence. This paper presents a few techniques for better analysis of the wall thinning data to address these issues. These techniques make use of the statistical methods, pattern recognition, and optimization to perform a robust data filtering and thinning rate estimation that accounts for measurement uncertainty. The techniques are discussed with application to a large database and an inspection program. The impact of these analytical improvements is presented in comparison with results of the simplified method of analysis. The results include both the margin on remaining life and the projected wall thinning rates, with implications for inspections.