XI.M35 (NUREG-2191 R0)

XI.M35 ASME CODE CLASS 1 SMALL-BORE PIPING

Program Description

This program is a condition monitoring program for detecting cracking in small-bore, American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Class 1 piping. The program augments the inservice inspections (ISI) specified by ASME Code, Section XI, for certain ASME Code Class 1 piping that is less than 4 inches nominal pipe size (NPS) and greater than or equal to 1 inch NPS.

Industry operating experience (OE) demonstrates that welds in ASME Code Class 1 small-bore piping are susceptible to stress corrosion cracking (SCC) and cracking due to thermal or vibratory fatigue loading. Such cracking is frequently initiated from the inside diameter of the piping; therefore, volumetric examinations are needed to detect cracks. However, ASME Code, Section XI, generally does not call for volumetric examinations of this class and size of piping. Specifically, ASME Code, Section XI, Subarticle IWB-1220, exempts all components that are less than or equal to 1 inch nominal pipe size (NPS) from volumetric examinations. In addition, with the exception of certain pressurized water reactor high pressure safety injection system piping components, ASME Code, Section XI, Table IWB-2500-1, calls for surface examinations and visual inspections during system leakage tests of piping components that are less than 4 inches NPS.

This program supplements the ASME Code, Section XI, examinations with volumetric examinations, or alternatively, destructive examinations, to detect cracks that may originate from the inside diameter of butt welds, socket welds, and their base metal materials. The examination schedule and extent is based on plant-specific OE and whether actions have been implemented that would successfully mitigate the causes of any past cracking. The program relies on a sample size as specified in Table XI.M35-1 as means to determine whether cracking is occurring in the total population of ASME Code Class 1 small-bore piping in the plant.

Evaluation and Technical Basis

1. Scope of Program: This program manages the effects of SCC and cracking due to thermal or vibratory fatigue loading for certain ASME Code Class 1 small-bore piping. For the purposes of this program, small-bore piping includes piping that is less than 4 inches NPS and greater than or equal to 1 inch NPS.

2. Preventive Actions: This is a condition monitoring program only; therefore, it has no preventive actions.

3. Parameters Monitored or Inspected: Cracking is detected through either destructive or nondestructive examinations of piping welds and base metal materials. The volume of these materials is examined to detect flaws or other discontinuities that may indicate the presence of cracks.

4. Detection of Aging Effects: A sample of ASME Code Class 1 small-bore piping welds is examined in accordance with the categories specified in Table XI.M35-1. The initial schedule of examinations, either one-time for Categories A and B or periodic for Category C, is based on plant-specific OE and whether actions that would successfully mitigate the causes of any past cracking have been implemented. Periodic examinations are implemented as per Category C if the one-time examinations detect any unacceptable flaws or relevant conditions. The scope of the examinations includes both full penetration (butt) welds and partial penetration (socket) welds.

The welds to be examined are selected from those locations that are determined to be the most risk significant and most susceptible to SCC and cracking due to thermal or vibratory fatigue loading. Other factors, such as plant-specific and industry OE, accessibility, and personnel exposure, can also be considered to select the most appropriate locations for the examinations. The guidelines from Electric Power Research Institute Technical Report 1011955(Archived), “Materials Reliability Program: Management of Thermal Fatigue in Normally Stagnant Non-Isolable Reactor Coolant System Branch Lines (MRP-146),” and Technical Report 1018330, “Materials Reliability Program: Management of Thermal Fatigue in Normally Stagnant Non-Isolable Reactor Coolant System Branch Lines– Supplemental Guidance (MRP-146S),” may be used to determine the locations that are most susceptible to thermal fatigue. Because more information can be obtained from a destructive examination than from a nondestructive examination, the applicant can take credit for each weld destructively examined as equivalent to having volumetrically examined two welds.

Table XI.M35-1. Examinations
Category	Plant Operating Experience	Mitigation	Examination Schedule	Sample Size	Examination Method
A	No age-related cracking^{(1) (2)}	Not applicable	One-time: completed within 6 years prior to the start of the subsequent period of extended operation	Full penetration (butt) welds: 3% of total population per unit, up to 10⁽⁴⁾ Partial penetration (socket) welds: 3% of total population per unit, up to 10 ⁽⁴⁾	Volumetric or destructive^{(5) (6)}
B	Age-related cracking⁽²⁾	Yes⁽³⁾	One-time: completed within 6 years prior to the start of the subsequent period of extended operation	Full penetration (butt) welds: 10% of total population per unit, up to 25 ⁽⁴⁾ Partial penetration (socket) welds: 10% of total population per unit, up to 25 ⁽⁴⁾	Volumetric or destructive^{(5) (6)}
C	Age-related cracking⁽²⁾	No	Periodic: first examination completed within the 6 years prior to the start of the subsequent period of extended operation with subsequent examinations every 10 years thereafter	Full penetration (butt) welds: 10% of total population per unit, up to 25 ⁽⁴⁾ Partial penetration (socket) welds: 10% of total population per unit, up to 25 ⁽⁴⁾	Volumetric or destructive^{(5) (6)}
NOTES: Must have no history of age-related cracking. Age-related cracking includes piping leaks or other flaws where fatigue or stress corrosion cracking are contributing factors. Actions must have been taken to mitigate the cause of the cracking. These actions, such as design changes, would generally go beyond typical repair or replacement activities. If welds that have been redesigned or repaired and the applicant could demonstrate through OE that no additional failures have been reported for the last 30 years, then the inspection sample size could follow the guidance in Category A. The welds to be examined are selected from locations that are determined to be the most risk significant and most susceptible to cracking. Other factors, such as plant-specific and industry OE, accessibility, and personnel exposure, can also be considered in selecting the most appropriate locations for the examinations. Volumetric examinations must employ techniques that have been demonstrated to be capable of detecting flaws and discontinuities in the examination volume of interest. Each partial penetration (socket) weld subject to destructive examination may be credited twice towards the total number of examinations because more information can be obtained from a destructive examination than from a nondestructive examination.

5. Monitoring and Trending: For plants that are either in Categories A or B, a one-time examination provides confirmation that cracking is not occurring or that it is occurring so slowly that it will not affect the component’s intended function during the subsequent period of extended operation. Periodic examinations provide for the timely detection of cracks for those plants that are in Category C. If a component containing flaws or relevant conditions is accepted for continued service by analytical evaluation, then it is subsequently reexamined to meet the intent of ASME Code, Section XI, Subarticle IWB-2420.

6. Acceptance Criteria: Examination results are evaluated in accordance ASME Code, Section XI, Paragraph IWB-3132.

7. Corrective Actions: Results that do not meet the acceptance criteria are addressed in the applicant’s corrective action program under those specific portions of the quality assurance (QA) program that are used to meet Criterion XVI, “Corrective Action,” of 10 CFR 50, Appendix B. Appendix A of the Generic Aging Lessons Learned for Subsequent License Renewal (GALL-SLR) Report describes how an applicant may apply its 10 CFR 50, Appendix B, QA program to fulfill the corrective actions element of this aging management program (AMP) for both safety-related and nonsafety-related structures and components (SCs) within the scope of this program.

The corrective actions are to include examinations of additional ASME Code Class 1 small-bore piping welds to meet the intent of ASME Code, Section XI, Subarticle IWB-2430. In addition, for those plants that are either in Categories A or B, periodic examinations are then implemented in accordance with the schedule specified in Category C.

8. Confirmation Process: The confirmation process is addressed through those specific portions of the QA program that are used to meet Criterion XVI, “Corrective Action,” of 10 CFR 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR 50, Appendix B, QA program to fulfill the confirmation process element of this AMP for both safety-related and nonsafety-related SCs within the scope of this program.

9. Administrative Controls: Administrative controls are addressed through the QA program that is used to meet the requirements of 10 CFR 50, Appendix B, associated with managing the effects of aging. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR 50, Appendix B, QA program to fulfill the administrative controls element of this AMP for both safety-related and nonsafety-related SCs within the scope of this program.

10. Operating Experience: Through-wall cracking in ASME Code Class 1 small-bore piping has occurred at a number of plants. Causes include SCC and thermal and vibratory fatigue loading as described in the U.S. Nuclear Regulatory Commission Information Notice 97-46, “Unisolable Crack in High-Pressure Injection Piping.” This program augments the ASME Code, Section XI, inspections to provide assurance that cracks will be detected before there is a loss of intended function. Licensee Event Reports (LERs) 259/2008-002 and LER 387/2012-007-00 provide a sample of relevant OE.

The program is informed and enhanced when necessary through the systematic and ongoing review of both plant-specific and industry OE including research and development such that the effectiveness of the AMP is evaluated consistent with the discussion in Appendix B of the GALL-SLR Report.

References

10 CFR Part 50, Appendix B, “Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.

10 CFR 50.55a, “Codes and Standards.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.

ASME. ASME Code Section XI, “Rules for Inservice Inspection of Nuclear Power Plant Components.” New York, New York: The American Society of Mechanical Engineers. 2008.

EPRI. Technical Report 1011955(Archived), “Materials Reliability Program: Management of Thermal Fatigue in Normally Stagnant Non-Isolable Reactor Coolant System Branch Lines (MRP-146).” Palo Alto, California: Electric Power Research Institute. June 2005.

_____. Technical Report 1018330, “Materials Reliability Program: Management of Thermal Fatigue in Normally Stagnant Non-Isolable Reactor Coolant System Branch Lines – Supplemental Guidance (MRP-146S).” Palo Alto, California: Electric Power Research Institute. December 2008.

Licensee Event Report 259/2008-002 and LER 259/2008-002-01, “ASME Code Class 1 Pressure Boundary Leak on an Instrument Line Connected to the Reactor Vessel.” https://lersearch.inl.gov/LERSearchCriteria.aspx. March 2009.

Licensee Event Report 387/2012-007-00, “Unplanned Shutdown Due to Unidentified Drywell Leakage.” https://lersearch.inl.gov/LERSearchCriteria.aspx. September 2012.

US NRC Information Notice 97-46, “Unisolable Crack in High-Pressure Injection Piping.” Washington, DC: U.S. Nuclear Regulatory Commission. July 1997.

XI.M35 (NUREG-2191 R0)

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