ML25135A417
| ML25135A417 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 05/16/2025 |
| From: | David Wrona Plant Licensing Branch II |
| To: | Krakuszeski J Duke Energy Progress |
| Purnell, B | |
| References | |
| EPID L-2025-LLR-0038 | |
| Download: ML25135A417 (11) | |
Text
May 16, 2025 Mr. John A. Krakuszeski Site Vice President Brunswick Steam Electric Plant Duke Energy Progress, LLC 8470 River Rd. SE (M/C BNP001)
Southport, NC 28461
SUBJECT:
BRUNSWICK STEAM ELECTRIC PLANT, UNIT 1 - PROPOSED ALTERNATIVE FOR ACCEPTANCE OF THROUGH-WALL FLAW IN REDUCER (EPID L-2025-LLR-0038)
Dear Mr. Krakuszeski:
By letter dated March 21, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25080A235), as supplemented by letter dated May 6, 2025 (ML25126A250), Duke Energy Progress, LLC (the licensee) submitted a request for a proposed alternative to certain requirements in Title 10 of the Code of Federal Regulations (10 CFR)
Section 50.55a, Codes and standards, for Brunswick Steam Electric Plant (Brunswick), Unit 1.
Specifically, the licensee proposed to use subarticle NB-3200, Design by Analysis, of the 2007 Edition with 2008 Addenda of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section III, for demonstration of the structural integrity and acceptance of the through-wall flaw in the small end transition zone of a reducer in the service water system until the next scheduled refueling outage. The licensees May 6, 2025, supplement was in response to U.S. Nuclear Regulatory Commission (NRC) staff request for additional information issued by email dated April 17, 2025 (ML25113A282).
The regulations in 10 CFR 50.55a(z) state, in part, that alternatives to the requirements in paragraphs (b) through (h) of 10 CFR 50.55a may be authorized by the NRC if the licensee demonstrates that: (1) the proposed alternative provides an acceptable level of quality and safety, or (2) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
At the time of the application and supplement, the flaw had not reached the small end transition zone of the reducer. On May 14, 2025, the licensee informed the NRC staff that ultrasonic testing results from a May 13, 2025, examination showed that the flaw had grown into the transition zone. This new information did not affect the NRC staffs review of the proposed alternative.
The NRC staff has reviewed the licensees application, as supplemented, and concludes, as set forth in the enclosed safety evaluation, that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the licensee to use the proposed alternative at Brunswick, Unit 1, in lieu of the ASME Code Section XI, requirements to repair or replace the degraded reducer, until the conclusion of the spring 2026 refueling outage which is currently planned to start on March 7, 2026.
J. Krakuszeski All other ASME Code,Section XI requirements for which an alternative was not specifically requested and authorized remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
If you have any questions, please contact Blake Purnell at 301-415-1380 or via email at Blake.Purnell@nrc.gov.
Sincerely, David Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-325
Enclosure:
Safety Evaluation cc: Listserv NATREON JORDAN Digitally signed by NATREON JORDAN Date: 2025.05.16 13:21:28 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED ALTERNATIVE FOR ACCEPTANCE OF THROUGH-WALL FLAW IN REDUCER DUKE ENERGY PROGRESS, LLC BRUNSWICK STEAM ELECTRIC PLANT, UNIT 1 DOCKET NO. 50-325
1.0 INTRODUCTION
By letter dated March 21, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25080A235), as supplemented by letter dated May 6, 2025 (ML25126A250), Duke Energy Progress, LLC (the licensee) submitted a request for a proposed alternative to certain requirements in Title 10 of the Code of Federal Regulations (10 CFR)
Section 50.55a, Codes and standards, for Brunswick Steam Electric Plant (Brunswick), Unit 1.
Specifically, the licensee proposed to use subarticle NB-3200, Design by Analysis, of the 2007 Edition with 2008 Addenda of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section III, for demonstration of the structural integrity and acceptance of the through-wall flaw in the small end transition zone of a reducer in the service water (SW) system until the next scheduled refueling outage. The licensees May 6, 2025, supplement was in response to U.S. Nuclear Regulatory Commission (NRC) staff request for additional information issued by email dated April 17, 2025 (ML25113A282).
At the time of the application and supplement, the flaw had not reached the small end transition zone of the reducer. On May 14, 2025, the licensee informed the NRC staff that ultrasonic testing (UT) results from a May 13, 2025, examination showed that the flaw had grown into the transition zone. This new information did not affect the NRC staffs review of the proposed alternative.
2.0 REGULATORY EVALUATION
The regulations in 10 CFR 50.55a(g)(4) state, in part, that ASME Code Class 1, 2, and 3 components (including supports) must meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, of the applicable editions and addenda of the ASME Code to the extent practical within the limitations of design, geometry, and materials of construction of the components.
The regulations in 10 CFR 50.55a(z) state, in part, that alternatives to the requirements in paragraphs (b) through (h) of 10 CFR 50.55a may be authorized by the NRC if the licensee demonstrates that: (1) the proposed alternative provides an acceptable level of quality and safety, or (2) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
3.0 TECHNICAL EVALUATION
3.1 Licensees Request 3.1.1 ASME Code Components Affected A concentric reducer in the conventional SW discharge header at Brunswick, Unit 1, that has developed a through-wall flaw.
3.1.2 Applicable ASME Code Edition and Addenda The code of record for the fifth inservice inspection interval of Brunswick, Unit 1, is the 2007 Edition through 2008 Addenda of the ASME Code,Section XI.
By teleconference held on February 12, 2025 (ML25043A229), the NRC verbally authorized the use of the 2021 Edition of ASME Code,Section XI, Nonmandatory Appendix C, Analytical Evaluation of Flaws in Piping, for all applicable pressure-retaining piping and components during the remainder of the Brunswick, Unit 1, fifth inservice inspection interval. On May 14, 2025 (ML25125A315), the NRC staff issued the associated safety evaluation for the verbal authorization.
3.1.3 Applicable ASME Code Requirements As discussed in the application, the ASME Code,Section XI, requirements applicable to this request originate in subarticles IWD-3100, IWD-3500, and IWA-4000 Paragraph IWD-3120(b) states, in part, that components whose examination reveals flaws that do not meet the standards of subarticle IWD-3400 shall be subjected to supplemental examination, or to a repair/replacement activity.
Subarticle IWD-3500, Acceptance Standards, describes acceptance standards, and states, in part, that the requirements of IWC-3500, Acceptance Standards, may be used.
Article IWA-4000, Repair/Replacement Activities, describes the repair/replacement activities to correct an unacceptable flaw. Discovery of an area below the design minimum wall thickness in the structural portion of an ASME Code Class 1, 2, or 3 component is direct evidence of a flaw in the component.
The ASME Code Case N-513-5, Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping and Gate Valves,Section XI, Division 1, provides analytical evaluation requirements for temporary acceptance of flaws in piping without performing a repair or replacement for a limited time, not exceeding the time to the next scheduled refueling outage. This code case has been incorporated by reference into 10 CFR 50.55a via Regulatory Guide 1.147, Revision 21, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1 (ML23291A003), with conditions. Section 3.4 of Code Case N-513-5 states that the evaluation of flaws in the small end transition zone of the reducer is outside scope of this code case.
3.1.5 Reason for Request On December 4, 2024, the licensee discovered a non-planar through-wall flaw in the Brunswick, Unit 1, SW system piping. The location of the flaw is in the straight portion of a 20-inch by 30-inch concentric reducer near the small end transition zone in the conventional SW discharge header.
The licensee has evaluated the flaw in accordance with ASME Code Case N-513-5 and has determined that the flaw met the acceptance criteria for the reducer to temporarily remain in service without repair or replacement. The licensee has been monitoring the flaw and, at the time the application was submitted, had identified that the flaw was growing towards the small end transition zone of the reducer. As noted above, the licensee recently discovered that the flaw had grown into the transition zone. The evaluation criteria in Code Case N-513-5 are not applicable to flaws in the small end transition zone of the reducer. Therefore, the reducer will need to be repaired or replaced in accordance with the ASME Code,Section XI. The license stated in the application that this would require the shutdown of the reactor.
3.1.5 Licensees Proposed Alternative The licensees proposed alternative is to use the design-by-analysis provisions in the 2007 Edition with 2008 Addenda of the ASME Code,Section III, NB-3200, to evaluate the structural integrity of the degraded reducer of the SW piping system. The licensee developed a finite element model (FEM) to calculate the stress field associated with a 100 percent through-wall flaw. From this evaluation, the licensee calculated the allowable flaw size to be a 10-inch by 10-inch hole.
As part of the proposed alternative, the licensee will perform the following compensatory actions consistent with Code Case N-513-5, Section 2:
(a)
Frequent periodic examinations of no more than 30-day Intervals shall be used to determine If flaws are growing and to establish the time, at which the detected flaw will reach the allowable size.
(b)
For through-wall leaking flaws, leakage shall be monitored daily to confirm the analysis conditions used in the evaluation remain valid.
(c)
If examinations reveal flaw growth rate to be unacceptable, a repair/replacement activity shall be performed.
(d)
Repair/replacement activities shall be performed no later than when the predicted flaw size from trending periodic Inspection is expected to exceed the acceptance criteria (10-inch in either the axial or circumferential direction), or during the next scheduled refueling outage, whichever occurs first. Repair/replacement activities shall be in accordance with IWA-4000 [of the ASME Code, Section XI].
The May 6, 2025, supplement further states that if the identified leakage exceeds the 10 [gallons per minute] gpm leak rate, repair/replacement activities will be performed to restore the integrity of the piping.
3.2 NRC Staffs Evaluation The NRC staff evaluated the licensees proposed alternative request pursuant to 10 CFR 50.55a(z)(2). The NRC staff focused on whether compliance with the specified requirements of 10 CFR 50.55a(g), or portions thereof, would result in hardship or unusual difficulty, without a compensating increase in the level of quality and safety.
3.2.1 Hardship Justification The ASME Code,Section XI, will require the licensee to repair or replace the reducer with the flaw in the small end transition zone. The licensee stated that repair of the degraded reducer cannot be performed with the unit online because the conventional SW system discharge header would need to be isolated and result in a loss of cooling water to the supported equipment. For this shutdown, the license stated that the reactor would enter Mode 5, and the reactor vessel head would be removed. The reactor cavity would be flooded to allow one loop of the residual heat removal (RHR) system to be placed in shutdown cooling and augmented spent fuel pool cooling. The licensee stated that an unscheduled plant shutdown would result in additional dose to personnel and additional plant risk. Based on this information, the NRC staff determined that compliance with the repair or replacement requirements of the ASME Code,Section XI, would result in hardship or unusual difficulty if an unplanned shutdown is required.
3.2.2 Flaw Evaluation Methodology The licensee stated that the original code of construction for the subject SW piping is the 1967 Edition of United States of America Standards (USAS) B31.1 with load combinations/allowable stresses from the 1967 Edition of the USAS B31.1.0 or the 1973 Edition of the American National Standards Institute B31.1. As such, the subject pipe is classified as a B31.1 component. The licensee further stated that the design by rule approach of B31.1 does not provide specific criteria for the evaluation of non-uniform wall thickness or local wall thinning.
Therefore, the licensee used the design-by-analysis approach of the ASME Code, Section Ill, NB-3200, to evaluate the degraded reducer of the SW pipe as part of the proposed alternative.
The 2007 Edition with 2008 Addenda of the ASME Code,Section III, is incorporated by reference in 10 CFR 50.55a(a), subject to the conditions in 10 CFR 50.55a(b). Therefore, the NRC staff determined that it is acceptable to use the analytical method of evaluating piping in the ASME Code,Section III, NB-3200.
The licensee stated that, given that there is at least one actual reading below minimum wall thickness (tmin) identified in the subject pipe, the primary stress design criteria for service level A (normal) and service level B (upset) conditions are based on the limit load design criteria of paragraph NB-3228.1, Limit Analysis, in the ASME Code,Section III. Paragraph NB-3228.1 states, in part, that:
The limits on General Membrane Stress Intensity (NB-3221.1), Local Membrane Stress Intensity (NB-3221.2), and Primary Membrane Plus Primary Bending Stress Intensity (NB-3221.3) need not be satisfied at a specific location if it can be shown by limit analysis that the specified loadings do not exceed two-thirds of the lower bound collapse load. The yield strength to be used in these calculations is 1.5Sm.
The licensee substituted the allowable stress (S) for the design stress intensity (Sm) to establish the yield stress. The licensee stated that this substitution is consistent with the design rules, as the yield stress definition is the equivalent of the local primary membrane stress, which uses an allowable stress of 1.5Sm for Class 1 components or 1.5S for Class 2, Class 3, and B31.1 components. The NRC staff determined that substituting the allowable stress value for the design stress intensity value is acceptable because this substitution is consistent with the design rules of the ASME Code,Section III.
The licensee stated that the subject pipe must support a total load of 150 percent of the applied maximum loads of the service levels A/B without the pipe plastically collapsing. That is, the membrane stress across an entire section for a given location does not exceed the defined yield stress of 1.5S. The NRC staff noted that the applied loading must be no more than two-thirds of the collapse load and the applied stresses must be within the material yield strength of 1.5S.
For the loading in the service level D faulted condition, the licensee used the limit load design criteria of paragraph F-1200(a) of the ASME Code,Section III, Nonmandatory Appendix F, which is based on paragraph F-1341.3. Paragraph F-1341.3 states in relevant part that static or equivalent static loads shall not exceed 90% of the limit analysis collapse load using a yield stress which is the lesser of 2.3Sm and 0.7Su, where Su is the ultimate stress. The licensee stated that the subject pipe must support a total load of 111.1 percent of the applied faulted loads without plastically collapse (i.e., the membrane stress across an entire section for a given location does not exceed the defined yield stress of 2.3S or 0.7Su).
The licensee applied additional uniform wall thinning on the FEM until the pipe structure meets the above limit load criteria or plastically collapses which will be at the point of numerical instability in terms of the FEM analysis. The total load, as a percentage of nominal load, will be compared to the required 150 percent of the service level A/B loads and 111.1 percent of the service level D loads.
The NRC staff noted that the allowable load on the component is established by applying design factors to the limit load such that the onset of gross plastic deformations (plastic collapse) will not occur. The NRC staff further noted that a limit-load analysis performed in accordance with the ASME Code,Section III, consists of using scaled loads (e.g., applying a factor of 1.5) and modeling elastic-plastic material behavior. If the analysis converges for the scaled loads, the design is considered acceptable. The simplified material model for the limit-load analysis method is defined as elastic-plastic with the yield point set equal to 1.5S, where S is the base allowable stress of the material. The NRC staff determined that the licensees methodology is based on the concept of the limit-load analysis. The NRC staff confirmed that the licensees methodology is consistent with the ASME Code,Section III, NB-3228.1, which specifies provision of limit-load analysis. Therefore, the NRC staff finds that the licensees methodology is acceptable.
Finite Element Model The licensee used a three-dimensional FEM to calculate the stress field associated with localized wall thinning. The FEM includes the reducer with a transition zone and a straight pipe at each end of the transition zone. The initial wall thinning is modeled as a hole with dimensions of 2.375 inches long by 1.5 inches wide, which was based on UT results from February 25, 2025. The wall thickness used for its evaluation was taken from the field examination report.
The licensee modeled an additional length of pipe on the end of the 30-inch pipe and an additional two-flange connection on the end of the 20-inch pipe to remove local effects of boundary conditions and applied load. The licensee stated: The connection of two flanges is modeled with a bonded surface between the contact areas instead of bolt connection. This does not affect the results since this surface boundary is far away from the area of interest. The free end of the modeled 30-inch pipe is fixed in the axial and circumferential directions. The radial direction is left unconstrained to allow for expansion due to internal pressure. The free end of the flange is left free to allow for applying the bending moment and torque loads. The NRC staff determined that the licensees FEM provides adequate boundary conditions and, therefore, is acceptable.
In its supplement letter, the licensee stated that the cement lining of the degraded pipe is unreinforced and does not possess the strength in tension necessary to be considered a structural component. The cement lining is not modeled in the FEM and not used in the pipe stress calculation. The licensee state, in part, that neglecting the stiffness of the cement liner is considered conservative because it increases the stress in the steel pipe. The licensee also stated, in part, that, the pipe stress calculation includes the weight of the cement lining in the weight per foot input value for the piping material plus contents. The forces and moments developed in the pipe stress analysis are used as input into the finite element analysis.
The NRC staff determined that since the cement lining does not provide any structural support to the subject piping, it is acceptable not to consider it in the FEM. The NRC staff further determined that including the weight of cement lining in the pipe stress analysis is acceptable because the weight of cement lining affects the overall pipe stresses. Therefore, the NRC staff finds that the licensees stress analysis with respect to the cement lining is acceptable.
The NRC staff noted that the hole modeled in the FEM is based on the field measurement on February 25, 2025. The information provided in the application shows the wall thinning has grown since the time the hole was discovered on December 4, 2024. The NRC staff was recently informed that the flaw has grown into the small end transition zone of the reducer.
However, the licensees analysis does not consider the growth of the wall thinning area (hole size), but rather the analysis was used to determine the maximum allowable hole size beyond which the licensee would take corrective actions.
The NRC staff determined that the licensees FEM is acceptable because the model includes the necessary boundary conditions, necessary pipe configuration, and the wall thickness from the measurements in the field.
Applied Loads The licensee applied the design pressure of 150 pound per square inch gauge to the interior pipe surfaces and an end-cap load of the FEM. To properly model the longitudinal stresses caused by pressure on the interior surface of the piping, the licensee applied the pressure induced end-cap load to the unconstrainted free end of the flange. Section 5.4, Piping Moment Loads, of the licensees analysis describes how the piping loads were applied to the FEM. The licensee stated that the two worst directions were applied because the moment loads do not include specific directionality.
Per ASME Code,Section III, Appendix F, the static or equivalent static loads shall not exceed two-thirds for the normal/upset condition or 90 percent for the faulted condition of the limit analysis collapse load. Therefore, the degraded location of the subject pipe must support a minimum applied load of 150 percent for the normal/upset condition or 111.1 percent for the faulted condition. The licensee applied all loads (piping loads, end-cap pressure, and internal pressure) simultaneously and a load factor of 1.6 (normal/upset) or 1.2 (faulted) for margin.
The NRC staff determined that the applied loads and associated margins have satisfied the ASME Code,Section III, Appendix F. Therefore, the NRC staff determined that the applied loading is acceptable because the licensee used the load combinations and load factors that are consistent with the ASME Code,Section III, NB-3200.
Analysis Results The licensee stated that the as-found wall thinning area of the subject pipe meets the criteria for both normal/upset and faulted conditions. The licensee further stated that the degraded pipe with a hole size of 10 inches by 10 inches meets the limit load design criteria of the ASME Code,Section III, NB-3228.1. The licensees proposed alternative states, in part, that repair/replacement activities shall be performed no later than when the predicted flaw size from trending periodic Inspection is expected to exceed the acceptance criteria (10-inch in either the axial or circumferential direction).
The NRC staff determined that the licensees proposed acceptance criterion is smaller and thus more conservative than what the limit load analysis would permit (i.e., a 10-inch by 10-inch hole). Therefore, the NRC staff finds that the licensees allowable hole size of 10 inches in either axial or circumferential direction is acceptable for maintaining the structural integrity of the degraded reducer because this hole size meets the limit load design criteria of the ASME Code,Section III, NB-3228.1.
3.2.3 Flooding Analysis In its supplemental letter, the licensee stated that the section of degraded piping will remain under vacuum with the design basis accident flow of 8000 gpm through the A-loop RHR heat exchangers. With the piping under vacuum conditions, leakage from the through-wall flaw is not postulated. In the unlikely event of external leakage from the through-wall flaw occurs, the licensee provided a flooding analysis and concluded that limiting the leakage from the through-wall flaw to less than 10 gpm is considered reasonable for protecting the North Core Spray Pump room from internal flooding. This limit is within the capacity of the area sump pump and provides margin to accommodate other leakage. The licensee further stated that if the identified leakage exceeds the 10-gpm leak rate, repair/replacement activities will be performed to restore the integrity of the piping.
The NRC staff reviewed the licensees flooding analysis and determined that a leak rate limit of 10 gpm for the subject degraded pipe is acceptable because:
(1) The degraded pipe segment is in the discharge side of the SW piping which does not significantly affect the functionality and operation of the SW piping.
(2) The SW piping can still provide necessary cooling to safety-related equipment.
(3) The 10-gpm leak rate limit is small compared to the design-basis accident analysis flow rate of 8000 gpm of SW piping.
(4) A 10-gpm leak will not flood the reactor building because the sump has sufficient capacity to pump water out of the pipe chase or the reactor building.
(5) Annunciator alarms will notify operators in case leakage does occur.
3.2.4 Inspection Strategy The proposed alternative includes compensatory actions to perform periodic examinations to determine flaw growth and to monitor for leakage. The licensee will perform repair/replacement activities in accordance with the ASME Code,Section XI, IWA-4000, if the flaw growth rate is unacceptable or the identified leakage exceeds 10 gpm. In any case, the licensee will perform repair/replacement activities no later when the predicted flaw size is expected to exceed 10 inches in the axial or the circumferential direction, or during the next scheduled refueling outage, whichever occurs first.
The NRC staff finds that the proposed compensatory actions provide reasonable assurance of the structural integrity of the degraded reducer in the SW system and that flooding will not be a concern until the proposed conditions for repair/replacement are met or until the next scheduled refueling outage, whichever comes first.
3.2.5 Summary The licensee has demonstrated that the structural integrity of the degraded reducer of the SW piping can be maintained with an allowable hole size up to 10 inches in the axial or the circumferential direction. The licensee has also demonstrated that the potential leakage from the through-wall flaw will not affect the safety-related structures, systems and components in the vicinity of the degraded pipe. As a compensatory measure, the licensee will perform periodic inspections and walkdowns to monitor leakage and flaw growth. Therefore, the NRC staff finds that the proposed alternative is acceptable because compliance with the ASME Code,Section XI, requirement to repair or replace the degraded reducer would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
4.0 CONCLUSION
As set forth above, the NRC staff has determined that complying with the requirements described in the licensees request would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. The proposed alternative provides reasonable assurance of the structural integrity of the degraded reducer in the SW pipe.
Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the use of the proposed alternative at Brunswick, Unit 1, until the conclusion of the spring 2026 refueling outage which is currently planned to start on March 7, 2026.
All other ASME Code,Section XI requirements for which an alternative was not specifically requested and authorized remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributors: A. Rezai, NRR J. Tsao, NRR Date of issuance: May 16, 2025
ML25135A417 NRR-028 OFFICE NRR/DORL/LPL2-2/PM NRR/DORL/LPL2-2/LAiT NRR/DNRL/NPHP/BC NAME BPurnell CAdams (SL)
MMitchell DATE 5/15/2025 5/16/2025 5/12/2025 OFFICE DORL/LPL2-2/BC NAME DWrona (NJordan for)
DATE 5/16/2025