ML25338A044
| ML25338A044 | |
| Person / Time | |
|---|---|
| Site: | Cooper  (DPR-046) |
| Issue date: | 12/10/2025 |
| From: | Tony Nakanishi Plant Licensing Branch IV |
| To: | Dent J Nebraska Public Power District (NPPD) |
| References | |
| EPID L-2025-LLR-0052 | |
| Download: ML25338A044 (0) | |
Text
December 10, 2025 Mr. John Dent, Jr.
Executive Vice President and Chief Nuclear Office Nebraska Public Power District Cooper Nuclear Station 72676 648A Avenue P.O Box 98 Brownville, NE 68321
SUBJECT:
COOPER NUCLEAR STATION - RELIEF REQUEST NO. RR6-01 PROPOSED ALTERNATIVE TO INSERVICE CODE REQUIREMENTS FOR RESIDUAL HEAT REMOVAL AND RESIDUAL HEAT REMOVAL SERVICE WATER BOOSTER PIPING (EPID L-2025-LLR-0052)
Dear Mr. Dent:
By letter dated May 12, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25132A198), Nebraska Public Power District (the licensee) requested relief from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, IWC-3122.2 and IWD-3122.2 at Cooper Nuclear Station (Cooper). The licensee submitted Relief Request RR6-01 for the U.S. Nuclear Regulatory Commissions (NRC) review and approval for the repair/replacement of residual heat removal (RHR) service water booster (RHRSWB) piping using 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, with exceptions.
Specifically, pursuant to Title 10 for the Code of Federal Regulations (10CFR) 50.55a(z)(2), the licensee submitted Relief Request RR6-01 on the basis that compliance with the specified ASME Code repair would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that that complying with the specified ASME Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the use of Relief Request RR6-01, as documented in the submittal dated May 12, 2025, for the duration of the Code of Record interval which commences on March 1, 2026, and ends February 28, 2050.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
J. Dent, Jr.
If you have any questions, please contact the Cooper Project Manager at 301-415-3719 or via email at Thomas.Byrd@nrc.gov.
Sincerely, Tony Nakanishi, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-298 Enclosure Safety Evaluation cc: Listserv TONY NAKANISHI Digitally signed by TONY NAKANISHI Date: 2025.12.10 12:21:59 -05'00'
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST NO. RR6-01 PROPOSED ALTERNATIVE TO INSERVICE CODE REQUIREMENTS FOR RESIDUAL HEAT REMOVAL AND RESIDUAL HEAT REMOVAL SERVICE WATER BOOSTER PIPING NEBRASKA PUBLIC POWER DISTRICT COOPER NUCLEAR STATION DOCKET NO. 50-298
1.0 INTRODUCTION
By letter dated May 12, 2025, (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25132A198), Nebraska Public Power District (NPPD, the licensee) requested relief from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, IWC-3122.2 and IWD-3122.2 at Cooper Nuclear Station (Cooper). The licensee submitted Relief Request RR6-01 for the U.S. Nuclear Regulatory Commissions (NRC) review and approval for the repair/replacement of residual heat removal (RHR) service water booster (RHRSWB) piping using ASME Code Case N-513-5, Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping Gate Valve Section XI, Division 1, with exceptions.
Specifically, pursuant to Title 10 for the Code of Federal Regulations (10CFR) 50.55a(z)(2),
Hardship without a compensating increase in quality and safety, the licensee submitted Relief Request RR6-01 on the basis that compliance with the specified ASME Code repair would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
2.0 REGULATORY EVALUATION
Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), Inservice Inspection standards requirements for operating plants, which states in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in ASME Code,Section XI.
The regulations in 10 CFR 50.55a(z), Alternatives to codes and standards requirements, states, that:
Alternatives to the requirements of paragraphs (b) through (h) of {10 CFR 50.55a} or portions thereof may be used when authorized by the Director, Office of Nuclear Reactor Regulation A proposed alternative must be submitted and authorized prior to implementation. The applicant or licensee must demonstrate that:
(1) Acceptable level of quality and safety. The proposed alternative would provide an acceptable level of quality and safety, or (2) Hardship without a compensating increase in quality and safety.
Compliance with the specified requirements of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Based on the above, and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request the use of an alternative and the NRC to authorize the proposed alternative.
3.0 TECHNICAL EVALUATION
3.1 Licensees Proposed Alternative 3.1.1 ASME Code Components Affected The licensee is requesting approval to apply ASME Code Case N-513-5 to the RHRSWB system piping having a maximum operating pressure of 490 per square inch gauge (psig) and a maximum operating temperature of 200 degrees Fahrenheit (°F).
The RHR and RHRSWB systems are standby systems that typically operate during testing or plant shutdown. The licensee stated that the RHR and RHRSWB systems are designed such that the RHRSWB operates at a higher pressure than the RHR. Under this design, if there is an internal leak within a RHR heat exchanger, RHRSWB water, which is raw water from the Missouri River, will leak into the RHR system.
The licensee stated the RHRSWB is designed to provide an adequate supply of cooling water to the RHR heat exchangers during postulated accident and transient conditions to remove the design RHR system heat load and at adequate pressure to prevent uncontrolled release of fission products to the environment due to a RHR heat exchanger tube failure.
The licensee stated that the RHRSWB system has exhibited a history of degradation similar to raw freshwater systems throughout the nuclear industry. Degradation requiring immediate action to address leakage of observed thinning in the system is generally due to localized corrosion mechanisms.
3.1.2 Applicable ASME Code Editon and Addenda The applicable Code for the Code of Record interval is the 2019 Edition of ASME Code,Section XI. Coopers Code of Record interval will start March 1, 2026, and end February 28, 2050.
3.1.3 Applicable ASME Code Requirements The licensee stated that the ASME Code,Section XI, IWC-3122.2 and IWD-3132.2 requires that a component with relevant conditions are corrected by a repair/replacement activity or by corrective measures to the extent necessary to meet the acceptance standards of Tables IWC-3410-1 and IWD-3410-1.
3.1.4 Reason for Request In its submittal, the license stated that, In accordance with 10 CFR 50.55a(z)(2), [NPPD] is requesting a proposed alternative from the requirement to perform repair/replacement activities for degraded RHRSWB piping, which has a maximum operating pressure in excess of 275 psig. Moderately degraded piping could require a plant shutdown with the required action statement timeframes to repair the observed degradation. Plant shutdown activities result in additional dose and plant risk that would be inappropriate when a degraded condition is demonstrated to retain adequate margin to complete the components function. The use of an acceptable alternative analysis method in lieu of immediate action for a degraded condition will allow NPPD to perform additional extent of condition examinations on the affected systems while allowing time for safe and orderly long term repair actions if necessary. Actions to remove degraded piping from service could have a detrimental overall risk impact by requiring a plant shutdown, thus requiring use of a system that is in standby during normal operation. Accordingly, compliance with the current code requirements results in a hardship without a compensating increase in the level of quality and safety.
3.1.5 Proposed Alternative The licensee is requesting approval to apply ASME Code Case N-513-5 to the RHRSWB system piping having a maximum operating pressure of 490 psig. The licensee stated that the operational and configuration limitations of Code Case N-513-5, paragraphs 1(a), (1b), and 1(d) shall apply. The maximum operating temperature of 200°F in paragraph 1(c) shall also apply.
3.1.6 Basis for Use The licensee stated that the ASME recognized that relatively small flaws could remain in service without risk to the structural integrity of a piping system and developed ASME Code Case N-513. The NRC approved Code Case N-513-5 with conditions in Regulatory Guide (RG) 1.147, Revision 21, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, March 2024 (ML23291A003). The two conditions are as follows:
(1) For the purposes of Section 5 of Code Case N-513-5, a flaw is a non-through wall planar flaw or a non-through-wall nonplanar flaw with a wall thickness less that 87.5 percent of the nominal wall thickness of the pipe or the design minimum wall thickness.
(2) For the purposes of section 5 of Code Case N-513-5, the term significant flaw means any flaw found during augmented examination performed per Section 5 of N-513-5 that has a depth greater than 75 percent of the pipe wall thickness or that does not satisfy the applicable requirements of the flaw evaluation per Section 3 of N-513-5. If a significant flaw as defined above is present, then the licensee must perform the additional augmented examination specified in Section 5.
RG 1.147 allows temporary acceptance of partial through-wall or through-wall flaws provided all conditions of the Code Case and NRC conditions are met. The temporary acceptance period is the time to the next refueling outage. The Code Case also requires the Owner to demonstrate system operability due to leakage.
The licensee stated that it will evaluate each leak using the NPPD Operability Evaluation process. The evaluation process must consider requirements or commitments established for the system, continued degradation and potential consequences, operating experience and engineering judgement. The licensee will consider, but not limited to, system make-up capacity, containment integrity with the leak not isolated, effects on adjacent equipment, and the potential for room flooding.
The licensee also stated that the effects of leakage may impact the operability determination, or the plant flooding analyses specified in paragraph 1(f) of ASME Code Case N-513-5. For a leaking flaw, the allowable leakage rate will be limited to 5 gallons per minute (gpm) to limit effects of jet thrust force even though structural evaluation of the subject piping and leakage effects would allow a much higher leakage rate than 5 gpm. The licensee stated that any leakage, if present, will be limited to the leakage allowed by the evaluation of 5 gpm, whichever is lower.
The licensee further states that during the temporary acceptance period, leaking flaws will be monitored daily as required by paragraph 2(f) of Code Case N-513-5 to confirm the analysis conditions used in the evaluation remain valid. Any significant change to the leakage rate is reason to question that the analysis conditions remain valid and would require re-inspection per paragraph 2(f) of the Code Case. Any reinspection must be performed in accordance with paragraph 2(a) of the Code Case.
3.1.7 Duration of Proposed Alternative The licensee stated in its submittal with respect to the provisions of ASME Code Case N-513-5 will be applied for the duration of the Code of Record interval which commences on March 1, 2026, and ends February 28, 2050. The licensee stated that a Section XI-compliant repair/replacement will be completed prior to exceeding the next scheduled refueling outage, or allowable flaw size, or leakage in excess of 5 gpm, whichever comes first. If a flaw is evaluated near the end of the interval and the next refueling outage is in the subsequent interval, the flaw may remain in service under this relief request until the next refueling outage.
3.2
NRC Staff Evaluation
The NRC staff evaluated the adequacy of the proposed alternative in maintaining the structural integrity of the piping components identified in ASME Code Case N-513-5. Code Case N-513-5 is conditionally approved for use in RG 1.147, Revision 21 with two conditions that provides alternative evaluation criteria for temporary acceptance of flaws, including through wall flaws, in moderate energy Class 2 and 3 piping. The licensee will apply ASME Code Case N-513-5 and RG 1.147, Revision 21 (or later NRC defined revision as applicable) for evaluation of RHRSWB piping flaws at Cooper if code repairs cannot reasonably be completed within the technical specifications required time limit. The licensee is requesting a proposed alternative from the requirement to perform repair/replacement activities of degraded RHRSWB piping, which has a maximum operating pressure in excess of 275 psig. The licensee stated that the Class 3 RHRSWB has a design pressure of 490 psig and an operating pressure of approximately 420 psig. The NRC staff evaluated the licensees ability to demonstrate system operability due to leakage, the design basis for each leak and evaluated using the licensees Operability Evaluation process, temporary acceptance period, structural integrity evaluations, and a jet thrust force evaluation.
In its submittal dated May 12, 2025, the licensee identified the piping to be Class 3, RHRSWB, and is utilized as a standby system that typically operates during testing or plant shutdown. The licensee further stated that the safety objective of the RHRSWB system is to provide cooling to the RHR system without an uncontrolled release of radioactive material to the environment. The RHRSWB system is designed to provide an adequate supply of cooling water to the RHR heat exchangers during postulated accidents and transient conditions to remove the design RHR system heat loads and at adequate pressure to prevent uncontrolled release of fission products to the environment due to a RHR heat exchanger tube failure. The NRC staff determined that the scope of the subject piping is appropriately defined and identified. Therefore, the NRC staff finds that the subject piping satisfies paragraph 1(a) of ASME Code Case N-513-5.
Paragraph 1(c) of ASME Code Case N-513-5 limits the maximum operating pressure to 275 psig. To justify the use of this code case on the RHRSWB piping with a maximum operating pressure of 490 psig, the licensee demonstrated in enclosure 2 of the May 12, 2025 submittal that (1) structural integrity of the RHRSWB piping is achieved by flaw evaluation, (2) structural integrity of the subject piping will be maintained by periodically inspecting and monitoring, and (3) in an effort to bolster defense in depth and avoid adverse consequences as a result of increasing the maximum operating pressure to 490 psig, the licensee is making one additional commitment to apply a 5 gpm leakage limit or lower as allowed by engineering evaluation. The licensee stated that this relief request is no longer valid for leakage greater than 5 gpm. Based on the flaw evaluation, periodic inspections and monitoring, and the commitment to apply a maximum 5 gpm leakage limit, the NRC staff finds the proposed alternative is acceptable with respect to paragraph 1(c) of ASME Code Case N-513-5.
Paragraph 1(f) of ASME Code Case N-513-5 requires that the Owner consider the effects of leakage in demonstrating system operability and performing plant flooding analysis. The licensee stated that each leak will be considered and evaluated using the NPPD Operability Evaluation process. The evaluation process must consider requirements or commitments established for the system, continued degradation and potential consequences, operating experience and engineering judgement. The evaluation process also considers but is not limited to system make-up capacity, containment integrity with the leak not isolated, effects on adjacent equipment, and the potential for room flooding. Based on the information provided above, the NRC staff finds it acceptable because the licensees operability evaluation will consider system makeup capacity, containment integrity with the leak not isolated, effects on the adjacent equipment, and the potential for room flooding.
The licensee further stated that for a leaking flaw, the allowable leakage rate will be limited to 5 gpm to limit effects of jet thrust force even though a structural evaluation of the subject piping and leakage effects would allow a much higher leakage rate than 5 gpm. The licensee committed that any leakage if present, will be limited to the leakage allowed by evaluation or 5 gpm, whichever is lower. As stated above, the NRC staff finds the proposed alternative is acceptable with respect to paragraph 1(f) of ASME Code Case N-513-5.
The licensee also stated that they will perform periodic inspection intervals as required by paragraph 2(e) of ASME Code Case N-513-5 and will provide evidence that a leaking flaw continues to meet the flaw acceptance criteria and that the flaw growth rate is such that the flaw will not grow to an unacceptable rate. The NRC staff finds this acceptable since the licensee will be following the requirements of Code Case N-513-5.
During the temporary acceptance period, the licensee stated that leaking flaws will be monitored daily as required by paragraph 2(f) of ASME Code Case N-513-5 to confirm the analysis conditions used in the evaluation remain valid. Any significant change in the leakage rate is reason to question that the analysis conditions remain valid and would require re-inspection per paragraph 2(f) of the Code Case. The licensee stated that the re-inspection must be performed in accordance with paragraph 2(a) of the Code Case. The NRC staff finds this acceptable since the licensee will be following the requirements as stated in Code Case N-513-5.
The licensee provided attachment 2 to RR6-01, which provides the technical basis for the proposed alternative to use ASME Code Case N-513-5 and scope expansions to higher-pressure limits. Table 3 of attachment 2 provides generic allowable axial and circumferential flaw size comparisons to nominal pipe sizes, not specific to Cooper. Table 3 illustrates the effect of pressure on the allowable through wall flaw size. The table shows that the plant specific allowable flaw size is a function of the applied loading at the specific location for which the code case will be applied. The licensee stated that the exact location for which the Code Case will be applied cannot be known at the time of this submittal. The NRC staff notes that Table 3 showed that with a higher operating pressure of 490 psig, the allowable flaw size based on structural consideration will be smaller than those at the Code Case limit of 275 psig. The licensee stated that minimum wall thickness evaluations showed that the allowable minimum pipe wall thickness will be higher for the 490 psig than for the 275 psig. However, the licensee concluded that the actual change in the minimum required pipe wall thickness is relatively low because the hoop stress at these pressures is small. The NRC staff reviewed the licensees evaluations and the results provided in tables 2 and 3 to be acceptable.
to RR6-01 also provided a Jet Thrust Force Evaluation. The figure in the attachment showed a comparison of jet thrust force for pressures of 275 psig and 490 psig over a range of through wall opening diameters. The licensee stated that a significant change in jet thrust force was only seen with large opening areas that would result in high leak rates that would challenge system functionality or local spray and/or compartment flooding requirements.
The NRC staff reviewed the evaluations and the figure provided and finds the results acceptable.
ASME Code Case N-513-5 requires a cover thickness requirement comparison. The Code Case provides a branch reinforcement method to evaluate nonplanar, through-wall flaws. The licensee stated that with the branch reinforcement approach, an opening is modeled such that its diameter fully bounds the leaking flaw. The licensee stated that Equation 9 of Code Case N-513-5 provides assurance against pressure blowout (i.e., ligament failure) by requiring an average cover thickness within the model opening. The licensee provided a figure which illustrated the average cover thickness required as a function of adjusted diameter for 275 and 490 psig. The average cover thickness with an assumed allowable stress of 17.1kilopounds per square inch (ksi) increased about 33 percent with a maximum operating pressure from 275 to 490 psig. The licensee noted that typically modeled openings have a diameter less than (<)
1 inch and the change in the required cover thickness is small (< 15 mils). The NRC staff has reviewed the evaluations and the licensee has satisfied the provisions in Code Case N-513-5.
The evaluation is considered acceptable.
For an augmented examination process, the licensee will follow the requirements as described in section 5 of ASME Code Case N-513-5. A sample size of at least five of the most susceptible and accessible locations will be examined within 30 days of detecting the original flaw.
In addition, the licensee stated that the implementation of ASME Code Case N-513-5 requires additional actions to be satisfied including observing leakage daily to confirm analysis conditions used in the evaluation remain valid, frequent periodic inspections to track flaw growth, daily walkdowns by plant personnel and augmented examinations to assess degradation of the affected system. In addition, larger leaks could be detected by the radwaste operators by observing changes to the floor drain and/or equipment drain collector tank levels. The NRC staff finds that the licensee does have adequate means and reasonable intervals to monitor the subject piping and report the leak before reaching the maximum limit leakage to 5 gpm.
4.0 CONCLUSION
As set forth above, the NRC staff determines that the proposed alternative as documented in the submittal dated May 12, 2025, provides reasonable assurance of structural integrity of the RHRSWB piping. The NRC staff concludes that complying with the specified ASME Code requirements would result in actions to remove degraded piping from service, which could require a plant shutdown to repair observed degradation. Plant shutdown activities result in additional dose and plant risk that would be inappropriate when a degraded condition is demonstrated to retain adequate margin to complete the components function. Therefore, the staff finds that allowing the licensee to operate the unit until a Section XI-compliant repair/replacement is completed prior to exceeding: (1) the next scheduled refueling outage, (2) the allowable flaw size, or (3) leakage in excess of 5 gpm, whichever comes first, is acceptable.
Therefore, the NRC staff concludes that complying with the specified ASME Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the use of Relief Request RR6-01, as documented in the submittal dated May 12, 2025, for the duration of the Code of Record interval which commences on March 1, 2026, and ends February 28, 2050.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: Eric Reichelt, NRR Date: December 10, 2025
- via eConcurrence NRR-028 OFFICE NRR/DORL/LPL4/PM*
NRR/DORL/LPL4/LA*
NRR/DNRL/NPHP/BC*
NAME TByrd PBlechman MMitchell DATE 12/9/2025 12/9/2025 12/9/2025 OFFICE NRR/DORL/LPL/BC*
NAME TNakanishi DATE 12/10/2025