ML25255A305
| ML25255A305 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/18/2025 |
| From: | Ilka Berrios NRC/NRR/DORL/LPL3 |
| To: | Paulhardt W Northern States Power Company, Minnesota |
| Wall S, NRR/DORL/LPL3 | |
| References | |
| EPID L-2025-LLR-0085 | |
| Download: ML25255A305 (1) | |
Text
September 18, 2025 PRAIRIE ISLAND NUCLEAR GENERATING PLANT, UNITS 1 AND 2 - AUTHORIZATION AND SAFETY EVALUATION FOR ALTERNATIVE REQUESTS 1-RR-5-16 AND 2-RR-5-16 (EPID L-2025-LLR-0085)
LICENSEE INFORMATION Recipients Name and Address:
Werner K. Paulhardt, Jr.
Site Vice President Northern States Power Company - Minnesota Prairie Island Nuclear Generating Plant 1717 Wakonade Drive East Welch, MN 55089 Licensee:
Northern States Power Company - Minnesota Plant Name and Unit:
PRAIRIE ISLAND NUCLEAR GENERATING PLANT (PRAIRIE ISLAND), UNITS 1 AND 2 Docket No.:
50-282 and 50-306 APPLICATION INFORMATION Submittal Date: September 11, 2025 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML25254A252 Supplement Date: September 16, 2025 Supplement ADAMS Accession No.: ML25259A243 Applicable Inservice Inspection (ISI) Program Interval and Interval Start/End Dates:
The Fifth ISI program interval at Prairie Island, Units 1 and 2, began on December 21, 2014, and is scheduled to end on December 20, 2025.
Alternative Provision: The applicant requested an alternative under Title 10 of the Code of Federal Regulations (10 CFR), Section 50.55a(z)(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.
=
Applicable Code Edition and Addenda===
ISI Program Code Record: American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, 2007 Edition with the 2008 Addenda.
Code of Record for Repair and Replacement: 2019 Edition with no Addenda Code of Record for Cooling Water system: ASME B31.1, 1967 Edition, Power Piping Applicable Code Requirements:
ASME Code,Section XI, IWD-3120(b) states that flaws in ASME Code Class components that do not meet the standards of IWD-3400 shall be subjected to supplemental examination or to a repair/replacement activity.
ASME Code Case N-513-5, Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping Section XI, Division 1, describes the expanded examinations required if unacceptable flaws are detected. ASME Code Case N-513-5 is conditionally approved for use in Regulatory Guide 1.147. The 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 than 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 examinations 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.
Brief Description of the Proposed Alternative:
In lieu of the required expanded examinations described as outlined in Section 5 of Code Case N-513-5, the licensee has requested authorization to apply alternative augmented examination requirements for this specific cooling water piping flaw at Prairie Island, Units 1 and 2, as detailed in the submittal and summarized below.
The licensee is requesting an alternate volumetric examination be used to assess degradation of the affected system, as described below:
(a) From an engineering evaluation, the most susceptible locations shall be identified. A sample size of at least five of the most susceptible and accessible locations, or, if fewer than five, all susceptible and accessible locations shall be examined within 30 days of detecting the flaw.
(b) When a location less than design minimum wall thickness is detected, an additional sample of the same size as defined in (a) shall be examined.
(c) This process shall be repeated within 15 days for each successive sample, until no locations unacceptable by Section 3 of Code Case N-513-5 are detected or until 100%
of susceptible and accessible locations have been examined.
In addition to repairing the through-weld flaw during the next Unit 2 refueling outage, additional portions of 4-2CL-90 will be replaced. The replacement will include a section from MV-32030 to the second upstream elbow.
For additional details on the licensees request, please refer to the documents located at the ADAMS Accession No. identified above.
REGULATORY EVALUATION Regulatory Basis: 10 CFR 50.55a(z)(2)
Paragraph 10 CFR 50.55a(z) states, in part, that alternatives to the requirements of 10 CFR 50.55a(b)-(h) may be used, when authorized by the Director, Office of Nuclear Reactor Regulation, if (1) the proposed alternatives would provide 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.
The licensee has submitted this request on the basis that compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in quality or safety.
STAFF EVALUATION The NRC staff has evaluated RRs 1-RR-5-16 and 2-RR-5-16 pursuant to 10 CFR 50.55a(z)(2).
Specifically, the NRC has reviewed flaw characterization, system operability, structural integrity, and hardship justification for the proposed alternative as follows.
The licensee is proposing to use an alternative to the augmented examination requirements of ASME Code Case N-513-5, Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping and Gate ValvesSection XI, Division 1. ASME Code Case N-513-5 is conditionally approved by the NRC and allows for the temporary acceptance of flaws (including through-wall flaws), in moderate energy Class 2 and 3 piping without performing repair or replacement activities in accordance with the ASME Code. On December 18, 2024, the licensee detected a through-wall flaw on a pipe-to-valve weld for the Prairie Island, Unit 2 cooling water supply line to the 22 turbine-driven auxiliary feedwater pump, located on the upstream weld of MV-32030. The subject line is a schedule 80, 4-inch nominal pipe size line, with a nominal wall thickness of 0.337-inch, and fabricated from carbon steel. The operating temperature and pressure are 85 degrees Fahrenheit (°F) and 125 pounds per square inch gauge (psig), respectively. The design temperature and pressure are 100 °F and 150 psig, respectively.
The licensee indicated that the cooling water system at Prairie Island is a raw water system, and as such it is subject to degradation due to Microbiologically Influenced Corrosion (MIC). The flaw was characterized and accepted for continued operation without repair or replacement until the next refueling outage, in accordance with ASME Code Case N-513-5. The through-wall flaw was characterized as one-inch in the circumferential direction and one-inch in the axial direction.
The licensee used the criteria of Code Case N-513-5 to determine the maximum acceptable flaw size for structural margin as 4.34-inch in the circumferential direction and 6.15-inch in the axial direction. The licensee has also taken steps to monitor the leakage of water through the existing flaw.
The licensee performed the required daily monitoring of the through-wall flaw for leakage and performed the required augmented examinations on January 2, 2025. The results of the five augmented examinations are provided in Table 1 of the submittal. The wall thickness values from the augmented examinations ranged from 12.5 percent to 15.7 percent of the nominal wall thickness.
The licensees review of its previous MIC inspection data for its cooling water system revealed that implementing the augmented examinations required by ASME Code Case N-513-5, as conditioned, could result in having to examine almost all cooling water piping welds for both units at Prairie Island. Performing these augmented examinations would require involving extensive scaffolding and expend significant engineering and nondestructive examination (NDE) resources.
Flaw Characterization The NRC noted that the actual flaw which is 1 inch axially by 1 inch circumferentially on the weld is bounded by the allowable 4.34-inch by 6.15-inch rectangular flaw based on the Code Case N-513-5 evaluation. Additionally, successive inspections of the weld flaw have shown the flaw has not grown by an appreciable amount. The NRC staff finds that the licensee has appropriately characterized the existing flaw, which is conservatively bounded by the allowable flaw size.
The thickness measurements performed as part of the original augmented inspections did find regions with greater than 75 percent reduction in wall thickness. This is mitigated by the original pipe design, which is significantly thicker than is required for these locations. Despite the significant wall thinning, the remaining wall thicknesses are well above the required minimum wall thicknesses, as shown in Table 1 of the submittal.
System Operability The NRC staff finds that the operability of the Prairie Island, Unit 2, cooling water supply line is not challenged due to the existing flaw. This is based on the relatively small observed leakage, which was initially quantified as approximately 3 drops per minute (dpm) on December 18, 2024. Over the last 9 months the licensee indicated that the observed leakage has varied from 1 to 4 dpm. The NRC staff finds that the operability and functionality of the degraded piping will not be significantly affected during the term of the proposed alternative, because the leakage is small (i.e. 1 to 4 dpm) and has been relatively stable over the last 9 months. In addition, the licensees daily monitoring of the leakage will ensure that the piping system is capable of performing its intended function.
Structural Integrity The licensee evaluated the degraded piping section in accordance with Code Case N-513-5, which indicated that the maximum acceptable flaw size for structural margin is 4.34-inch in the circumferential direction and 6.15-inch in the axial direction. Based on the initial characterization of the flaw, the existing flaw is conservatively bounded by the allowable flaw. Additionally, subsequent examinations have confirmed that the flaw has essentially remained unchanged since initial characterization. The NRC staff does not expect the flaw to grow significantly during the time the alternative will be in use. Furthermore, if the flaw does grow unexpectedly, the licensees periodic monitoring and examinations are expected to detect the flaw growth prior to challenging the structural integrity of the piping. The NRC staff finds that the licensees flaw evaluation has adequately demonstrated the structural integrity of the subject piping.
Hardship Justification The NRC staff finds that without the proposed alternative the licensee could potentially be forced to examine essentially all of the cooling water welds for both units at Prairie Island. As stated by the licensee this would require extensive plant engineering and NDE resources.
The NRC staff noted that the activity would also require extensive use of scaffolding, which would increase the risk to plant personnel. While this hardship does not involve significant radiological dose to workers, the results showing all measured wall thicknesses being above the design minimum wall thicknesses shows that the change in safety for this alternative is very small. Additionally, the licensee has already scheduled alternative augmented examinations for the subject cooling water piping along with replacement activities for cooling water piping with unacceptable degradation. These planned activities will take place during the next refueling outage which is scheduled to begin on October 10, 2025. The NRC staff therefore finds that in this case the required augmented examinations would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
CONCLUSION As described above, the NRC staff has determined that alternative requests 1-RR-5-16 and 2-RR-5-16 for Prairie Island, Units 1 and 2, as described in the licensees submittal dated September 11, 2025, as supplemented by letter dated September 16, 2025, will provide an acceptable level of quality and safety. The NRC has also determined that the licensee has justified that compliance with the specified ASME BPV Code,Section XI 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 the regulatory requirements set forth in 10 CFR 50.55a(z)(2).
The NRC staff authorizes the use of proposed alternatives 1-RR-5-16 at Prairie Island, Unit 1, and 2-RR-5-16 at Prairie Island, Unit 2, for the remainder of the Fifth ISI Interval.
All other ASME BPV 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: Varoujan Kalikian and Stephen Cumblidge Date: September 18, 2025 Ilka Berrios, Acting Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation cc: Listserv ILKA BERRIOS Digitally signed by ILKA BERRIOS Date: 2025.09.18 11:50:38 -04'00'
- via eConcurrence NRR-028 OFFICE NRR/DORL/LPL3/PM*
NRR/DORL/LPL3/LA*
NRR/DNRL/NPHP/BC NAME SWall SLent MMitchell DATE 9/17/2025 9/17/2025 9/17/2025 OFFICE NRR/DORL/LPL3/BC (A)*
NRR/DORL/LPL3/PM*
NAME IBerrios SWall DATE 9/18/2025 9/18/2025