L-PI-25-041, CFR 50.55a Request Nos. 1-RR-5-16 and 2-RR-5-16, Proposed Alternative to Conditions for Applying Code Case N-513-5, Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping and Gate
| ML25254A252 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/11/2025 |
| From: | Currier B Northern States Power Company, Minnesota, Xcel Energy |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-PI-25-041 | |
| Download: ML25254A252 (1) | |
Text
1717 Wakonade Drive Welch, MN 55089 L-PI-25-041 10 CFR 50.55a September 11, 2025 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Prairie Island Nuclear Generating Plant Units 1 and 2 Docket Nos. 50-282 and 50-306 Renewed Facility Operating License DPR-42 and DPR-60 10 CFR 50.55a Request Nos. 1-RR-5-16 and 2-RR-5-16, Proposed Alternative to Conditions for Applying 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"
References:
- 1) 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," Approval Date: April 18, 2018.
- 2) Regulatory Guide 1.147, Revision 21, "Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1," Issue Date: March 2024.
Pursuant to 10 CFR 50.55a(z)(2), Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), requests U.S. Nuclear Regulatory Commission (NRC) authorization of an alternative to the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, 2007 Edition through 2008 Addenda for the Prairie Island Nuclear Generating Plant (PINGP) Unit 1 and Unit 2.
NSPM is requesting NRC authorization of the proposed alternative for the 5th ten-year interval of the PINGP lnservice Inspection (ISI) Program. Specifically, NSPM is requesting authorization to apply ASME Code Case N-513-5 (Reference 1) with Conditions different than provided in Table 2 of Regulatory Guide 1.147, Revision 21 (Reference 2). The enclosed proposed alternative for Unit 1 (1-RR-5-16) and Unit 2 (2-RR-5-16) provides the basis and supporting information for the proposed alternative.
PINGP is currently in the 5th ten-year interval, which began on December 21, 2014, and is currently scheduled to end December 20, 2025. NSPM requests authorization of this 10 CFR 50.55a request by September 19, 2025.
Document Control Desk L-Pl-25-041 Page 2 Summary of Commitments This letter makes no new commitments and no revisions to existing commitments.
If there are any questions or if additional information is required, please contact Mr. Jeff Kivi at 612-33z 88 or L
@xcelenergy.com.
Bryan Cu,'ff-Plant Manager, Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, USNRC Project Manager, Prairie Island, USNRC Resident Inspector, Prairie Island, USNRC
L-PI-25-041 NSPM Enclosure Page 1 of 11 10 CFR 50.55a Request 1-RR-5-16 (PINGP Unit 1) 10 CFR 50.55a Request 2-RR-5-16 (PINGP Unit 2)
Proposed Alternative to Reactor Vessel Inservice Inspection (ISI) Intervals Proposed Alternative in Accordance with 10 CFR 50.55a(z)(2)
Hardship or Unusual Difficulty without Compensating Increase in Level of Quality or Safety
- 1. American Society of Mechanical Engineers (ASME) Code Component(s) Affected Component:
Prairie Island Nuclear Generating Plant Cooling Water Piping to Auxiliary Feedwater Code Class:
3
Reference:
ASME Section XI Code Case N-513-5
==
Description:==
Cooling Water Piping leading to the Auxiliary Feedwater pumps suction Size:
Pipe Diameter - 4 inches Nominal Thickness - 0.337 inches Design Conditions:
Temperature - 100 degrees F Pressure - 150 psig Operating Conditions:
Temperature - 85 degrees F Pressure - 125 psig Materials:
Carbon steel, ASTM A-106 GR. B (pipe)
Carbon steel, ASTM A-234 GR. WPB (fitting)
Carbon steel, SA-105 (valve body)
2. Applicable Code Edition and Addenda
Prairie Island Nuclear Generating Plant (PINGP) is in its 5th ASME Section XI Code ISI interval which began on December 21, 2014 and ends on December 20, 2025. The current applicable edition of ASME Section XI for the ISI interval is the 2007 Edition with the 2008 Addenda [Reference 1]. The current applicable edition of ASME Section XI for the Repair and Replacement program is the 2019 Edition with no Addenda [Reference 2].
The code of record for the PINGP Cooling Water system is USAS/ASME B31.1, 1967 Edition, Power Piping. [Reference 3].
L-PI-25-041 NSPM Enclosure Page 2 of 11
3. Applicable Code Requirement
In accordance with ASME Code Section XI, 2007 Edition with the 2008 Addenda, subparagraph IWD-3120(b) 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.
In accordance with IWD-3400, the requirements of IWC-3400 may be used. IWC-3410 indicates the acceptance standards referenced in Table IWC-3410-1 shall be applied to determine acceptability for service. Table IWC-3410-1 indicates the acceptance standards for examination category C-H pressure retaining components are given in IWC-3516.
In accordance with IWC-3516 the standards of IWB-3522 may be applied. IWB-3522 indicates the following relevant conditions shall meet IWB-3142 and IWA-5250 prior to continued service:
a) Any through-wall or through-weld, pressure retaining material leakage from insulated or noninsulated components; b) Leakage in excess of limits established by the owner for mechanical connections or from components provided with leakage limiting devices; c) Areas of general corrosion of a component resulting from leakage; d) Discoloration or accumulated residue on surfaces of components, insulation, or floor areas that may be evidence of borated water leakage; e) Leakage or flow test results from buried components in excess of limits established by the owner.
For the subject component item, a) applies as it was discovered that through-weld leakage was occurring on the upstream weld of MV-32030 (22 turbine-driven auxiliary feedwater pump suction cooling water supply motor operated valve). Therefore, the requirements of IWB-3142 and IWA-5250 shall apply prior to continued service. IWA-5250 states the sources of leakage shall be located and evaluated by the owner for corrective actions as follows:
- 1) Buried components with leakage losses in excess of limits acceptable for continued service shall meet the requirements of IWB-3142, IWC-3132, or IWD-3120, as applicable.
- 2) If leakage occurs at a bolted connection in a system borated for the purpose of controlling reactivity, one of the bolts shall be removed, VT-3 examined, and evaluated in accordance with IWA-3100. The bolt selected shall be the one closest to the source of leakage. When the removed bolt has evidence of degradation, all remaining bolting in the connection shall be removed, VT-3 examined, and evaluated in accordance with IWA-3100.
- 3) Components requiring corrective action shall have repair/replacement activities performed in accordance with IWA-4000 or corrective measures performed where the relevant condition can be corrected without a repair/replacement activity.
L-PI-25-041 NSPM Enclosure Page 3 of 11
- 4) If boric acid residues are detected on components, the leakage source and the areas of general corrosion shall be located. Components with local areas of general corrosion that reduce the wall thickness by more than 10% shall be evaluated to determine whether the component may be acceptable for continued service, or whether repair/
replacement activities will be performed.
Of these conditions, only 3) applies to this discovered through-weld leak as this system is not buried and is not borated.
IWB-3142 outlines acceptance via: (IWB-3142.1) visual examination, (IWB-3142.2) supplemental examination, (IWB-3142.3) corrective measures or repair/replacement activities, (IWB-3142.4) and analytical evaluation. Through-weld leakage is a relevant condition unacceptable via visual or supplemental examination. Similarly, the ASME Section XI Code does not include analytical evaluation criteria for acceptance of through-weld flaws between pressure retaining components, Resultingly, acceptance may only be made via repair/replacement activities in accordance with ASME Section XI. IWA-4000 describes the repair/replacement activities to correct an unacceptable flaw.
Code Case N-513-5, "Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping Section XI, Division 1," [Reference 4] has been conditionally approved for use by the U.S. Nuclear Regulatory Commission (NRC) in Regulatory Guide 1.147, "lnservice Inspection Code Case Acceptability, ASME Section XI, Division 1," Revision 21 [Reference 5]. This code case provides analytical evaluation rules for temporary acceptance of flaws in piping systems deemed to not be high energy based on system temperature and pressure. This temporary analytical acceptance allows for deferral of repair and replacement activities until the next planned refueling outage. For PINGP Unit 2, this next outage is scheduled to begin October 10, 2025.
In addition to analytical acceptance criteria for flaws, Code Case N-513-5 outlines flaw monitoring criteria for the time period between flaw discovery and flaw repair. It also outlines the following augmented examination criteria in Section 5 which is a parallel to standard scope expansion criteria in ASME Section XI:
An augmented volumetric examination or physical measurement to assess degradation of the affected system shall be performed as follows:
(a) From the 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 flaw is detected, an additional sample of the same size as defined in (a) shall be examined.
L-PI-25-041 NSPM Enclosure Page 4 of 11 (c) This process shall be repeated within 15 days for each successive sample, until no significant flaw is detected or until 100% of susceptible and accessible locations have been examined.
With respect to Section 5, the NRC put the following conditions on the use of Code Case N-513-5 in NRC Regulatory Guide (RG) 1.147, Revision 21:
(1) For the purposes of Section 5 of Code Case N-513-5, a "flaw" is a nonthrough-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.
4. Reason for Request
Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), is requesting a proposed alternative to the Augmented Examination requirements as they are written in Section 5 of Code Case N-513-5 for this through-weld leak on the cooling water supply to the 22 auxiliary feed water pump. Specifically, NSPM is proposing alternative augmented examination requirements to those written in Section 5 of Code Case N-513-5 until the flaw can be repaired during next Unit 2 refueling outage scheduled to start October 10, 2025. During the refueling outage the detected through-weld leak will be repaired via a partial replacement of the affected line (4-2CL-90). For the duration of the proposed alternative, augmented examinations will be as described in Section 5 of this request.
On December 18, 2024 a through-weld flaw was discovered on the PINGP Unit 2 cooling water supply to the 22 turbine driven auxiliary feed water pump. The flaw is located on line 4-2CL-90 on the upstream weld of MV-32030. The line is NPS 4 schedule 80 carbon steel as noted in Section 1 of this request. The initially reported images associated with this through-weld flaw can be seen below in Figure 1. A diagram indicating the location of this through-weld flaw can be seen in Figure 2.
L-PI-25-041 NSPM Enclosure Page 5 of 11 Figure 1: Image of initially reported flaw from 12/18/25.
Figure 2: Flaw location relative to main cooling water supply header.
L-PI-25-041 NSPM Enclosure Page 6 of 11 The cooling water system at PINGP is a raw water system and is more commonly referred to as the service water system at other plants. As such, it is subject to Microbiologically Influenced Corrosion (MIC) degradation mechanisms.
During the period of discovery, the flaw was characterized and accepted for continued operation using Code Case N-513-5. The initial flaw was bounded by inspection to be no greater than one inch in the circumferential direction and one inch in the axial direction. The N-513-5 evaluation indicated that the maximum acceptable flaw size was 4.34 inches in the circumferential direction and 6.15 inches in the axial direction.
As part of this discovery phase, the MIC inspection results were reviewed for the cooling water system, and it was deemed MIC was the most likely cause of the through-weld flaw. As such, the growth rate of the flaw was expected to be very low with no meaningful growth until the next scheduled refueling outage. The flaw was accepted with N-513-5 for continued service and not immediately repaired.
Resultingly, in accordance with N-513-5 a monitoring and inspection plan was implemented for the detected through-weld flaw. This included daily leakage monitoring by operations logged in the narrative log. It also included follow up flaw characterization exams scheduled at a frequency not to exceed 30 days between successive exams. Over the past nine months the daily monitoring of the flaw has shown that that the leakage has changed from three drops per minute when initially reported to as high as four drops per minute down to less than 1 drop per minute most recently. The successive through-weld flaw inspections have shown that the flaw has not grown by an appreciable amount during this time.
In addition to monitoring this flaw, the first five augmented examinations required by Section 5 of Code Case N-513-5 were performed on the affected system on January 2, 2025, 15 days after the initial flaw was detected. This included four straight pipe sections and one elbow, all upstream of the leak on the same line. The results of these first five augmented exams can be seen below in Table 1.
Location Nominal wall thickness Measured minimum wall thickness (% nominal)
Design minimum wall thickness (% nominal)
ND-160393 P4 0.337 inches 0.259 inches (76.9%)
0.042 inches (12.5%)
ND-160393 P5 0.337 inches 0.151 inches (44.8%)
0.048 inches (14.2%)
ND-160393 P5A 0.337 inches 0.151 inches (44.8%)
0.048 inches (14.2%)
ND-160393 P7 0.337 inches 0.060 inches (17.8%)
0.045 inches (13.4%)
ND-160393 E8 0.337 inches 0.300 inches (89%)
0.053 inches (15.7%)
Table 1: Summary results for first five augmented examinations.
All of the inspected areas measured above the design minimum wall thickness, which ranges from 12.5% to 15.7% the nominal wall thickness. Four out of five inspections measured below
L-PI-25-041 NSPM Enclosure Page 7 of 11 87.5% nominal wall thickness. One of those four results had a flaw that measured greater than 75% of the pipe wall thickness.
NSPMs initial interpretation of the conditions RG 1.147, Table 2, for Code Case N-513-5 was that none of the above inspection locations should be considered flaws because they were above the design minimum wall thickness. Similarly, none of these would be considered significant flaws as all were above the applicable requirements of the flaw evaluation per Section 3 of N-513-5 and, thus, all are acceptable for continued service by Code Case N-513-5. As a result, no additional augmented examinations were performed in response to these initial augmented results.
Subsequent discussion with the NRC staff led to the conclusion that the conditions within RG 1.147 should be interpreted differently. That is, a detected area should be considered a flaw if either the measured wall thickness is below the 87.5% of the nominal wall thickness or if it is below the design minimum wall thickness, whichever is more limiting. Similarly, a detected area should be considered a significant flaw if either the measured defect depth is greater than 75% the nominal wall thickness or cannot be accepted via Code Case N-513-5, whichever is more limiting. Therefore, additional augmented examinations are required to be performed based on the results of the initial augmented examinations.
Further review of the previous MIC inspection data for the Cooling Water system showed that many of these inspections revealed wall thinning below 87.5% nominal wall thickness and some exams revealed flaws greater than 75% wall thickness. These indications are acceptable in accordance with the MIC program because the piping system is robust, having wall thicknesses far greater than the design minimum wall thickness. When selecting the most susceptible and accessible locations to examine per code case N-513-5, these locations necessarily must be selected because they are areas that contain known flaws and significant flaws as defined by RG 1.147. As a result, implementing augmented examinations with the acceptance criteria outlined Section 5 of Code Case N-513-5 creates a scenario where it is unlikely to get a sample of five augmented exams where additional scope expansion is not required.
Much of the cooling water system is original to the plant with some piping having been replaced as degradation reached an unacceptable level. An unintended consequence of cooling water piping being conservatively designed results when defining a flaw and significant flaw based on the nominal wall thickness instead of the stresses the pipe must withstand, which creates a scenario where acceptable and known degradation must be deemed unacceptable.
Contributing to the improbability of performing five augmented examinations where none of the results would be considered a flaw or significant flaw is the idea of factory deviation from nominal wall thickness. As noted by the staff in the Regulatory Analysis for Final Rule for RG 1.84, Revision 39, RG 1.147, Revision 20, and RG 1.192 Revision 4 (NRC ADAMS Accession No. ML21194A128), "The NRC notes that the pipe wall thickness at the time of the
L-PI-25-041 NSPM Enclosure Page 8 of 11 plant construction may deviate from the nominal pipe wall thickness slightly as part of manufacturing process. The generally accepted deviation is 12.5 percent of the nominal pipe wall thickness or the design minimum wall thickness." This indicates that if the augmented exams outlined in N-513-5 were to be performed on newly replaced pipe, there is a chance that the pipe would be considered to contain flaws as defined in RG 1.147 by having expected deviation from nominal wall thickness during pipe fabrication.
In terms of the scope of the augmented examinations, there is a similar 4-inch schedule 80 cooling water supply to each of the of the four auxiliary feed water pumps (two per unit) susceptible to similar degradation. Moreover, although the cooling water system is divided into two trains, these trains are normally cross connected to increase redundancy. Similarly, these trains are common to both PINGP Unit 1 and 2. As a result, following the augmented examination conditions outlined in Section 5 of Code Case N-513-5 with the definitions in RG 1.147 as written would result in an inspection of essentially all of the cooling water system for both Unit 1 and 2. Performing these inspections where piping exceeds the design minimum wall thickness requires scaffolding, engineering planning, and significant NDE resources which results in a hardship or unusual difficulty without a compensating increase in the level of quality and safety.
5. Proposed Alternative and Basis for Use
NSPM is requesting authorization to apply alternative augmented examination requirements that differ from those outlined in Section 5 of Code Case N-513-5. Specifically, for this cooling water flaw, NSPM is requesting the augmented volumetric examination used to assess degradation of the affected system be implemented as follows:
(a) From the 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.
These requirements are aligned with the language of Code Case N-513-6 [Reference 6]
which states the following:
(a) If an augmented examination detects a planar flaw, the flaw may be evaluated in accordance with this Case to show acceptance for continued operation until the next scheduled refueling outage. Timing of successive examinations shall be based on the requirements of 2(e).
L-PI-25-041 NSPM Enclosure Page 9 of 11 (b) If an augmented examination location indicates a non-through-wall nonplanar wall loss with a predicted thickness at the next scheduled refueling outage of less than 0.875tnom, the location may be evaluated in accordance with this Case to show acceptance for continued operation until the next scheduled refueling outage. Timing of successive examinations shall be based on the requirements of 2(e).
(c) Alternatively, a non-through-wall flaw may be analytically evaluated in accordance with the procedures of IWC-3100 for Class 2 items, in accordance with the procedures of IWD-3100 for Class 3 items, or in accordance with the Construction Code to justify continued operation for an operating period that may extend beyond the next scheduled refueling outage. The predicted flaw size for the evaluated period and timing of successive examinations or repair or replacement activities shall be based on this evaluation.
Similarly, prior to Code Case N-513, Generic Letter (GL) 90-05 [Reference 7] was used to allow for temporary non-Code repairs on ASME Code Class 1, 2, and 3 piping [3]. As a part of this guidance, the NRC included guidelines to perform a flaw evaluation for "through-wall flaws" and "wall thinning." As a part of this guidance, GL 90-05 requires that users assess the overall degradation of the system by an augmented inspection. Enclosure 1 of GL 90-05 states the following:
Flaws detected in the augmented inspection should be characterized and evaluated. If any flaw is detected having a minimum measured wall thickness "tmeas" less than the code-required minimum wall thickness "tmin" in the augmented inspection sample, inspection of an additional sample of the same size should be performed.
From this Generic Letter, additional examinations were only required when the results of an augmented inspection were below the design minimum wall thickness and had limited relation to the nominal wall thickness.
Cooling Water is routinely monitored as part of the PINGP MIC Program. Under this program 124 inspections have been performed over the past four outages to monitor MIC degradation within susceptible systems. This includes 18 exams planned for the next refueling outage specifically on the Cooling Water system. This is effectively 3.5 rounds of augmented examination scope expansion under N-513-5 as written. Per FP-PE-SW-01, the MIC program will perform additional examinations when a standard inspection reveals a location less than the design minimum wall thickness as part of the extent of condition process.
In addition to repairing the through-weld flaw during the next Unit 2 refueling outage, additional portions of 4-2CL-90 will be replaced with known degradation. The replacement will include a section from MV-32030 to second upstream elbow. It will also include from the cooling water supply to the second downstream elbow. An outline of this replacement can be seen in Figure 3.
L-PI-25-041 NSPM Enclosure Page 10 of 11 Figure 3: Planned piping replacement for 4-2CL-90 Therefore, the proposed alternative of the augmented examinations described above is an acceptable alternative to applying the Conditions that Table 2 of RG 1.147, Revision 21 places on the use of Code Case N-513-5, compliance with which would result in a hardship without a compensating increase in quality or safety.
6. Duration of Proposed Alternative
NSPM requests authorization of the proposed alternative by September 19, 2025, in order to minimize the hardship created by additional augmented examinations as described herein.
The proposed alternative would remain in effect until completion of the Prairie Island Nuclear Generating Plant Unit 2 Fall 2025 refueling outage scheduled to begin October 10, 2025. This duration shall not exceed the end of the PINGP 5th ISI interval that occurs December 20, 2025.
7. Precedents
None.
L-PI-25-041 NSPM Enclosure Page 11 of 11
- 8. References
- 1. ASME Boiler and Pressure Vessel Code,Section XI, 2007 Edition through 2008 Addenda, American Society of Mechanical Engineers, New York.
- 2. ASME Boiler and Pressure Vessel Code,Section XI, 2019 Edition, American Society of Mechanical Engineers, New York.
- 3. USAS/ASME B31.1, 1967 Edition, Power Piping.
- 4. 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," Approval Date: April 18, 2018.
- 5. Regulatory Guide 1.147, Revision 21, "Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1," Issue Date: March 2024.
- 6. Code Case N-513-6, "Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping and Gate ValvesSection XI, Division 1," Approval Date: October 28, 2021.
- 7. NRC Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping," June 15, 1990.