NLS2025023, 10 CFR 50.55a Request for Sixth and Seventh Twelve-Year Inservice Inspection Intervals
| ML25132A198 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 05/12/2025 |
| From: | Dia K Nebraska Public Power District (NPPD) |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NLS2025023 | |
| Download: ML25132A198 (1) | |
Text
N Nebraska Public Power District "Alway therc when.you neetl us" 10 CFR 50.55a NLS2025023 May 12,2025 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Subject 10 CFR 50.55a Request for Sixth and Seventh Twelve-Year Inservice Inspection Intervals Cooper Nuclear Station, Docket No. 50-298, Renewed License No. DpR-46 The purpose of this letter is for the Nebraska Public Power District (NPPD) to request that the Nuclear Regulatory Cbmmission (NRC) grant use of a proposed alternative to certain inservice code requirements for Cooper Nuclear Station (CNS) pursuant to 10 CFR 50.55a. The 10 CFR 50.55a request pertains to the application of American Society of Mechanical Engineers (ASME)
Code Case N-513-5 at a higher system operating pressure. The applicable ASME Code for the sixth and seventh Inservice Inspection (ISI) intervals and the ISI progrcm will be the2019 Edition of Section XI. The request is applicable to the Code of Record (COR) interval which commences on March 1,2026 and ends February 28,2050.
The COR interval will encompass the next two consecutive Inservice Inspection Intervals (6th and 7th) and Containment Inservice Inspection interval (4ft and 5th) as described in 10 CFR 50.55a(y)(2), which are being extended from l0 years to 12 years each based on the implementation of conditionally acceptable ASME Section XI Code Case N-921, "Altemative 12-yr Inspection Interval Duration,Section XI, Division 1." As required per Regulatory Guide 1.147, Revision 21, "Inservice Inspection Code Case Acceptability, ASME Section XI, Division," the conditions required to implement code case N-921 will be met. CNS will be implementing an acceptable edition of the ASME Section XI Code (2019 Edition), the code case will be implemented at the beginning of an interval, and the requirernents of N-921 will have been satisfied.
CNS has elected to update to the 2019 Edition of the ASME Section XI Code for the upcoming COR Interval, which was the latest edition incorporated by reference in 10 CFR 50.55a(a)(1)(ii) 18 months prior to the start of the new ISI interval. Per 10 CFR 50.55a(gxaxiv), NRC approval is not required when updating to the latest edition before the start of an ISI interval. NPPD requests approval of this relief request by March I,2026, in support of the start of the next COR ISI interval. contains Relief Request RR6-01. Attachment 2 contains a technical basis discussion for the proposed altemative.
COOPER NUCLEAR STATION 72676 648A Ave / P.O. Box 98 / Brownvilte, NE 68321 http://www.nppd.com
ldv NLS2025023 Page2 of3 A formal licensee commitment is being made by this submittal and is included on the last page of this cover letter.
Should you have any questions concerning this matter, please contact Linda Dewhirst, Regulatory Affairs & Compliance Manager, at(402) 825-5416.
Sincerely, Dia Site Vice President Attachments l. 10CFR50.55aRequestNo. RR6-01, Revision0, CooperNuclearStation Request to Use Code Case N-513-5 at a Higher System Operating Pressure
- 2. Cooper Nuclear Station Technical Basis for Proposed Alternative to Use ASME Code Case N-513-5 and Scope Expansion to a Higher Pressure Limit cc: Regional Administrator d attachments USNRC - Region IV Cooper Project Manager w/ attachments USNRC - NRR Plant Licensing Branch IV Senior Resident Inspector w/ attachments USNRC - CNS NPG Distribution w/ attachments CNS Records d attachments
NLS2025023 Page 3 of3 This table identifies an action discussed in this letter which NPPD commits to perform. Any other actions discussed in this submittal are described for the NRC's information and are not regulatory commitments.
COMMITMENT/COMMITMENT NO TYPE (check one)
SCHEDULED COMPLETION DATE ONE-TIME ACTION CONTINUING COMPLIANCE Commitment # - NLS2025023-01 When evaluating RHRSWB piping flaws, a maximum of 5 gpm leakage limit is applied, or lower if limited by the engineering evaluation. Relief Request RR6-01 is no longer valid for leakages greater than 5 gpm or as limited by the engineering evaluation.
X Within 60 days of NRC approval of Relief Request RR6-01
NLS2025023 Page I of5 10CFR50.55a Request No. RR6-01, Revision 0 Cooper Nuclear Station Request to Use Code Case N-513-5 at a Higher System Operating Pressure Proposed Alternative in Accordance with 10 CFR 50.55^(2)(2)
Hardship Without a Compensating Increase in Quality and Safety American Societv of Mechanical Ensineers (ASME) Code Component(s) Affected All ASME,Section XI, Class 3 Residual Heat Removal (RHR) Service Water Booster (RHRSWB) system piping with a maximum operating pressure less than or equal to 490 psig and a maximum operating temperature less than 200oF at the Cooper Nuclear Station (CNS).
The RHR and RHRSWB Systems are designed such that RHRSWB operates at a higher pressure than RHR. The RHR and RHRSWB Systans are standby systems that typically operate during testing or plant shutdown. 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 safety objective of the RHRSWB System is to provide cooling to the RHR Systern 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 accident and transient conditions to remove the design RHR System heat load and at adequate pressure to prevent uncontrolled release offission products to the environment due to a RHR heat exchanger tube failure.
RHRSWB System at CNS has exhibited a history of degradation similar to raw fresh water systems throughout the nuclear industry. Degradation requiring immediate action to address leakage or observed thinning in the system is generally due to localized corrosion mechanisms.
Applicable Code Edition and Addenda
CNS's applicable Code for the Code of Record (COR) interval is the 2019 Edition of Section XI. CNS's COR interval will start March 1,2026 and end February 28,2050.
Applicable ASME Code Requirements ASME Code,Section XI, IWC-3122.2 and IWD-3 132.2 requires that a component with relevant conditions is acceptable for continued service ifthe relevant conditions are corrected by arepairheplacement activity or by corrective measures to the extent necessary to meet the acceptance standards of Table IWC-3410-1 and Table IWD-3410-1.
Reason for Request
In accordance with 10 CFR 50.55a(z)(2), Nebraska Public Power District (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.
NLS2025023 Page2 of 5 10CFR50.55a Request No. RR6-01, Revision 0 Cooper Nuclear Station Request to Use Code Case N-513-5 at a Higher System Operating Pressure Moderately degraded piping could require a plant shutdown within the required action statement timeframes 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 component's 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.
Proposed Alternative and Basis for Use NPPD is requesting approval to apply 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," to the RHRSWB Systern piping having a maximum operating pressure of 490 psig. The operational and configuration limitations of Code Case N-513-5, paragraphs 1(a), l(b), and 1(d), shall apply. The maximum operating temperature of 200'F in paragraph 1(c) shall also apply. Application of the Code Case will avoid accruing additional personnel radiation exposure and increased plant risk associated with a plant shutdown to comply with the cited Code requirements.
The Nuclear Regulatory Commission (NRC) issued Generic Letter 90-05 (Reference l),
"Guidance for Performing Temporary Non-Code Repair of ASME Code Class I,2, and3 Piping (Generic Letter 90-05)," to address the acceptability of limited degradation in moderate energy piping. The generic letter defines conditions that would be acceptable to utilize temporary non-code repairs with NRC approval. The ASME recognized that relatively small flaws could remain in service without risk to the structural integrity of a piping system and developed Code Case N-513. NRC approval of Code Case N-513 versions in Regulatory Guide I.I47, "Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1 (Reference 2)," 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 has historically been the time to the next scheduled refueling outage. The Code Case also requires the Owner to demonstrate system operability due to leakage.
The design basis is considered for each leak 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 judgment. As required by the Code Case, the evaluation process
NLS2025023 Page 3 of5 10CFR50.55a Request No. RR6-01, Revision 0 Cooper Nuclear Station Request to Use Code Case N-513-5 at a Higher System Operating Pressure 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.
Leakage rate is not typically a good indicator of overall structural stability, where the allowable through-wall flaw sizes are often on the order of inches. The periodic inspection interval defined using paragraph 2(e) of Code Case N-513-5 provides 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 size.
The effects of leakage may impact the operability determination or the plant flooding analyses specified in paragraph 1(0. For a leaking flaw, the allowable leakage rate will be limited to 5 grm to limit effects ofjet thrust force even though a structural evaluation of the subject piping and leakage effects would allow a much higher leakage rate than 5 gpm. Any leakage, if present, will be limited to the leakage allowed by the evaluation or 5 gpm, whichever is lower.
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. Significant change in the leakage rate is reason to question that the analysis conditions remain valid, and would require re-inspection per paragraph2(f) of the Code Case.
Any re-inspection must be performed in accordance with paragraph 2(a) of the Code Case.
The design pressure of the Class 3 RHRSWB system is 490 psig with a maximum operating pressure of approximately 420 psig. The background, history, and effects of using Code Case N-513-5 at a conservative pressure value of 490 psig in lieu of the current 275 psiglimitation provided in the Code Case are contained in Attachment 2. A review of previous NRC submittals identified that the NRC has previously granted specific relief for leaks on high energy systems (Attachment 2). NPPD is seeking relief for general application for limited degradation in the RHRSWB System raw water piping for a maximum operating pressure of 490 psig (conservative). Raw water piping degradation is a well understood phenomenon and the evaluation methods in Code Case N-513-5 are widely applied by the industry in raw water piping systems that operate at apressure less than or equal to 275 psig without incident.
The structural aspects of raising the allowable operating pressure to 490 psig were evaluated as discussed in Attachment2. It was determined that Code Case allowable flaw sizes by both the Linear Elastic Fracture Mechanics and branch reinforcement methods used in Code Case N-513-5 were smaller as would be expected. The effects ofjet thrust force were evaluated and it was determined there was little difference in force for a 0.50" diameter flaw size at275 psig versus 490 psig. The study also determined that jet thrust force increases with increasing leakage rate and that it is appropriate to limit the application of this relief request to 490 psig.
NLS2025023 Page 4 of5 10CFR50.55a Request No. RR6-01, Revision 0 Cooper Nuclear Station Request to Use Code Case N-513-5 at a Higher System Operating Pressure provides
- 1) A review of relevant NRC approved relief requests
- 2) A structural integrity evaluation that includes:
Design minimum wall thickness comparison Code Case N-513-5 allowable flaw size comparison Code Case N-513-5 cover thickness requirement comparison
- 3) A jet thrust force evaluation CNS will follow all requirements of Code Case N-513-5. With regard to augmented examination process as described in Section 5 of the Code Case, 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 summary, NPPD will apply ASME Code Case N-513-5 and RG 1.147, Revision 2l (orlater NRC defined revision as applicable) for evaluation of RHRSWB piping flaws at CNS if Code repairs cannot reasonably be completed within the Technical Specifications required time limit.
NPPD will apply a490 psig maximum operating pressure in lieu of the 275 psigmaximum operating pressure defined in paragraph 1(c) of the Code Case. Code Case N-513-5 utilizes technical evaluation approaches that are based on principals that are accepted in other Code documents already acceptable to the NRC. In order to bolster defense-in-depth and avoid adverse consequences as a result of increasing the maximum operating pressure to 490 psig, NPPD is making one additional commitment to apply a 5 gpm leakage limit or lower as allowed by the engineering evaluation. This Relief Request is no longer valid for leakages greater than 5 gpm.
The application of this Code Case, along with the additional commitment above, will maintain acceptable structural and leakage integrity while minimizing plant risk and personnel exposure by minimizing the number of plant transients that could be incurred if degradation is required to be repaired based on ASME Section XI acceptance criteria only.
Duration Of Proposed Alternative The proposed alternative is for use of Code Case N-513-5 for Class 3 RHRSWB piping and components within the scope of the Code Case and the request herein. A Section XI compliant repairlreplacernent 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. This relief request will be applied for the duration of the Code of Record interval which commences on March 1, 2026 and ends February 28,2050. If a flaw is evaluated near the end of the interval and the
NLS2025023 Attachment I Page 5 of5 10CFR50.55a Request No. RR6-01, Revision 0 Cooper Nuclear Station Request to Use Code Case N-513-5 at a Higher System Operating Pressure next refueling outage is in the subsequent interval, the flaw may remain in service under this relief request until the next refueling outage.
Precedents US NRC letter to "Peach Bottom Atomic Power Station, Units 2 and 3, and Quad Cities Nuclear Power Station, Units I and2 - Relief from the Requirements of the ASME Code," dated March 19, 20t 5 (MLl 5043 A496).
US NRC letter to Cooper Nuclear Station - "Requests for Relief Associated with the Fifth 10-year Inseruice Inspection Interval Program," dated July 31, 2018 (MLIS183 A32S).
References
- 1. NRC Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME code class 1,2, and 3 Piping (Generic Letter 90-05)," dated June I 5,1990.
- 2. NRC Regulatory Guide 1.147, "lnseryice Inspection Code Case Acceptability, ASME Section XI, Division 1, Revision2l," dated March 2024.
NLS2025023 Page I of9 Cooper Nuclear Station Technical Basis for Proposed Altemative to Use ASME Code Case N-513-5 and Scope Expansion to a Ifigher Pressure Limit
NLS2025023 Page2 of9 INTRODUCTION Backsround of Code Case N-513-5 Code Case N-513-5 [] provides evaluation rules and criteria for the temporary acceptance of flaws, including through-wall flaws, in moderate energy piping. Moderate energy piping is defined as those piping systems where the maximum operating pressure and temperature do not exceed 275 psig and 200oF, respectively. The provisions of the Code Case are focused on preventing gross failure of the affected pipe for a temporary period while permitting leakage within the plant's Technical Specification. The Code Case provides rules for the evaluation of degraded pipe and tube for a short operating period, with inspection and monitoring requirements of the degraded condition as part of the overall integrity assessment.
The Code Case, N-513-5 [1], is conditionally approved by the Nuclear Regulatory Commission (NRC) in Regulatory Guide 1.147 [2]. The two conditions deal with the terms "flaw" and "significant flaw" in Section 5. Since the introduction of this Code Case, many utilities have used it as a basis for continued operation of degraded piping in moderate energy systems and that has resulted in significantly fewer relief requests to the NRC. Consequently, the industry has benefited from substantial cost savings while maintaining safety. To date, there have been no known instances where the use of the Code Case has resulted in any safety issues in the industry.
Backsround of Enerw Definition The genesis of Code Case N-513 was NRC Generic Letter (GL) 90-05 [3]. Prior to Code Case N-513, this GL was the only available guidance for plants regarding operational leakage in moderate energy piping. The definition of moderate energy piping in Code Case N-513 is consistent with GL 90-05. The scope of GL 90-05 is limited to Class 3 piping but does address moderate and high energy systerns. While non-code repairs are allowed by GL 90-05 (with NRC review) for the temporary period of operation prior to Code compliant repairlreplacement, an additional requirement for the repair having load-bearing capability is necessary for high energy pipe applications. Both GL 90-05 and the latest approved revision of Code Case N-513 are identified as methods available to evaluate the structural integrity of piping with discovered leakage in the NRC Inspection Manual[a].
The definition of moderate energy (the 200'F and275 psig limits) was first introduced in an NRC letter from A. Giambusso to licensees in 1972 to address postulated piping breaks in fluid systems outside containment (discussed in the Branch Technical Position 3-3 of the current Standard Review Plan [5]). The importance of the 200'F temperature limit being below the boiling point of water at atmospheric pressure is clear in the definition of moderate energy piping. However, the basis for the pressure limit of 275 psig is less evident. In conversations with the NRC, there appears to be a link between the 275 psig limit and the recommended working pressure limit for Class 150 pipe flanges and flanged fittings given in the American Society of Mechanical Engineers (ASME) 816.5 standard
[6]. This basis is not known to be documented.
NLS2025023 Page 3 of9 A technical approach and basis are presented herein to justifu the scope expansion ofCode Case N-513-5 to a higher pressure limit. The Class 3 Residual Heat Removal Service Water (RHRSW) system at Cooper Nuclear Station (CNS) has a design pressure of 490 psig [7]. It is desired to expand the scope of N-513-5 such that it may be generically applied to this specific system as it has experienced part-wall degradation in the past.
TECHNICAL BASIS FOR CODE CASE N.513.5 SCOPE EXPANSION TO IIIGHER PRESSURE LIMIT The technical approach includes several elements that support the following objectives:
(i) show NRC precedent in approving relief requests for through-wall leakage in piping or components operating at high energy, (ii) show that structural integrity is not overly impacted when comparing flaw evaluations between piping systems operating at275 and 490 psig, and (iii) show that possible jet thrust forces resulting from leaking flaws are not of concern. An outline of the technical basis detailing the elements used to support these objectives follows:
a a
Review of Relevant NRC Approved Relief Requests Structural Integrity Evaluation o
Design minimum wall thickness comparison o
Code Case N-513-5 allowable flaw size comparison o
Code Case N-513-5 cover thickness requirement comparison Jet Thrust Force Evaluation a
Note that the evaluation rules and acceptance criteria of Code Case N-513 -4 are not limited by pressure and remain valid for the higher limit of 490 psig.
Review of Relevant Relief Reouests A detailed search of the NRC ADAMS database was conducted to identiff any relief requests for continued operation of degraded high energy piping or components. The search results are summarizedinTable 1.
Table 1: Summary of Relevant Relief Requests ADAMS Accession Number Plant /
Operating Condition Description Status ML072140851 McGuire, Ul /
2.500 psie Application for continued operation of leakine valve Approved MLr01440381 San Onofre U2, U3 l<275 psig (275" Fl Generic application for continued operation of high temperature through-wall leakine pipe Approved MLr 5043A496 Exelon Plants /
375 psie Generic request to use Code Case N-513-3 at a hieher pressure limit Approved:r
MLl81834325 Cooper/
490 psie Request to use Code Case N-513-4 at higher pressure limit Approved NLS2025023 Page 4 of9
- In addition to the N-513-3 requirements, the NRC imposed an allowable leak rate limit of 5 gpm.
Structural Intesritv Evaluation Design minimum wall thickness comparison ASME 831.1 [8] defines the design minimum wall thickness required for pressure loading, tm, zrS (without a corrosion allowance):
tm (1) where:
p Design pressure Do Outside pipe diameter S Material allowable stress.
Substituting maximum operating pressure for design pressure in Equation 1, the minimum required wall thickness increases 77Yo with an increase in pressure ftom275 to 490 psig.
While this percentage increase appears significant, the actual change in minimum required wall thickness is relatively low as the hoop stress at these pressures is small. Table 2 shows a comparison of minimum wall thicknesses for various pipe sizes.
Table 2: Minimum Required Wall Thickness Comparison Nominal Pioe Size t-n for 275 psie* (in) tmin for 490 psie* (in) 6-inch 0.053 0.094 12-inch 0.102 0.181 l8-inch 0.t44 0.255
- A material allowable stress of 17.1 ksi is assumed Code Case N-513-5 flaw size comoarison Code Case N-513-5 allows for nonplanar, through-wall flaws to be evaluated as two independent planar through-wall flaws, one orientated in the axial direction and one orientated in the circumferential direction (i.e., a planar flaw characterization approach).
The Code Case acceptance criteria require the flaw region be bounded by the area defined by the allowable axial and circumferential flaw sizes. Several example N-513-5 calculations were conducted illustrating the influence of higher pressure on allowable flaw size. Following the Code Case guidance, a linear elastic fracture mechanics evaluation PDo 2(s+0.4p)
NLS2025023 Page 5 of9 was performed for various carbon steel pipe sizes to determine the maximum allowable flaw sizes at maximum operating pressures of 275 and 490 psig. Table 3 summarizes the results and several notes are provided giving more details regarding the analysis inputs.
The influence of the higher pressure is clearly seen, and a greater impact is observed in the axial direction as expected since pressure hoop stress is twice the axial membrane stress due to pressure. While the higher pressure does decrease the allowable flaws sizes, the effect does not impact the functionality or validity of the Code Case approach.
Table 3: Allowable Axial and Circumferential Flaw Size Comparison Notes: - Piping material assumed,4106 Grade B; standard schedule thickness.
- Applied bending moment for each pipe size results in a stress ratio of about 0.25 at275 psig.
- Allowable flaw sizes based on Service Level B structuralfactors.
- Analysis based on a conservative lower shelf toughness value of 45 in-lb/in2.
Code Case N-513-5 cover thickness requirement comparison Code Case N-513-5 provides a branch reinforcement method to evaluate nonplanar, through-wall flaws. As part of the branch reinforcement approach, an opening is modeled such that its diameter fully bounds the leaking flaw. In practice, there could exist a remaining wall ligament within the modeled opening. Equation 9 of Code Case N-513-5 provides assurance against pressure blowout (i.e., wall ligament failure) by requiring an average cover thickness, L,uue, within the modeled opening:
t".uu, > 0.353d..j (2)
Nominal Pipe Size Axial Direction (in)
Circumferential Direction (in) 275 psig 490 psig
%L 275 psig 490 psig
%L 6-inch Mb:40 in-kips 3.1 1.9 39%
2.8 2.1 2s%
l2-inch Mb: 170 in-kips 3.0 1.5 s0%
J.J 2.1 36%
18-inch Mb:260 in-kips 2.2 0.9 59%
J.J 1.8 44%
= !,u/
g 0,06
,Js L-
- n,1<
.Jr c.04 e;.:
5^^-
u LUt t
NLS2025023 Page 6 of9 where:
- dadj Modeled opening diameter
- P Maximum operating pressure
- S Material allowable stress The average cover thickness requirement increases about 33%o with an increase in maximum operating pressure from275 to 490 psig. Figure I illustrates the average cover thickness required as a function of adjusted diameter for 275 and 490 psig. Note that a material allowable stress of 17.1 ksi is assumed. Typically, modeled openings have a diameter < I inch and the change in the required cover thickness is small (< 15 mils).
c.l0 o.09 u.u6
."t*--275 psig
+-49C psig
-a"/
.".,4!'
,..r)'
r'
{'.""
0 025 0.s Adit,s:ed 0iarr:eter' { inl
- 0. ?5 Figure 1: Required Cover Thickness vs. Adjusted Diameter for 275 and 490 psig a.a2 l.n!
c00
Where:
NLS2025023 PageT of 9 Jet Thrust Force Evaluation Part3.6.2 of the NRC Standard Review Plan [9] provides a simplified dynamic analysis model to quantifu the jet thrust force, T, of water from a pipe break. The following equation is given:
T: KpA (3)
Thrust coefficient (2.0 for subcooled, nonflashing water)
Systern pressure prior to pipe break Pipe break area.
Figure 2 shows a comparison ofjet thrust force for pressures of 27 5 and 490 psig over a range of through-wall opening diameters. For small through-wall opening diameters (< 0.5 in), the difference in the jet thrust force is small. It is unlikely larger openings would be tolerated by the plant as this would result in excessive leakage and possibly impact system operability or room flooding abatement.
/
.. *t'- 2 75 psig
...l-490 psrg
/
/
/
.1, 025 C.5 A75 Trrrirgh-Wait {si3 !13as:er iinl Figure 2; Jet Force vs. Through-wall Hole Diameter for 275 and 490 psig K
P A
5
,J 8CC 7,30 5UU 5CO "l0c 5vt.]
!'tlt i.3c I
I
NLS2025023 Page 8 of9 ADDITIONAL REOUIREMENTS Implementation of Code Case N-513-5 requires additional actions be satisfied by the plant including observing leakage daily to confirm analysis conditions used in the evaluation remain valid, frequent periodic inspections to track flaw growth and augmented examinations to assess degradation of the affected system. Furthermore, the following additional requirement is included as part of the Relief Request in order to bolster defense-in-depth and avoid adverse consequences:
- 1. Limit leakage to 5 gpm CONCLUSIONS This attachment provides a technical basis for proposed alternative use of ASME Section XI Code Case N-513-5 and its scope expansion to a higher pressure limit from 275 to 490 psig for cNS, specific to the RHRSW system. The discussion provided herein demonstrates that the use of Code Case N-513-5 with the higher pressure limit will reduce plant burden without any adverse effect on safety. The technical basis is comprised of four primary elements.
First, relevant NRC relief requests were reviewed. They demonstrate that there is precedent for the temporary acceptance of leaking flaws in Class 213 piping and components at pressures higher than275 psig.
Second, a structural integrity evaluation was performed to determine the impact of the increased pressure on design minimum wall thickness, N-513-5 allowable flaw sizes and the N-513-5 cover thickness requirement. While the influence of the higher pressure was observed in the evaluation, structural integrity can still be demonstrated. The functionality and validity of the Code Case methods at higher pressure were confirmed.
Finally, jet thrust forces were estimated for a leaking pipe at 275 and490 psig. A significant change in jet thrust forces was only seen with large opening areas that would result in high leak rates, i.e., rates that would challenge system functionality or local spray andl or compartment flooding requirements.
As part of the generic Relief Request, the additional requirement of a limit on discovered leakage of 5 gpm is introduced in order to bolster defense-in-depth.
REFERENCES
- 1. 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," Cases of the ASME Boiler and Pressure Vessel Code, April 18, 2018.
NLS2025023 Page 9 of9
- 2. Regulatory Guide 1.147, "Inseryice Inspection code case Acceptability, ASME Section XI, Division 1," Revision 21, March 2024.
- 3. NRC Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2, and 3 Piping," (June 15, 1990).
- 4. NRC Inspection Manual, Chapter 0326, "Operability Determinations &
Functionality Assessments for Conditions Adverse to Quality or Safety," Issue Date:
t2/03ns.
- 5. NRC Standard Review Plan, NUREG-0800, Branch Technical Position 3-3, "Protection Against Postulated Piping Failures in Fluid Systems Outside Containment," Revision 3, March 2007.
- 7. Nebraska Public Power District Design Calculation No. NEDC 02-05I, Revision 1, "Design Temperature Determination for SW, CW, WW, & CS Piping; Design Pressure Determination for SW-I piping," September 17,2002, SI File No.
170040s.20 t.
- 9. NRC Standard Review Plan, NUREG-0800, Part3.6.2, "Determination of Rupture Locations and Dynamic Effects Associated with the Postulated Rupture of Piping,"
Revision 2,March 2007.
- 10. NRC Relief Request Approval and Safety Evaluation Report for Exelon Generation Company Fleet Request for Proposed Altemative to Use ASME Code Case N-513-4, ADAMS Accession No. ML16230A237.