ML25127A174
| ML25127A174 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 05/15/2025 |
| From: | Tony Nakanishi Plant Licensing Branch IV |
| To: | Heflin A Arizona Public Service Co |
| Orders, William | |
| References | |
| EPID L-2024-LLR-0070 | |
| Download: ML25127A174 (9) | |
Text
May 15, 2025 Mr. Adam Heflin Executive Vice President/
Chief Nuclear Officer Mail Station 7605 Arizona Public Service Company P.O. Box 52034 Phoenix, AZ 85072-2034
SUBJECT:
PALO VERDE NUCLEAR GENERATING STATION UNITS 1, 2, AND 3 -
ALTERNATIVE REQUEST ALLOWING REMOVAL OF THE LOWER ACCEPTANCE CRITERIA THRESHOLD FROM SOLENOID-OPERATED VALVES (EPID L-2024-LLR-0070)
Dear Mr. Heflin:
By letter dated October 22, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24296B215), Arizona Public Service Company (the licensee) submitted Alternative Request VRR-03 to the U.S. Nuclear Regulatory Commission (NRC) proposing the use of an alternative to specific inservice testing (IST) requirements in the 2012 Edition of the American Society of Mechanical Engineers (ASME) Operation and Maintenance of Nuclear Power Plants, Division 1, OM Code: Section IST (OM Code) for the fourth IST interval program at Palo Verde Nuclear Generating Station, Units 1, 2, and 3 (Palo Verde).
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1),
Acceptable level of quality and safety, the licensee proposed to implement Alternative Request VRR-03 on the basis that the alternative would provide an acceptable level of quality and safety in lieu of certain requirements in the ASME OM Code as incorporated by reference in 10 CFR 50.55a, Codes and standards, for specific valves at Palo Verde.
In Alternative Request VRR-03, the licensee proposes to remove the lower acceptance criteria threshold for certain solenoid-operated valves at Palo Verde. The licensee asserts that the proposed alternative provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1).
As described in the enclosed safety evaluation, the NRC staff has determined, based on current information, that Alternative Request VRR-03 proposing to use only the upper acceptance criteria of ASME OM Code, subsection ISTC, subparagraph ISTC-5152(a) (+25 percent) and subparagraph ISTC-5152(b) (+50 percent) for performance monitoring of solenoid-operated valves within the scope of this request, provides an acceptable level of quality and safety in lieu of the ASME OM Code requirements for both upper and lower stroke-time acceptance criteria.
The NRC staff has also determined that Alternative Request VRR-03 will provide reasonable assurance that the subject valves will be operationally ready to perform their safety functions for the duration of the alternative request. Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements in 10 CFR 50.55a(z)(1). Therefore, the
NRC staff authorizes Alternative Request VRR-03 for the remaining duration of Palo Verde Nuclear Generating Station, Units 1, 2, and 3, fourth IST interval program, which is scheduled to end on January 14, 2028.
All other ASME OM Code requirements as incorporated by reference in 10 CFR 50.55a for which relief or an alternative was not specifically requested, and granted or authorized (as appropriate), in the subject request remain applicable.
If you have any questions, please contact the Palo Verde Project Manager, William Orders, at 301-415-3329 or by email at William.Orders@nrc.gov.
Sincerely, Tony Nakanishi, Chief Plant Licensing Branch LPL4 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. STN 50-528, STN 50-529, and STN 50-530
Enclosure:
Safety Evaluation cc: Listserv TONY NAKANISHI Digitally signed by TONY NAKANISHI Date: 2025.05.15 12:19:24 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE REQUEST VRR-03 PALO VERDE NUCLEAR GENERATING STATION, UNITS 1, 2, AND 3 ARIZONA PUBLIC SERVICE COMPANY DOCKET NOS. 50-528, 529, AND 530
1.0 INTRODUCTION
By letter dated October 22, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24296B215), Arizona Public Service Company (APS, the licensee) submitted Alternative Request VRR-03 to the U.S. Nuclear Regulatory Commission (NRC) proposing the use of an alternative to specific inservice testing (IST) requirements in the 2012 Edition of the American Society of Mechanical Engineers (ASME) Operation and Maintenance of Nuclear Power Plants, Division 1, OM Code: Section IST (OM Code) for the fourth IST interval program at Palo Verde Nuclear Generating Station, Units 1, 2, and 3 (Palo Verde).
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1),
Acceptable level of quality and safety, the licensee proposed to implement Alternative Request VRR-03 on the basis that the alternative would provide an acceptable level of quality and safety in lieu of certain requirements in the ASME OM Code as incorporated by reference in 10 CFR 50.55a for specific valves at Palo Verde.
In Alternative Request VRR-03, the licensee proposes to remove the lower acceptance criteria threshold for certain solenoid-operated valves (SOVs) at Palo Verde. The licensee asserts that the proposed alternative provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1). The NRC staff describes its review of Alternative Request VRR-03 for Palo Verde in this safety evaluation.
2.0 REGULATORY EVALUATION
The regulations in 10 CFR 50.55a(f)(4), Inservice testing standards requirement for operating plants, state, in part:
Throughout the service life of a boiling or pressurized water-cooled nuclear power facility, pumps and valves that are within the scope of the ASME OM Code must meet the inservice test requirements (except design and access provisions) set forth in the ASME OM Code and addenda that become effective subsequent to editions and addenda specified in [10 CFR 50.55a(f)(2) and (3)] and that are incorporated by reference in 10 CFR 50.55a(a)(1)(iv), to the extent practical within the limitations of design, geometry, and materials of construction of the components.
The regulations in 10 CFR 50.55a(z), Alternatives to codes and standards requirements, state:
Alternatives to the requirements of paragraphs (b) through (h) of this section 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.
3.0 TECHNICAL EVALUATION
3.1 Licensees Alternative Request VRR-03 Applicable ASME OM Code Edition The applicable Code of Record for the fourth IST interval program at Palo Verde is the 2012 Edition of ASME OM Code, as incorporated by reference in 10 CFR 50.55a.
Applicable ASME OM Code Components The licensees submittal dated October 22, 2024, lists the valves at Palo Verde within the scope of Alternative Request VRR-03. The scope includes 4 SOVs at each of the three Palo Verde units for a total of 12 valves at the Palo Verde site. Table 1 in this safety evaluation summarizes the valve information:
Table 1 Valve Description Code Class OM Category Operator and Valve Type RCBHV0108 Pressurizer vent valve 1
B SOV 1-inch globe valve RCBHV0109 Pressurizer vent valve 1
B SOV 1-inch globe valve SIAUV0660 Safety injection combined recirculation to refueling water tank isolation valve 2
B SOV 4-inch globe valve SIBUV0659 Safety injection combined recirculation to refueling water tank isolation valve 2
B SOV 4-inch globe valve Applicable OM Code Requirements The IST requirements in the 2012 Edition of the ASME OM Code, as incorporated by reference in 10 CFR 50.55a, related to this alternative request are as follows:
ASME OM Code, subsection ISTC, Inservice Testing of Valves in Light-Water Reactor Nuclear Power Plants, paragraph ISTC-5150, Solenoid-Operated Valves, subparagraph ISTC-5152, Stroke Test Acceptance Criteria, states the following:
Test results shall be compared to reference values established in accordance with para. [paragraph] ISTC-3300, ISTC-3310, or ISTC-3320.
(a) Valves with reference stroke times of greater than 10 sec [seconds] shall exhibit no more than +/-25% change in stroke time when compared to the reference value.
(b) Valves with reference stroke times of less than or equal to 10 sec shall exhibit no more than +/-50% change in stroke time when compared to the reference value.
(c) Valves that stroke in less than 2 sec may be exempted from subpara.
[subparagraph] ISTC-5152(b). In such cases the maximum limiting stroke time shall be 2 sec.
Licensees Proposed Alternative In Alternative Request VRR-03, the licensee requests the NRC to authorize this alternative to use only the upper acceptance criteria of ASME OM Code, subsection ISTC, subparagraph ISTC-5152(a) (+25 percent) and subparagraph ISTC-5152(b) (+50 percent) for performance monitoring of SOVs within the scope of this request. The licensee asserts that the upper acceptable criteria will provide an acceptable level of quality and safety because the lower acceptance criteria have no technical basis in SOV operation. The licensee has requested authorization of Alternative Request VRR-03 for the remaining duration of the fourth IST interval program, which is scheduled to end on January 14, 2028.
Licensees Reason for Request In its submittal dated October 22, 2024, the licensee provided the following reason for its request:
There is no technical basis for applying the lower acceptance criteria (-25%
and -50%) to Table 1 SOVs. Rapid stroking is not a failure mode for these valves. APS is requesting to apply only the upper acceptance of ISTC-5152(a)
(+25%) or ISTC-5152(b) (+50%) to Table 1 SOVs. The Table 1 valves either apply ISTC-5152(a), ISTC-5152(b), or have the potential to do so in the future as indicated by plant operating experience.
Licensees Basis for Use of Proposed Alternative The NRC staff summarizes the basis provided by the licensee for Alternative Request VRR-03 in the following paragraphs:
The licensee states that it can provide an acceptable level of quality and safety in detecting and monitoring degradation of the valves within the scope of Alternative Request VRR-03 by eliminating the ASME OM Code requirement to apply a lower acceptance criteria threshold. The licensee asserts that SOV operation below the -25 percent or -50 percent threshold - stroking too fast - is not a failure mode for SOVs. Only the upper acceptance criteria of ISTC-5152(a) and ISTC-5152(b) provides insight into the SOV condition.
The license asserts that there is no increase in quality or safety from applying the lower acceptance criteria of ISTC-5152(a) or ISTC-5152(b) to the subject SOVs. The licensee considers the retest and analysis requirements for stroking too fast to be burdensome because they cannot identify valve degradation. The licensee believes that the proposed alternative provides an acceptable level of quality and safety because SOV operating experience, design, and vendor and industry guidance show that only increasing stroke time indicates valve degradation.
The licensee states that potential SOV failure modes show all vendor-identified failures contribute to increased valve stroke times. None of the indication, electromagnetic, or mechanical issues can reasonably be expected to result in decreasing stroke time. The licensee references industry guidance on SOV troubleshooting that shows only increasing stroke times indicate SOV failure.
The licensee states that the SOV design prevents operating too fast (less than -25 percent or less than -50 percent of a reference value). For example, the licensee states that SOVs do not have valve packing such that packing friction does not influence valve stroke time performance.
The licensee asserts that stem-disk separation in SOVs is not applicable based on industry guidance. The licensee states that operating experience provides no evidence that flow-induced vibration has caused SOV stem-disk failures. The licensee does not consider that stem pins could back out to cause SOV stem-disk separation.
The licensee states that the Palo Verde corrective maintenance history shows no evidence of maintenance in response to rapid stroking (less than -25 percent or less than -50 percent of a reference value). Since 2020, the licensee states that it has issued 24 corrective maintenance work orders against the 12 valves within the scope of the request. More than half of the work orders address increasing stroke times or fail-to-operate events. Nearly half of the work orders support ASME OM Code program management and do not reflect an equipment issue. The remaining work orders applied to seat leakage. Therefore, the licensee considers rapid stroking of SOVs to not be driving corrective maintenance activities at Palo Verde.
In its submittal, the licensee requests that the NRC authorize Alternative Request VRR-03 to use only the upper acceptance criteria of ASME OM Code, subsection ISTC, paragraphs ISTC-5152(a) (+25 percent) and ISTC-5152(b) (+50 percent) for performance monitoring of SOVs within the scope of the request. The licensee asserts that the upper acceptable criteria for SOV testing provide an acceptable level of quality and safety, without the lower acceptance criteria for SOV stroke time.
3.2
NRC Staff Evaluation
The 2012 Edition of the ASME OM Code, subsection ISTC, paragraph ISTC-5150, subparagraph ISTC-5152, as incorporated by reference in 10 CFR 50.55a, requires SOV test results shall be compared to reference values established in accordance with paragraph ISTC-3300, ISTC-3310, or ISTC-3320. Subparagraph (a) in paragraph ISTC-5152 requires that valves with reference stroke times of greater than 10 seconds shall exhibit no more than
+/-25 percent change in stroke time when compared to the reference value. Subparagraph (b) in paragraph ISTC-5152 requires that valves with reference stroke times of less than or equal to 10 seconds shall exhibit no more than +/-50 percent change in stroke time when compared to the reference value.
In Alternative Request VRR-03, the licensee requests that the NRC authorize the use of only the upper acceptance criteria of ASME OM Code, subsection ISTC, subparagraph ISTC-5152(a) (+25 percent) and subparagraph ISTC-5152(b) (+50 percent) for performance monitoring of SOVs within the scope of this request. The licensee asserts that the upper acceptable criteria will provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1). The licensee states that only the upper acceptance criteria of ISTC-5152(a) and ISTC-5152(b) provide insight into the SOV condition. The licensee asserts that SOV operation below the -25 percent or -50 percent threshold (i.e., stroking too fast) is not a failure mode for SOVs. The licensee considers the retest and analysis requirements for an SOV stroking too fast to be burdensome without a safety benefit.
In its submittal, the licensee provided the results of its evaluation of the performance history of SOVs to support Alternative Request VRR-03. For example, potential SOV failure modes show all vendor-identified failures contribute to increased valve stroke times. Further, industry guidance on SOV troubleshooting referenced by the licensee shows that only increasing stroke times can indicate SOV degradation. Although faster stroke times of SOVs might cause pressure or flow transients that might impact system operation, the licensee did not identify any significant plant transients from its performance history search for SOVs stroking faster than their design value.
The SOV design reduces the potential for SOVs to operate faster than the reference value. For example, SOVs do not have valve packing such that packing friction does not influence valve stroke-time performance. The industry information regarding SOV performance found that stem-disk separation in SOVs is unlikely. Further, operating experience does not provide evidence that flow-induced vibration is a concern for SOV stem-disk failures.
With respect to corrective maintenance history, the licensee reports that the SOVs at Palo Verde show no evidence of maintenance in response to rapid stroking (less than -25 percent or less than -50 percent of a reference value). Since 2020, the licensee has only identified increasing stroke times as a performance concern. The licensee did not identify any maintenance at Palo Verde that was the result of shorter stroke times for the SOVs within the scope of this request.
The NRC staff evaluated the information provided by the licensee in Alternative Request VRR-03 and reviewed available NRC information with respect to SOV operating experience, design, and vendor and industry guidance. From its review, the staff determined that the information supports that only increasing stroke time has revealed SOV degradation. Further, the staff notes that ASME OM Code, subsection ISTC, paragraph ISTC-5151, Valve Stroke Testing, subparagraph (d), requires that any abnormality or erratic action during SOV stroke testing shall be recorded and an evaluation shall be made regarding the need for corrective action. Therefore, the staff finds that this ASME OM Code requirement will ensure that the licensee addresses faster stroke times for SOVs within the scope of Alternative Request VRR-03 at Palo Verde.
In summary, the NRC staff evaluated the justification provided by the licensee in Alternative Request VRR-03 in support of the proposal to use only the upper acceptance criteria of ASME OM Code, subsection ISTC, subparagraph ISTC-5152(a) (+25 percent) and subparagraph ISTC-5152(b) (+50 percent) for performance monitoring of SOVs within the scope of this request rather than both the upper and lower stroke-time acceptance criteria. Together with the ASME OM Code requirement in paragraph ISTC-5151 for monitoring abnormal or erratic SOV operation, the NRC staff finds that the proposed alternative provides an acceptable level of quality and safety, based on SOV operating experience, design, and vendor and industry guidance, for the remainder of the current IST program interval. Therefore, the staff finds that Alternative Request VRR-03 meets the requirements of 10 CFR 50.55a(z)(1).
4.0 CONCLUSION
As described above, the NRC staff has determined, based on current information, that Alternative Request VRR-03 proposing to use only the upper acceptance criteria of ASME OM Code, subsection ISTC, subparagraph ISTC-5152(a) (+25 percent) and subparagraph ISTC-5152(b) (+50 percent) for performance monitoring of SOVs within the scope of this request, provides an acceptable level of quality and safety in lieu of the ASME OM Code requirements for both upper and lower stroke-time acceptance criteria. The NRC staff has also determined that Alternative Request VRR-03 will provide reasonable assurance that the subject valves will be operationally ready to perform their safety functions for the duration of the alternative request. Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes Alternative Request VRR-03 for the remaining duration of Palo Verde Nuclear Generating Station, Units 1, 2, and 3, fourth IST interval program, which is scheduled to end on January 14, 2028.
All other ASME OM Code requirements as incorporated by reference in 10 CFR 50.55a for which relief or an alternative was not specifically requested, and granted or authorized (as appropriate), in the subject request remain applicable.
Principal Contributors: Thomas G. Scarbrough, NRR Nicholas Hansing, NRR Date: May 15, 2025
- concurrence via e-concurrence OFFICE NRR/DORL/LPL4/PM*
NRR/DORL/LPL4/LA*
NRR/DEX/EMIB/BC*
NRR/DORL/LPL4/BC*
NAME WOrders PBlechman SBailey TNakanishi DATE 5/7/2025 5/13/2025 5/13/2025 5/15/2025