ML24296B215
| ML24296B215 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 10/22/2024 |
| From: | Horton T Arizona Public Service Co |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 102-08872-TAH/MDD | |
| Download: ML24296B215 (1) | |
Text
10 CFR 50.55a A member of the STARS Alliance LLC Callaway
- Diablo Canyon
- Palo Verde
- Wolf Creek 102-08872-TAH/MDD October 22, 2024 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Subject:
Palo Verde Nuclear Generating Station Units 1, 2, and 3 Docket Nos. STN 50-528, 50-529, and 50-530 Renewed Operating License Nos. NPF-41, NPF-51, NPF-74 Transmittal of Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Pursuant to 10 CFR 50.55a(z)(1), Arizona Public Service Company (APS) requests Nuclear Regulatory Commission (NRC) approval of the enclosed alternative request for removal of the lower acceptance criteria threshold from solenoid-operated valves (SOVs).
This proposed alternative applies to subparagraphs (a) and (b) in ISTC-5152, Stroke Test Acceptance Criteria of the American Society of Mechanical Engineers (ASME), Operations and Maintenance of Nuclear Power Plants (OM) Code, 2012. This paragraph establishes acceptance criteria for in-service testing of SOVs.
Pursuant to 10 CFR 50.55a(z)(1), plants may request authorization to implement alternative methodologies that deviate from regulatory requirements provided they achieve an acceptable level of quality and safety.
No new commitments are being made to the Nuclear Regulatory Commission by this letter.
Should you need further information regarding this submittal, please contact Michael D.
DiLorenzo, Department Leader Nuclear Regulatory Affairs - Licensing at (623) 393-3495.
Sincerely, Todd Horton Senior Vice President Nuclear Regulatory & Oversight MDD/WEH/cr Todd A. Horton Sr. Vice President Nuclear Regulatory & Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7602 Tel: 623.393.6418 Horton, Todd (Z10098)
Digitally signed by Horton, Todd (Z10098)
Date: 2024.10.22 13:37:24
-07'00'
102-08872-TH/MDD ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Valve Relief Request - 03 Page 2
Enclosure:
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves cc:
J. D. Monninger NRC Region IV Regional Administrator W. T. Orders NRC NRR Project Manager for PVNGS L. N. Merker NRC Senior Resident Inspector for PVNGS
Page 1 of 6 Enclosure Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Page 2 of 6
- 1. ASME Code Components Affected This alternative applies to four solenoid-operated valves per unit as identified in Table 1. The request applies to all three Palo Verde Nuclear Generating Station (PVNGS) units, totaling 12 valves.
2. Applicable Code Edition and Addenda
Arizona Public Service Company (APS) applies ASME OM-2012 (reference 1) for the fourth 120-month in-service testing interval, which began January 15, 2018, and ends on January 14, 2028.
3. Applicable Code Requirement
This proposed alternative applies to ISTC-5152, Stroke Test Acceptance Criteria. This paragraph establishes acceptance criteria for stroke time testing of solenoid-operated valves (SOVs).
Specifically, subparagraphs ISTC-5152(a) and ISTC-5152(b) establish the acceptance criteria for SOVs with reference values. Valves with reference values greater than 10 seconds are required to stroke within +/-25% of the reference value. Valves with reference values less than or equal to 10 seconds are required to stroke within +/-50% of the reference value.
4. Reason for 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.
5. Proposed Alternative and Basis for Use
APS can provide an acceptable level of quality and safety in detecting and monitoring Table 1 valve degradation by eliminating the OM Code requirement to apply a lower acceptance criteria threshold. The acceptance criteria are defined in ISTC-5152(a) and ISTC-5152(b) as a tolerance band centered on the reference value (+/-25% or +/-50%). Operation below the -25% or -50%
threshold - stroking too fast - is not a failure mode of solenoid-operated valves. Only the upper acceptance criteria of ISTC-5152(a) and ISTC-5152(b) provides insight into valve condition.
When applying ISTC-5152(a) or ISTC-5152(b) to Table 1 valves, APS:
Shall establish valve acceptance criteria as:
o
+25% for valves with a reference stroke time greater than 10 seconds o
+50% for valves with a reference stroke time less than or equal to 10 seconds Shall not establish valve acceptance criteria:
o
-25% for valves with a reference stroke time greater than 10 seconds o
-50% for valves with a reference stroke time less than or equal to 10 seconds There is no increase in quality or safety from applying the lower acceptance criteria of ISTC-5152(a) or ISTC-5152(b) to Table 1 valves. Valves that stroke faster than the acceptance criteria (less than -25% or less than -50% of a reference value) are subject to ISTC-5153, Stroke Test Corrective Action. Specifically, ISTC-5153(b) requires inoperability or re-test and evaluation.
Valves that do not meet the acceptance criteria must be declared inoperable or immediately
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Page 3 of 6 retested. Retested valves require a performance analysis. The retest and analysis requirements for stroking too fast are burdensome because they cannot identify valve degradation.
The proposed alternative provides an acceptable level of quality and safety because operating experience, SOV design, and vendor and industry guidance show that only increasing stroke time indicates valve degradation.
Vendor-provided failure mode analyses show the PVNGS SOVs can fail due to indication, electromagnetic or mechanical issues. Indication issues include failure to illuminate and failure to extinguish, both of which would manifest as a longer stroke time because operators end timing based on indicating lights changing state. Electromagnetic failures include lack of magnetic field strength, insufficient voltage, and lack of power, all of which would result in longer stroke times or fail-to-operate events due to insufficient motive force. Mechanical issues, such as internal binding, a failed internal spring, or foreign material, would result in longer stroke times because they either remove the motive force or increase internal friction. This summary of potential failure modes shows all vendor-identified failures contribute to increased valve stroke times. None of the indication, electromagnetic, nor mechanical issues can reasonably be expected to result in decreasing stroke time because they impede the operators ability to measure stroke time or impede the valves ability to operate.
Industry guidance on SOV troubleshooting shows only increasing stroke times can indicate failure.
EPRIs Solenoid Valve Maintenance Guide (reference 2) explains that stroke time failures and fail-to-operate events (also known as shift failures) share common causes.
Shift and stroke time failures are related in that a shift failure is simply an infinite stroke time failure. They were analyzed separately to ensure that other mechanisms were not involved; no additional mechanisms were identified. Therefore, troubleshooting of shift and stroke time failures is identical.
The EPRI analysis shows SOV performance degrades only by increasing stroke times. The passage shows that mechanisms that lead to stroke time failures can also cause failure-to-operate events.
The analysis provides no examples of failure modes that exhibit decreasing stroke times.
Therefore, SOVs that stroke too fast are showing improved performance capability, not degradation.
SOV design prevents operating too fast (less than -25% or less than -50% of a reference value) as a failure mechanism. Examples:
Indicating switches are set at the extremes of full-stroke travel as implemented by maintenance work instructions. This precludes the valve from displaying as fully closed or fully open when the stem and disk are at mid-position (any position that is not fully open nor fully closed). In the event of a partial valve stroke or inadequate seating, control room operators would identify this as a fail-to-operate event. Indicating lights would show the valve in mid stroke, and operators would be unable to meet the criteria for completing a stroke time test.
SOVs do not have valve packing. In an air-operated valve, loose packing could result in an excessively rapid stroke time. Contrary to this, an SOVs stem is housed completely inside the valve bonnet. SOVs have no packing friction to influence valve stroke time performance.
Stem-disk separation is not applicable. Table 1 valves have a pinned stem-disk connection.
EPRIs Susceptibility of Valve Applications to Failure of the Stem-to-Disk Connection finds that pinned connections are essentially not susceptible to stem-disk separation (reference 3). EPRIs determination applies to butterfly valves and is instructive for pinned stem-disk
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Page 4 of 6 connections in SOVs. The EPRI report identifies two failure modes of pinned connections: 1) shear failure due to flow-induced vibration, and 2) pins backing out (due to flow-induced vibration or inadequate locking).
o Shear failure: Operating experience provides no evidence that flow-induced vibration is causing SOV stem-disk failures. The EPRI guide identifies one instance of SOV stem-disk failure. Event ID 160783 states that a rubber valve disk separated from the stem due to incorrect assembly of the valve internals. Searching INPOs IRIS database for shear SOV and SOV flow induced vibration identifies 26 total results.
None of the 26 results describe SOV pin issues. To ensure broad results, searches were performed as collections of terms, not phrases.
o Pins backing out: The EPRI guide states that non-susceptibility to stem-disc separation for pinned butterfly valves requires implementing industry guidance to stake the pins in place. In an SOV, pins do not require staking because internal clearances limit pin travel. Pins cannot fully back out. A pin that backs out would rub against the valve body interior, increasing friction and potentially increasing stroke time.
Corrective maintenance history shows no evidence of maintenance in response to rapid stroking (less than -25% or less than -50% of a reference value). Since 2020, APS has issued 24 corrective maintenance work orders against the 12 valves in the scope of Table 1. Corrective maintenance work order categories include:
fail-to-operate (17%)
loss of indication (17%)
increasing stroke time trends (17%)
stroke time testing exceeds the limiting value (4%)
seat leakage (4%)
test condition change per ISTC-3320 (42%)
More than half of the work orders (54%) address increasing stroke times or fail-to-operate events.
Nearly half of the work orders (42%) support ASME OM Code program management and do not reflect an equipment issue. The remaining 4% of work orders are for seat leakage, which is not reasonably detectable via stroke time testing. Rapid stroking of Table 1 valves is not driving corrective maintenance. The maintenance history shows only increasing stroke times are an equipment performance concern.
ASME OM-2012 includes a precedent for applying only upper test criteria to SOVs. ISTC-5152(c) states:
Valves that stroke in less than 2 sec may be exempted from subpara. ISTC-5152(b).
The paragraph establishes only an upper performance threshold for valves that stroke in less than 2 seconds. There is no lower threshold, an acknowledgement that evaluating performance below a certain threshold provides no additional safety assurance. ISTC-5152(c) expressly excludes select SOVs from the requirements of ISTC-5152(b) with the benefit of not re-testing or evaluating performance as required by ISTC-5153(b).
APS recommends NRC approve this request to use only the upper acceptance criteria of ISTC-5152(a) (+25%) and ISTC-5152(b) (+50%) for performance monitoring on the solenoid-operated valves in Table 1. The upper acceptable criteria provide an acceptable level of quality and safety because the lower acceptance criteria have no technical basis in SOV operation.
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Page 5 of 6
6. Duration of Proposed Alternative
The proposed alternative shall be used for the duration of Palo Verdes fourth 120-month in-service testing interval, which ends January 14, 2028.
Summary:
Applying only the upper acceptance criteria of ISTC-5152(a) and ISTC-5152(b) for solenoid-operated valves provides an acceptable level of quality and safety. SOVs have no failure modes that can reasonably be expected to result in faster-than-normal operation. This is sufficient basis to support an alternative request to NRC per 10 CFR 50.55a(z).
References:
- 1. Operation and Maintenance of Nuclear Power Plants, ASME OM-2012
- 2. NP-7414, Solenoid Valve Maintenance Guide, December 2003 (EPRI)
- 3. Technical Report 3002019621, Susceptibility of Valve Applications to Failure of the Stem-to-Disk Connection, September 2020 (EPRI)
Attachment:
Table 1
Valve Relief Request (VRR) - 03: Alternative Request Allowing Removal of the Lower Acceptance Criteria Threshold from Solenoid-Operated Valves Page 6 of 6 Attachment Table 1 Valve Description Code class Code category Operator type Valve type Valve size (inches)
Valve function Safety position RCBHV0108 Pressurizer vent valve 1
B Solenoid Globe 1
Active Open & Closed RCBHV0109 Pressurizer vent valve 1
B Solenoid Globe 1
Active Open & Closed SIAUV0660 Safety injection combined recirculation to refueling water tank isolation valve 2
B Solenoid Globe 4
Active Open & Closed SIBUV0659 Safety injection combined recirculation to refueling water tank isolation valve 2
B Solenoid Globe 4
Active Open & Closed