ML24296B214
| ML24296B214 | |
| 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 | |
| 102-08871-TAH/MDD | |
| Download: ML24296B214 (1) | |
Text
10 CFR 50.55a A member of the STARS Alliance LLC Callaway
- Diablo Canyon
- Palo Verde
- Wolf Creek 102-08871-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) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercise Tested Once per Refueling Cycle Pursuant to 10 CFR 50.55a(z)(1), Arizona Public Service Company (APS) requests Nuclear Regulatory Commission (NRC) approval of the enclosed request for an alternative test frequency of certain valves; specifically, to be tested once per refueling cycle in lieu of the quarterly exercise required by paragraph ISTC-3510 of the American Society of Mechanical Engineers (ASME), Operations and Maintenance of Nuclear Power Plants (OM) Code, 2012.
In accordance with paragraph ISTC-3510 of the OM Code, active category A and B valves shall be exercised quarterly. For the valves in Enclosure 1, Table 1, Palo Verde Nuclear Generating Station (PVNGS) is requesting NRC approval to perform exercise testing once per refueling cycle in lieu of the quarterly exercise required by ISTC-3510.
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 13:34:35 -07'00'
102-08871-TAH/MDD ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Valve Relief Request - 02 Page 2
Enclosure:
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 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
1 of 8 Enclosure Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 2 of 8
- 1. ASME Code Components Affected The components applicable to this request are identified in Table 1 (see Attachments). The request applies to all three Palo Verde Nuclear Generating Station (PVNGS) units; unit numbers are omitted from the valve identification numbers.
2. Applicable Code Edition and Addenda
Arizona Public Service Company (APS) applies ASME OM-2012 for the fourth 120-month in-service testing interval, which began January 15, 2018, and ends on January 14, 2028. Palo Verde applies Code Case OMN-20 to test periods specified in ASME OM Code.
3. Applicable Code Requirement
This proposed alternative applies to ISTC-3510, Exercise Test Frequency, stating: Active Category A, Category B, and Category C check valves shall be exercised nominally every 3
[months]
4. Reason for Request
APS desires to perform once-per-refueling cycle exercise and stroke-time testing of the valves in Table 1. Refueling cycles are 18 months, which is greater than the 3-month frequency required by ISTC-3510.
5. Proposed Alternative and Basis for Use
For the valves in Table 1, APS is requesting NRC approval to perform exercise testing once per refueling cycle in lieu of the quarterly exercise required by ISTC-3510. This alternative to ISTC-3510 provides an acceptable level of quality and safety because the valves are designed, operated, and controlled to ensure safety functions are met. Risk impacts from changing Table 1 valves to a once-per-cycle test period have been evaluated and are found to be acceptably small. The maintenance and performance history demonstrates the organization is assuring acceptable operation. This request is applicable to all three units at Palo Verde. Code Case OMN-20 will apply to the 18-month test period for the valves in Table 1.
Design The valves in Table 1 are normally closed with an active safety function to close. They have no safety function in the open position and are designed to fail in the closed position. To meet these functions, the Table 1 valves are designed to close through passive release of stored energy upon operation of control room hand switches. For an air-operated valve (AOV), control solenoids deenergize to remove air pressure from the diaphragm and the main spring drives the valve closed. For a solenoid-operated valve (SOV), the main solenoid deenergizes allowing an internal spring to drive the valve closed. Taking a hand switch to open energizes the control solenoids or main solenoid, respectively, driving the valve open. Taking a hand switch to close removes the power, allowing passive closure via the internal spring. Therefore, based on valve design, every stroke of a Table 1 valve validates the safety function to close and the ability to fail closed upon loss of power.
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 3 of 8 Operation The valves in Table 1 remain closed unless taken out of their safety position for testing or plant control. The valves were verified to be normally closed by reviewing applicable piping and instrumentation diagrams. The valves are only operated under procedural control, either for testing or to achieve a specific plant control evolution. To perform exercise testing, the valves are taken out of their safety position to establish test conditions, and then tested in the closing direction.
After testing, the valves remain in their normally closed safety position. To perform plant operations such as containment depressurization and safety injection tank level maintenance, operators open the valves via established operating procedures, and close the valves again once the evolution is complete. Procedures are written to return the valves to their normally closed position in accordance with the Palo Verde status control program. Any failure to return the valve to its normal position violates the status control program and is dispositioned in accordance with the corrective action program.
Table 1 valve usage varies depending on plant status and operational needs. Valve position data from the second quarter of 2024 (April, May and June) was reviewed to assess the frequency and duration for which Table 1 valves are out of their safety position. The data varies widely. In the online units, the main steam isolation valve bypass valves (SGEUV0169 and SGEUV0183) were operated only for their quarterly tests. Meanwhile, the reactor drain tank containment isolation valves (CHAUV0560 and CHBUV0561) were operated regularly for tank level maintenance, and the containment power access purge exhaust valves (CPAUV0004B & CPBUV0005B) were open for hours to allow containment depressurization. No valve was open for more than 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> with a unit online. Unit 3, which had a refueling outage, showed nearly every valve was cycled more during the quarter than its comparable valve in the two online units. This shows outages place a greater demand on Table 1 valves. During the refueling outage, some Unit 3 valves were open for multiple days or had no position data available (lasting nearly two weeks in one outlier case). This is an expected result of systems and components being taken out of service to support refueling outage maintenance. Before returning a valve to service, performance is reconfirmed through formal exercise testing as required by the in-service testing program. Post-maintenance and periodic testing in refueling outages provides confidence in each valves ability to perform its safety function during the coming operating cycle. Similarly, each successful valve closure performed to support operational activities validates a valves ability to close and meet its safety function to fail-safe in the closed position.
The out-of-safety-position durations observed in the second quarter of 2024 are consistent with their active classification under ASME OM Code and NRC guidance. NUREG-1482 states that passive valves open for days, weeks or months are expected to be classified as active, whereas passive valves removed from their safety position temporarily under administrative control need not be classified as active (Section 2.4.2, Valves, Active/Passive). The guidance indicates duration out of safety position is a key input to active/passive determination. This is relevant because ASME OM Code specifies exercise test requirements for active valves. The position data review discussed above shows Table 1 valves are operated consistent with NRC guidance for active valve classification under ASME OM Code. The Table 1 valves are classified as active. APS continues to apply active valve test methodology to the valves in the scope of this request.
Control Administrative controls ensure Table 1 valves are returned to their closed safety position after each use. Operators rely on control room indications to ensure closure. The Table 1 valves have remote position indication and must meet ISTC-3700, Position Verification Testing. APS complies with
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 4 of 8 ISTC-3700 and the supplemental indication requirements of 10 CFR 50.55a(b)(3)(xi) to provide reasonable assurance that control room indication correctly reflects valve position for valves in the in-service testing program. In the unlikely event that a Table 1 valve fails to close on demand, the condition is identified immediately. Fail-to-close events are self-revealing because an open valve does not present closed indication. When the expected closed indication is not displayed after valve use, operators are trained to enter the condition into the corrective action program and follow the required Technical Specifications actions. Procedural requirements and control room indicating lights provide an acceptable means of verifying Table 1 valves are closed when control room hand switches are taken to close.
Risk Evaluation Extending valve exercise testing to 18 months from quarterly produces an acceptably small increase in risk. Nine of the 28 valves in Table 1 are in the PVNGS probabilistic risk assessment (PRA) model. These valves contribute to changes in core damage frequency (CDF) and large early release frequency (LERF). Based on the test period extension, the total change in CDF is 5.2x10-10 per year while the total change in LERF is 1.63x10-8 per year). These are less than permitted in Regulatory Guide 1.174 (reference 8). The guide requires risk increases to remain less than 1x10-6 per year for CDF and less than 1x10-7 per year for LERF. Nineteen of the 28 valves in Table 1 are not in the PRA model. These valves are excluded because they have negligible risk impact. For each of the 28 valves in Table 1, the change in risk from extending to an 18-month test frequency is acceptably small. Nine of the 28 valves produce a total change in CDF and LERF that remains less than allowed under Regulatory Guide 1.174, while the remaining 19 valves have negligible risk impact.
Maintenance and Performance History The Table 1 valves routinely demonstrate acceptable testing performance. The in-service testing performance trends were reviewed and found to show stable performance. There are no degrading trends.
Maintenance history shows few corrective maintenance work orders result from quarterly in-service testing. Most corrective maintenance is identified outside of the in-service testing program. Since 2020, APS has processed 42 corrective maintenance work orders on the 84 valves (28 per unit) in the scope of this request. Of the 42, three work orders (7%) resolved issues during in-service testing. Of the three, two work orders resolved component issues that prevented establishing test conditions, and one work order resolved a stroke time outside the OM Code acceptance criteria. In the past 3 1/2 years, only 1 of the 84 valves in this request has shown in-service test results that warranted maintenance. Therefore, the population of valves subject to this request routinely demonstrate acceptable performance in quarterly testing.
The valves maintenance history also shows the majority of corrective maintenance on Table 1 valves comes from testing and inspection outside of the in-service testing program. More than 70%
of the 42 work orders since 2020 addressed issues identified by programs outside OM Code. These include:
21 work orders addressed system medium leaks (valve packing, body-to-bonnet, and valve seat) 8 work orders addressed non-IST test results (poor local leak rate performance, AOV diagnostics, and electrical testing)
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 5 of 8 1 work order implemented a vendor-recommended preventive maintenance task for bonnet re-torque These issues could not reasonably be detected by OM Code testing because the test methodology is not sensitive to these issues. Valve stroke time performance trends since 2020 show normal performance variation with no degrading trends. Non-in-service testing and inspections are supporting reasonable assurance of Table 1 valve operational readiness.
The maintenance history shows nearly 20% of the 42 corrective maintenance work orders came from operational issues and component interaction (walk-downs, maintenance observations, etc.).
Operational issues resulted in 3 work orders (7%), while AOV air leaks generated 5 work orders (12%). The operational issues were dual indication, a slow-to-actuate in the non-safety opening direction, and loss of electrical power. They were identified by operators during plant evolutions or via control room alarms. The five corrective maintenance work orders addressing AOV air leaks were generally identified during maintenance. The Table 1 AOVs are air-to-open, spring-to-close operators. For these valves, loss of air pressure assists in fulfilling the safety function to close. OM Code acceptance criteria include a lower threshold on AOV performance. Performance trends were reviewed and stroke times remained stable before the air leaks were identified and remained so until the issues were resolved. Therefore, Palo Verde equipment interactions such as walk-downs, maintenance and operational activities are identifying component issues at a lower threshold than required by OM Code test criteria.
The 84 valves in the scope of this request are being maintained to support operational readiness.
Corrective maintenance over the last 3 1/2 years shows just 7% was related to in-service test performance. Most valve issues are identified by non-IST activities (operations, maintenance, and other tests and inspections) and are resolved such that component performance is maintained satisfactory.
OM Code Precedents A once-per-refueling cycle stroke time test provides an acceptable level of quality and safety because it is consistent with precedents in OM Code and the Palo Verde Inservice Testing Program.
Per ISTC-3520, plants may perform valve exercise during cold shutdowns or refueling outages in cases where it is impracticable to exercise a valve with the nuclear unit operating at power. The Palo Verde Inservice Testing Program already includes AOVs and SOVs that are tested at cold shutdown and refueling conditions based on impracticability. Cold shutdown and refueling outage conditions are normally achieved only once per cycle and the valves are effectively tested on a once-per-refueling cycle frequency. While it is practicable to test the valves in Table 1 with the unit online, a once-per-refueling cycle test provides an acceptable level of quality and safety because the valves are normally closed, and the fail-safe functionality is verified each time the valve is cycled to support plant operations.
Conclusion APS recommends NRC approve this request to perform once-per-refueling cycle exercise testing of Table 1 valves in lieu of the 3-month requirement in ISTC-3510. The once-per-refueling cycle test for these valves provides an acceptable level of quality and safety because the valves are designed, operated, and controlled such that their safety functions are routinely met and verified. The risk evaluation shows the changes in CDF and LERF are acceptably small. Performance and maintenance history shows the valves are managed effectively because issues are identified and resolved before they can be detected by OM Code testing.
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 6 of 8
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:
Once-per-refueling cycle exercise testing of the valves in Table 1 provides an acceptable level of quality and safety. The valves have a safety function to close and fail closed. The valves are normally closed and are therefore normally in their safety position unless operated per approved procedures that ultimately return the valve to its closed position. Hence, administrative controls are in place to ensure the valve is returned to its closed position. If a valve is operated during an operating cycle, its fail-safe closure function is inherently verified by returning the valve to its normally closed position. This is sufficient basis to support an alternative request to NRC per 10 CFR 50.55a(z)(1).
References:
- 1. Operations and Maintenance of Nuclear Power Plants, ASME OM-2012
- 2. Code Case OMN-20, Inservice Test Frequency (2022)
- 3. Guidelines for Inservice Testing at Nuclear Power Plants, NUREG-1482, revision 3
- 4. An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Regulatory Guide 1.174, revision 3 Attachments:
Table 1-1 Table 1-2
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 7 of 8 Table 1-1 Valve Description Code category Code class Valve type Operator type Valve function Normal position Safety position Fail-safe position In PRA model?
CHNUV0527 Makeup to charging volume control tank bypass isolation valve B
3 Gate Air Active Closed Closed Closed Yes CHAUV0580 Reactor makeup water to reactor drain tank outboard containment isolation valve (penetration no. 45)
A 2
Gate Air Active Closed Closed Closed Yes CHAUV0560 Reactor drain tank outlet inboard containment isolation valve (penetration no. 44)
A 2
Globe Air Active Closed Closed Closed Yes CHBUV0561 Reactor drain tank inboard containment isolation valve (penetration no. 44)
A 2
Globe Air Active Closed Closed Closed Yes CPAUV0004A Containment power access purge supply outboard containment isolation valve (penetration no. 78)
A 2
Butterfly Air Active Closed Closed Closed Yes CPAUV0004B Containment power access purge exhaust inboard containment isolation valve (penetration no. 79)
A 2
Butterfly Air Active Closed Closed Closed Yes CPBUV0005A Containment power access purge supply inboard containment isolation valve (penetration no. 78)
A 2
Butterfly Air Active Closed Closed Closed Yes CPBUV0005B Containment power access purge exhaust outboard containment isolation valve (penetration no. 79)
A 2
Butterfly Air Active Closed Closed Closed Yes GAAUV0001 High pressure nitrogen supply header outside containment isolation valve (penetration no. 30)
A 2
Globe Solenoid Active Closed Closed Closed Yes SGBHV0200 Chemical injection isolation valve (penetration no. 11)
B 2
Globe Solenoid Active Closed Closed Closed No SGBHV0201 Chemical injection isolation valve (penetration no. 12)
B 2
Globe Solenoid Active Closed Closed Closed No SGEUV0169 Main steam isolation valve bypass valve (penetration no. 2)
B 2
Globe Air Active Closed Closed Closed No SGEUV0183 Main steam isolation valve bypass valve (penetration no. 3)
B 2
Globe Air Active Closed Closed Closed No SIBUV0631 Safety injection tank 1A fill/drain isolation valve B
2 Globe Air Active Closed Closed Closed No
Valve Relief Request (VRR) - 02: Alternative Request Allowing Normally Closed Valves with a Safety Function to Close to be Exercised Once per Refueling Cycle 8 of 8 Table 1-2 Valve Description Code category Code class Valve type Operator type Valve function Normal position Safety position Fail-safe position In PRA model?
SIBUV0638 Safety injection tank 1A check valve leakage test line isolation valve B
1 Globe Air Active Closed Closed Closed No SIBUV0641 Safety injection tank 1B fill/drain isolation valve B
2 Globe Air Active Closed Closed Closed No SIBUV0648 Safety injection tank 1B check valve leakage test line isolation valve B
1 Globe Air Active Closed Closed Closed No SIAUV0682 Safety injection tank fill line containment isolation valve (penetration no. 28)
A 2
Globe Air Active Closed Closed Closed No SIBUV0611 Safety injection tank 2A fill/drain isolation valve B
2 Globe Air Active Closed Closed Closed No SIBUV0618 Safety injection tank 2A check valve leakage test line isolation valve B
1 Globe Air Active Closed Closed Closed No SIBUV0621 Safety injection tank 2B fill/drain isolation valve B
2 Globe Air Active Closed Closed Closed No SIBUV0628 Safety injection tank 2B check valve leakage test line isolation valve B
1 Globe Air Active Closed Closed Closed No SSBUV0200 Hot leg sample line outboard containment isolation valve (penetration no. 42C)
A 2
Globe Solenoid Active Closed Closed Closed No SSBUV0201 Pressurizer surge line sample line outboard containment isolation valve (penetration no. 42A)
A 2
Globe Solenoid Active Closed Closed Closed No SSBUV0202 Pressurizer steam space sample line outboard containment isolation valve (penetration no. 42B)
A 2
Globe Solenoid Active Closed Closed Closed No SSAUV0203 Hot leg sample line inboard containment isolation valve (penetration no.
42C)
A 2
Globe Solenoid Active Closed Closed Closed No SSAUV0204 Pressurizer surge line sample line inboard containment isolation valve (penetration no.
42A)
A 2
Globe Solenoid Active Closed Closed Closed No SSAUV0205 Pressurizer steam space sample line inboard containment isolation valve (penetration no.
42B)
A 2
Globe Solenoid Active Closed Closed Closed No