05000528/LER-2023-001-01, Reactor Trip Following a Main Turbine Trip
| ML23256A370 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 09/13/2023 |
| From: | Harbor C Arizona Public Service Co |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 102-08671-CDH/DHD LER 2023-001-01 | |
| Download: ML23256A370 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| 5282023001R01 - NRC Website | |
text
10 CFR 50.73 A member of the STARS Alliance, LLC Callaway
- Diablo Canyon
- Palo Verde
- Wolf Creek Cary D. Harbor Vice President Regulatory & Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel: 623.393.7953 102-08671-CDH/DHD September 13, 2023 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Subject:
Palo Verde Nuclear Generating Station Unit 1 Docket No. STN 50-528 /Renewed License No. NPF-41 Licensee Event Report Supplement 2023-001-01 Enclosed, please find the Licensee Event Report (LER) 50-528/2023-001-01 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER supplement provides additional information that was identified through PVNGS evaluation of Unit 1s unplanned reactor trip that occurred on April 8, 2023. The initial LER 2023-001-00 for the Unit 1 reactor trip was provided to the Nuclear Regulatory Commission (NRC) on June 7, 2023.
In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the NRC Regional Office, NRC Region IV, and the Senior Resident Inspector.
No new commitments are being made to the NRC by this letter. Should you need further information regarding this submittal, please contact Michael DiLorenzo, Department Leader, Nuclear Regulatory Affairs, at (623) 393-3495.
Sincerely, CDH/DHD/cr Enclosure: Unit 1 Licensee Event Report 2023-001-01 cc:
J. Monninger NRC Region IV Regional Administrator S. P. Lingam NRC NRR Project Manager for PVNGS L. N. Merker NRC Senior Resident Inspector for PVNGS Harbor, Cary (Z16762)
Digitally signed by Harbor, Cary (Z16762)
Date: 2023.09.13 16:04:17
- - 07'00'
ENCLOSURE Unit 1 Licensee Event Report 2023-001-01
Abstract
Palo Verde Nuclear Generating Station (PVNGS) Unit 1
00528 6
Unit 1 Reactor Trip Following a Main Turbine Trip 04 08 2023 2023 001 01 09 13 2023 1
100
Michael DiLorenzo, Department Leader, Nuclear Regulatory Affairs 623-393-3495 E
TG Motor GE Yes X
TL Relay GE Yes
On April 8, 2023 at approximately 2144 MST, following a turbine trip due to the loss of control oil header pressure, both 13.8 kV non-class buses de-energized, which caused a loss of power to the Reactor Coolant Pumps and resulted in a reactor trip. The B auxiliary feedwater pump was started to feed both steam generators due to the de-energization also having led to the main feed pumps tripping on low suction pressure. A Main Steam Isolation Signal was manually actuated due to a loss of circulating water flow to the condenser, which isolated the main steam header and required the use of Atmospheric Dump Valves for heat removal.
The cause of the reactor trip was the result of the turbine generator primary and backup reverse power relays failing to actuate due to not reaching the trip setpoint timing. There was high reactive load on the system grid. This resulted in the reverse power relays not actuating. Corrective actions include enhancement of the excitation system, modification of the existing fast bus transfer logic upon a turbine trip, and a design change to replace the electro-mechanical reverse power relays with microprocessor-based relay equivalents for the main generator in all three units.Page of
- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
052 050
7. CORRECTIVE ACTIONS
IMMEDIATE INTERIM CORRECTIVE ACTIONS:
All three units will be in a split-bus operation, as operational risk allows, until the design modification to address the root cause is implemented. In a split-bus arrangement, one 13.8 kV non-class bus is aligned to the UAT, and the other is aligned to offsite power. This action will preclude the possibility of a complicated reactor trip should the reverse power relays fail to actuate.
The degraded cable splice connection on the A CO pump was repaired.
Addition of hydraulic fluid to the CO reservoir, replacement of the normal level indicating switch, removing the jumper for hydraulic fluid reservoir alarm on panel B06B, and retrieval/reinstallation of the missing float parts from the CO reservoir for the normal level indicating switch were completed for the B CO pump hydraulic fluid low level condition.
CORRECTIVE ACTIONS IDENTFIED DURING EVALUATION:
The corrective actions to prevent recurrence include tracking the implementation of a design change in each unit to zero the reactive power (VARs) post turbine trip and to conditionally remove the line drop compensation. The design change will also enhance the excitation system, i.e., EX2100E, software to provide a fast bus transfer to offsite power (through NAN-S03 and NAN-S04) upon verification of main turbine stop or control valves and intercept or intermediate stop valves being closed. A separate design change to replace the electro-mechanical reverse power relays with microprocessor-based relay equivalents for the main generator is in progress.
The site specification for 480 VAC connections will be updated to add adequate guidance to prevent degradation of cable splice connection insulation in junction boxes.
A design change will be implemented to address the level float arm assembly set screw in the CO reservoir and maintenance instructions will be updated to ensure normal level indicating switch functionality during normal system operations and during testing. Additional actions are to revise site documents/procedures to ensure process gaps with identifying, prioritizing, and correcting deficient equipment conditions are addressed as well as implementing training and learnings for the use of troubleshooting and CO system operating experience.
8. PREVIOUS SIMILAR EVENTS
No similar events have been reported by PVNGS in the last three years due to the same direct cause.
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Palo Verde Nuclear Generating Station Unit 1 00528 2023 001 01