05000445/LER-2022-001, (Cpnpp), Reactor Trip Due to Turbine Trip Greater than 50% Power
| ML22307A292 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 11/03/2022 |
| From: | Sewell S Luminant, Vistra Operations Company |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| CP-202200378, TXX-22083, T 254 .897.6113 LER 2022-001-00 | |
| Download: ML22307A292 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4452022001R00 - NRC Website | |
text
II Luminant CP-202200378 TXX-22083 November 3, 2022 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Steven Sewell Senior Director Engineering &
Regulatory Affairs
Subject:
Comanche Peak Nuclear Power Plant (CPNPP) Docket No. 50-445 Reactor Trip Due To Turbine Trip Greater Than 50% Power Licensee Event Report 445/22-001-00
Dear Sir or Madam:
Ref Comanche Peak Nuclear Power Plant (Vistra Operations Company LLC)
P.O. Box 1002 6322 North FM 56 Glen Rose, TX 76043 T 254.897.6113 10 CFR 50.73 Enclosed is Licensee Event Report (LER) 1-22-001-00, "Reactor Trip Due To Turbine Trip Greater Than 50% Power" for Comanche Peak Nuclear Power Plant (CPNPP) Unit 1.
This letter contains no new regulatory commitments.
If you have any questions regarding this submittal, please contact Ryan Sexton at (430) 203-3009 or ryan.sexton@vistracorp.com.
Attachment c (email) -
Scott Morris, Region IV [Scott.Morris@nrc.gov]
Dennis Galvin, NRR [Dennis.Galvin@nrc.gov]
Sincerely, Steven Sewell John Ellegood, Senior Resident Inspector, CPNPP Uohn.Ellegood@nrc.gov]
Neil Day, Resident Inspector, CPNPP [Neil.Day@nrc.gov]
Abstract
On September 05, 2022, Unit 1 was operating at approximately 100% power. At 2345, a spurious failure of the channel 1 shaft displacement signal coupled with a previous failure of the channel 2 shaft displacement signal caused a trip of the Unit 1 turbine through the turbine trip logic. This led to a Unit 1 reactor trip due to a turbine trip with reactor power greater than 50%. Entry into the Emergency Plan was not required.
The cause of this event was the spurious, momentary failure of the channel 1 main turbine shaft displacement signal coupled with the complete failure of the channel 2 main turbine shaft displacement signal. These excessive displacement signals satisfied the shaft displacement trip logic for the turbine. The cause of the signal failures was determined to be inadequate protection of the shaft displacement probe cabling against chafing. Corrective actions include replacement of all three channels of shaft probes, cables, and drivers, as well as defeating the shaft displacement trip through the next refueling outage. Additional corrective actions included revising the shaft displacement probe installation instructions and developing permanent modifications for the routing of the turbine shaft displacement probes for implementation during the next refueling outage.
All times are in Central Daylight Time (CDT).
I. DESCRIPTION OF REPORTABLE EVENT On September 05, 2022, Unit 1 was operating at approximately 100% power. At 2345, a failure occurred of the cabling for the Unit 1 Main Turbine shaft displacement probe channel 1 coupled with a previous failure of the channel 2 shaft displacement probe, resulted in a shaft displacement signal that satisfied the trip logic for the shaft displacement automatic trip of the Unit 1 Main Turbine and Reactor. The Auxiliary Feedwater System (AFW) automatically started as expected.
A. REPORTABLE EVENT CLASSIFICATION
The event is reportable under 10 CFR 50.73(a)(2)(iv)(A), "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section." The Reactor Protection System (RPS) automatically actuated and the AFW started as designed due to low-low steam generator water level following trip.
B. PLANT CONDITION PRIOR TO EVENT On September 05, 2022 Unit 1 was operating in Mode 1 at approximately 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE INOPERABLE AT THE START OF THE EVENT AND CONTRIBUTED TO THE EVENT There were no structures, systems, or components which were inoperable prior to the event which contributed to the event. Prior to the main turbine trip, the main turbine and generator were performing their design functions.
D. NARATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES On September 05, 2022, Unit 1 was operating at approximately 100% power. At 2345 a failure occurred on the Unit 1 Main Turbine shaft displacement probe channel 1 [EIIS:(TA)(DET)(CHA)(CBL)] coupled with a previous failure of the channel 2 shaft displacement probe, resulted in a shaft displacement automatic trip of the Unit 1 Main Turbine. Because reactor power at the time of the turbine trip was approximately 100% power, the Westinghouse permissive P-9 signal was unblocked (power> 50%) as designed and when the turbine tripped an automatic reactor trip was actuated.
Reactor operators in the control room responded to the trip in accordance with Emergency Operating procedures. Entry into the Emergency Plan was not required. Operators responded to the event and safely placed Unit 1 in Mode 3.
E.THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE, OR PROCEDURAL PERSONNEL ERROR Operators (Utility, Licensed) in the Unit 1 Control Room received a "RX> 50% PWR TURB TRIP" alarm and confirmed automatic Unit 1 reactor and turbine trips.
II. COMPONENT OR SYSTEM FAILURES A. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE The cause of this event was failure of the channel 1 Main Turbine shaft displacement probe coupled with a previous failure of the channel 2 shaft displacement probe, satisfied the shaft displacement trip logic for the Turbine. The cause of this failure was unwanted chafing of the shaft displacement probe cabling against the turbine pedestal, which led to the failure of the shaft displacement probe.
B. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT The event was first noticed by the control room operators as both the unit 1 reactor and turbine automatically tripped. The turbine trip was initiated by satisfying the trip logic for indications of excessive shaft displacement. The cabling for the shaft displacement probe channels, in the Unit 1 Main Turbine, were chafing against the turbine pedestal, which ultimately led to the failure of the channel 1 turbine shaft displacement probe and the previous failure of the channel 2 shaft displacement probe, which satisfied the excessive shaft displacement trip logic to initiate an automatic trip of the Unit 1 Turbine.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS WITH MULTIPLE FUNCTIONS This event did not involve systems or secondary functions which were affected by the main turbine shaft displacement probe failure.
D. FAILED COMPONENT INFORMATION
The Unit 1 Main Turbine shaft displacement probe is a Connection Technology Center model PRO part number DP100103-00-06-10-01. As the probe is a non-critical system, CPNPP performed a PM at a frequency of 550 days
+/-25%, to look for degradation in the probe or any of its associated components, which will lead to a replacement. At the time of the failure, the probe had been in service approximately 4 months.
Ill. ANALYSIS OF THE EVENT A. SAFETY SYSTEM RESPONSES THAT OCCURRED The Reactor Protection System responded as designed to the automatic turbine trip by initiating an automatic reactor trip.
The Auxiliary Feedwater System started as designed due to low-low steam generator water level following the trip.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY This event did not involve the inoperability of any safety component or system.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT A turbine trip is an American Nuclear Society Condition II event (Faults of Moderate Frequency). When the main turbine tripped the reactor was automatically tripped and the Auxiliary Feedwater System automatically started to provide feedwater to the steam generators.
Automatic turbine trip and automatic reactor trip were exercised and the Auxiliary Feedwater System started automatically as expected and all plant safety systems responded as designed during the resultant transient. This event had no impact on nuclear safety, reactor safety, radiological safety, environmental safety or the safety of the public. This event has been evaluated as not meeting the definition of a safety system functional failure per 10 CFR 50.73(a)(2)(v).
IV. CAUSE OF THE EVENT
The cause of this event was the failure of the channel 1 Main Turbine shaft displacement probe, which satisfied the shaft displacement trip logic for the Turbine. The cause of this failure was inadequate protection of the shaft displacement probe cabling.
V. CORRECTIVE ACTIONS
All three of the Unit 1 Main Turbine shaft displacement probes installed during the trip went through troubleshooting to find specific failures, and ultimately all three were replaced to facilitate startup. Subsequently, after startup, CPNPP operators noticed that they were receiving spikes from multiple shaft displacement probe channels that were not indicative of actual performance. Therefore, CPNPP has defeated the shaft displacement automatic trip for the Unit 1 Main Turbine through the next refueling outage, to avoid a similar trip. All other turbine protection features remain functional, and CPNPP has established alternative monitoring methods for shaft displacement.
Future corrective actions include, developing and scheduling the implementation of a modification to the routing and installation of the Main Turbine shaft displacement probe cabling to preclude the possibility of cable chafing, as well as revising the operations procedure to provide added instructions for resolving Main Turbine digital alarms.
VI. PREVIOUS SIMILAR EVENTS
There have been no previous similar reportable events at CPNPP in the past three years. Page 4
of 4