05000413/LER-2020-001, Valid Actuation of the Unit 1 Reactor Protection System and Auxiliary Feedwater System
| ML20104A020 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 04/13/2020 |
| From: | Simril T Duke Energy Carolinas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RA-20-0102 LER 2020-001-00 | |
| Download: ML20104A020 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 4132020001R00 - NRC Website | |
text
(.,DUKE ENERGY.
RA-20-0102 April 13, 2020 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555
Subject:
Duke Energy Carolinas, LLC Catawba Nuclear Station, Unit 1 Docket No. 50-413 Licensee Event Report (LER) 413/2020-001-00 Tom Simril Vice President Catawba Nuclear Station Duke Energy CN01VP 14800 Concord Road York, SC 29745 o: 803.701.3340 f: 803. 701.3221 10 CFR 50.73 Pursuant to 10 CFR 50.73(a)(1) and (d), attached is LER 413/2020-001-00, entitled "Valid Actuation of the Unit 1 Reactor Protection System and Auxiliary Feedwater System."
This report is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A).
This LER is preliminary and will be supplemented upon completion of the cause analysis. Duke Energy will provide a supplement to this LER within 30 days.
There are no regulatory commitments contained in this letter or its attachment.
This event is considered to be of no significance with respect to the health and safety of the public.
If questions arise regarding this LER, please contact Sherry E. Andrews of Regulatory Affairs at (803) 701-3424.
Sincerely, Tom Simril Vice President, Catawba Nuclear Station Attachment
United States Nuclear Regulatory Commission Page 2 April 13, 2020 xc (with attachment):
L. Dudes Regional Administrator U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, GA 30303 M. Mahoney NRC Project Manager (CNS)
U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mailstop O-8G9A Rockville, MD 20852 J. Austin (without enclosure)
NRC Senior Resident Inspector
Abstract
At 1800 [EST] hours on February 12, 2020, with Unit 1 in Mode 1 at approximately 100 percent power, Catawba Unit 1 experienced an automatic reactor trip that was triggered by a self-revealing generator lockout signal.
Degraded brushes in the generator exciter failed to establish adequate excitation to keep the generator online.
The trip was not complex, with all systems responding normally post-trip. Because of the reactor trip, the Auxiliary Feedwater system actuated as expected due to an Anticipated Transient Without Scram Mitigation System Actuation Circuitry signal.
The preliminary cause is that the station did not establish and maintain an effective single point vulnerability preventative maintenance mitigation strategy for the turbine exciter. This LER will be supplemented upon finalization of the cause analysis. There was no impact to the health and safety of the public.
BACKGROUND The following information is provided to assist readers in understanding the event described in this LER. Applicable Energy Industry Identification [EIIS] system and component codes are enclosed within brackets. Catawba Nuclear Station unique system and component identifiers are contained within parentheses.
This event is being reported under the following criterion:
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)." The applicable 10 CFR 50.73(a)(2)(iv)(B) systems include the Reactor Protection System and the Auxiliary Feedwater System.
Reactor Protection System [JC] (IPE):
The Reactor Trip System (RPS) automatically limits reactor operation to within a safe region by shutting down the reactor whenever the limits of the region are approached. Whenever a direct process or calculated variable exceeds a setpoint the reactor will be shutdown in order to protect against either gross damage to fuel cladding or loss of system integrity which could lead to release of radioactive fission products into the containment. The various reactor trip circuits automatically open the reactor trip breakers whenever a condition monitored by the Reactor Trip System reaches a preset level. The reactor will trip on an anticipatory Turbine Trip. The anticipatory reactor trip on turbine trip is actuated by a low pressure signal from two-out of-four stop valve electro-hydraulic fluid pressure switches, or by valve closed signals from four-out-of-four turbine steam stop valve limit switches.
Engineering Safety Feature Actuation System [JE] (ISE):
The Engineered Safety Features Actuation System is a functionally defined system. The equipment which provides the actuation functions is listed below
- 1. Process Instrumentation and Control System
- 2. Solid State Logic Protection System
- 3. Engineered Safety Features Test Cabinet
- 4. Manual Actuation Circuits Auxiliary Feedwater System [BA] (CA):
The Auxiliary Feedwater System (CA) assures sufficient feedwater supply to the steam generators (S/G), in the event of loss of the Condensate/Feedwater System, to remove energy stored in the core and primary coolant. The CA System may also be required in some other circumstances such as evacuation of the main control room or cooldown after a loss-of-coolant accident for a small break, including maintaining a water level in the steam generators following such a break.
EVENT DESCRIPTION
Timeline of Events:
1/20/20: Inspection of both Unit 1 and Unit 2 exciter and generator brushes was performed.
2/3/20: Inspection of both Unit 1 and Unit 2 exciter and generator brushes was performed. Unit 1 work order completion comments state: Performed monthly inspection of brushes, sight glasses, panels, and shaft using strobe. All items SAT.
2/12/20 1800: Catawba Unit 1 turbine tripped as a result of loss of excitation to the main generator field. A Zone G Lockout occurred which tripped the Generator Protection Circuit Breakers A and B and the exciter. The turbine trip caused an automatic reactor trip. Because of the reactor trip, the CA System actuated as expected due to an AMSAC signal. The CA pumps started as designed when the valid system actuation was received.
2/16/20 0410: Catawba Unit 1 back on-line. The degraded brushes, brush holders, and housings were replaced. The collector ring was machined to clean off any residual damage.
2/17/20 0137: Catawba Unit 1 reached 100% power.
The generator Zone G Lockout signal was a result of degraded Alterex brushes that were no longer able to supply excitation to the main generator field. The brushes had either been worn down too far to make proper contact with the collector plate, even with a spring force driving the brush toward the collector plate, or carbon buildup caused binding that prevented movement of the brush. Without proper contact, the current flow through the degraded brushes would have decreased, and as a result, the remaining brushes with good contact would have seen an increase in current. The brushes that were still conducting became overheated and were damaged. This caused damage to the collector ring. At some point, sustained electrical arcing developed between the degraded exciter brushes and the collector plate resulting in a flashover event. Damage was sustained by the brushes, brush holders, housing, and collector ring from the flashover.
Because of this damage, there was a decrease in current flow to the exciter, which resulted in a decreasing Generator Field. The Generator Terminal Voltage started to rapidly decrease. The AVR attempted to make up for the Voltage drop by increasing the AVR current output. This increased the heating on the brushes and they started to fail. As a result, the Generator Terminal Voltage dropped to 18.5kV and initiated a trip of the Loss of Field Relay (40) Relay. This led to the Turbine / Generator Trip and ultimately the Reactor Trip.
CAUSAL FACTORS A root cause analysis is currently in progress for the Unit 1 reactor trip event. This LER will be supplemented upon finalization of the cause analysis. The preliminary cause is that the station did not establish and maintain an effective single point vulnerability preventative maintenance mitigation strategy for the turbine exciter.
CORRECTIVE ACTIONS
Immediate Actions:
- 1. Degraded brushes, brush holders, and housings within the Catawba Unit 1 generator exciter were replaced. The associated collector ring was machined to remove residual damage.
(Complete)
- 2. Inspect the exciter and generator brushes in Units 1 and 2. (Complete)
Interim and Subsequent Actions:
- 1. Combine maintenance activities for brush inspection and brush replacement into one work order package to permit immediate, timely field replacement if an issue is identified.
(Complete)
- 2. Ensure Catawba procedure is aligned with industry and vendor recommendations for Alterex brush inspection criteria. Issue a procedure for Catawba Alterex inspection criteria.
(Complete)
Planned Actions:
- 1. The LER supplement will address planned actions.
SAFETY ANALYSIS
At 1800 [EST] hours on February 12, 2020, with Unit 1 in Mode 1 at approximately 100 percent power, the reactor tripped following a main turbine trip. The Catawba Unit 1 scram did not impact the health and safety of the public and there is no safety consequence to this event. The generator lockout signal was caused by a failure in a non-safety related component and did not impact the ability of the reactor to safety shutdown and maintain a safe shutdown condition. The trip was uncomplicated with all systems responding normally post-trip. The crew performed appropriately for the condition. The unit was safely shutdown on February 12 and was restarted on February 16. Unit 2 was not affected.
ADDITIONAL INFORMATION
A review of Catawba License Event Reports from the past 3 years based on the preliminary cause did not identify any similar occurrences. Currently this is not considered a recurring event.