05000483/LER-2024-001, Reactor Trip Due to Turbine Trip
| ML24095A326 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 04/04/2024 |
| From: | Ameren Missouri, Union Electric Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24095A324 | List: |
| References | |
| ULNRC-06860 LER 2024-001-00 | |
| Download: ML24095A326 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(6) |
| 4832024001R00 - NRC Website | |
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Abstract
On February 1 5, 2024 at 0247, with the reactor at 1 00% power, a failure of the C-phase surge capacitor downstream of low-side XI, winding of the unit auxiliary transformer (UAT) occurred, which led to the main generator protection logic being actuated and a resultant turbine trip with both generator output breakers immediately opening. The turbine-generator trip then immediately led to a reactor trip and an automatic initiation of the auxiliary feedwater system. Safety systems functioned as required, with the exception of a position indication issue on one of the feedwater isolation valves, which was confirmed closed. The Operations staff responded to the event in accordance with applicable plant procedures. Formal troubleshooting and testing confirmed that the C-phase surge capacitor for the affected 3-phase system had faulted to ground, causing the trip. No other power delivery equipment was found failed. The three surge capacitors for the X UAT low-voltage winding, as well as the three surge capacitors connected to the Y UAT low-voltage winding, were replaced and power factor tested successfully, which allowed the UAT to be returned to service. The failed capacitor has been sent to a laboratory for further failure analysis.
1
. DESCRIPTION OF STRUCTURE(S), SYSTEM(S), AND COMPONENT(S):
The systems and components affected by this event include the reactor trip system, the aUxiliary feedwater system, the unit main generator/turbine, and a surge capacitor associated with the unit auxiliary transformer (UAT).
The reactor trip system (i.e., reactor protection system) at Callaway Plant initiates a unit shutdown, based on the values of selected unit parameters, to protect against violating the core fuel design limits and reactor coolant system pressure boundary during anticipated operational occurrences and to assist the Engineered Safety Features systems in mitigating accidents.
The main generator/turbine has a number of protective trip functions in order to protect this asset from irreparable damage. A turbine trip effects a reactor trip when reactor power is greater than 50% rated thermal power.
The UAT extracts power from the output of the main generator (at the 25-ky level) to normally supply the non-safety I 3.8-kV buses, PAOI and PAO2, via two separate secondary (Y and X) windings. Surge capacitor MAO5A is connected to the secondary circuit that normally supplies bus PAOI
. Surge capacitor MAO5B is connected to the secondary circuit that normally supplies bus PAO2.
The auxiliary feedwater system ensures that the reactor coolant system (RCS) can be cooled down (via the steam generators) to less than 350 degrees Fahrenheit from normal operating conditions, in the event of a loss of off-site power.
2. INITIAL PLANT CONDITIONS
Callaway was at I 00% power/MODE I at the time of this event.
3. EVENT DESCRIPTION
On February 1 5, 2024 at 0247, with the reactor at 100% power, a failure of the C-phase surge capacitor downstream of low-side X winding of the unit auxiliary transformer (UAT) occurred, which led to main generator protection logic (386-1/
G and 486-1/U lockout relays) being actuated, resulting in a turbine trip (as indicated by main control room annunciation I 13F, Unit Trip Turbine Trip), a fasttransfer of PAOI and PAO2 from the UAT to the startup transformer, and both generator output breakers immediately opening. A reactor trip immediately occurred in response to the turbine trip. In addition, an automatic auxiliary feedwater system actuation occurred as expected after the reactor trip. Safety systems thus functioned as required, with the exception of a position indication issue on one of the feedwater isolation valves, which was confirmed closed. The Operations staff responded to the event in accordance with applicable plant procedures.
An ENS notification (ENS 56968) was made for this event.
4. ASSESSMENT OF SAFETY CONSEQUENCES
There were no actual nuclear, radiological, or personnel safety impacts associated with this event. The potential impact was on nuclear safety with respect to challenging the reactor trip system as well as any potential challenges to the plant due to the transient associated with the reactor trip. However, all safety systems functioned as designed, with the exception of a position indication issue on one of the feedwater isolation valves, which was confirmed closed. The reactor automatically tripped (i.e., shut down) per design following the main generator/turbine trip.
5. REPORTING REQUIREMENTS
This LER is submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A) to report a reactor protection system actuation and an auxiliary feedwater actuation.
Specifically, 10 CFR 50.73(a)(2)(iv)(A) states in part, The licensee shall report:
(A) Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)
(2)(iv)(B) ofthis section..
(B) The systems to which the requirements of paragraph (a)(2)(iv)(A) of this section apply are:
(1 ) Reactor protection system (RPS) including: reactor scram or reactor trip.
(6) PWR auxiliary or emergency feedwater system.
The reactor trip system was actuated on February 15, 2024 at 0247 (in response to a turbine trip). This fulfills the reporting requirement of I 0 CFR 50.73(a)(2)(iv)(A) by actuation of the system specified in 10 CFR 50.73(a)(2)(iv)(B)(1 ). In addition, a valid automatic auxiliary feedwater system actuation occurred after the turbine trip. This fulfills the reporting requirement of 10 CFR 50.73(a)(2)(iv)(A) by actuation ofthe system specified in 10 CFR 50.73(a)(2)(iv)(B)(6).
6. CAUSE OF THE EVENT
A post-trip plant walk-down identified the failed C-phase surge capacitor MAO5B. Formal troubleshooting and testing confirmed that the C-phase surge capacitor had failed and no other power delivery equipment was found failed.
7. CORRECTIVE ACTIONS
The MAO5A and MAO5B surge capacitors (6 total) were replaced and power factor tested successfully, which allowed the UAT to be returned to service. The failed capacitor has been sent to a laboratory for further failure analysis.
8. PREVIOUS SIMILAR EVENTS
On July 26, 2013, electrical faults caused damage to the isophase bus in the auxiliary transformer and main generator neutral connection box which caused a generator trip and subsequent reactor trip. This event was reported per LER 201 3-008.
On December 3, 2014, a turbine trip occurred, when the main generator excitation transformer faulted to ground. This resulted in a reactor trip (classified as uncomplicated). Safety systems performed as designed. This event was reported per LER 2014-006.
On September 27, 2020, a main generator fault occurred that led to a turbine trip and reactor trip. A fault in the 25-kV portion of the generator output actuated the main generator protection system, which in turn caused the turbine trip (with reactor power above the P-9 set-point) and an automatic reactor trip. This event was reported per LER 2020-006.Page 4
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