05000318/LER-2023-002, Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer
| ML24008A240 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 01/08/2024 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24008A238 | List: |
| References | |
| LER 2023-002-00 | |
| Download: ML24008A240 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3182023002R00 - NRC Website | |
text
Abstract
On November 7, 2023, the Unit 2 reactor automatically tripped due to a Reactor Protection System actuation based on an undervoltage condition. Specifically, the U-4000-22 Unit Service Transformer was lost due to an electrical bus fault. This resulted in a loss of both Control Element Drive Mechanism Motor Generator Sets, causing the reactor trip bus undervoltage condition. The Main Feedwater pumps tripped; therefore, Auxiliary Feedwater was manually initiated to supply both Steam Generators. The 2B Emergency Diesel Generator automatically started and restored its bus. Heat removal remained via the normal turbine bypass valves to the main condenser. The cause of the U-4000-22 transformer fault was due to a short to ground through Current Transformer (CT) secondary wiring.
Inadequate work practices lead to insufficient clearance between the installed CT wire and the live 13 kV bus bar.
The CT wire installed during the open phase modification in 2017 was routed too close to the bus bar. An extent of condition review was performed applicable to CT wiring installed during the open phase modification. Issue reports were generated to perform inspection and repairs. One additional CT wire was identified and corrected in the U-4000-22 voltage regulator. No CT wires were found to be in contact with adjacent components for U-4000-21 or U-4000-11. The wiring was replaced, rerouted, and secured. Extent of condition will be performed on U-4000-12 open phase CTs in the 2024 Unit 1 refueling outage.
(See Page 2 for required number of digits/characters for each block)
(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) Page 3 of 3
D. CAUSE OF EVENT
The method of discovery for this event was self-revealing and is documented in the sites Corrective Action Program under IR04716036. The Unit 2 reactor automatically tripped (scrammed) on Reactor Trip Bus Undervoltage due to a loss of U-4000-22 Transformer. The cause of the U-4000-22 transformer fault is the A phase of the 13kV Voltage Regulator shorted to ground through the Current Transformer (CT) secondary wiring due to insufficient clearance. Inadequate work practices lead to insufficient clearance between the installed CT wire and the live 13kV bus bar. The CT wire was routed too close to the bus bar such that induced heat allowed the wire to sag and contact the 13kV bus bar. The short to ground through the CTs secondary wiring was detected by the U-4000-22 transformers differential protective relaying, tripping the high-side feeder breaker to the Voltage Regulator and the U-4000-22 Transformer, along with its associated low-side 4kV breakers, thus isolating the fault.
E. SAFETY ANALYSIS
The subject event satisfies the criteria in NUREG-1022, Revision 3, for any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10CFR50.73, paragraph (a)(2)(iv)(B).
Specifically, for this event, the Reactor Protection System, the 2B Emergency Diesel Generator, and the Auxiliary Feedwater System. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A). There were no safety consequences as a result of the event. All safety systems functioned and operated as designed.
F. CORRECTIVE ACTIONS
The open phase CTs and its wiring for the U-4000-22 and U-4000-21 Voltage Regulators was installed in 2017 during a refueling outage as part of the Open Phase Detection modification. The open phase CTs and its wiring for U-4000-11 and U-4000-12 Voltage Regulators was installed in 2016 as part of the Open Phase Detection Modification for Unit 1. An extent of condition review was performed applicable to the voltage regulator CT wiring installed during the Open Phase Detection modification. Issue reports were generated to perform inspection and repairs. One additional CT wire was identified and corrected in the U-4000-22 voltage regulator.
No CT wires were found to be in contact with adjacent components for U-4000-21 or U-4000-11; however, to increase margin, the wiring was replaced, rerouted, and secured. Inspection and repairs, if required, for the U-4000-12 open phase CTs is currently scheduled for the 2024 Unit 1 refueling outage.
G. PREVIOUS OCCURRENCES
A review of Calvert Cliffs events was performed. No events were identified from this review for an automatic reactor trip from the Reactor Protection System due to loss of a unit service transformer.
H. COMPONENT FAILURE DATA
Component IEEE 803 IEEE805 FUNCTION ID SYSTEM ID Voltage Regulator Current Transformer XCT EA