05000387/LER-2021-003-01, Automatic Reactor Scram Due to Main Turbine Trip Caused by a C Isophase Bus Ground Fault
| ML21315A010 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 11/11/2021 |
| From: | Cimorelli K Susquehanna |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 2021-003-01 | |
| Download: ML21315A010 (4) | |
| 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 |
| 3872021003R01 - NRC Website | |
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November 11, 2021 Kevin Cimorelli Site Vice President Attn: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Susquehanna Nuclear, LLC 769 Salem Boulevard Berwick, PA 18603 Tel. 570.542.3795 Fax 570.542.1504 Kevin.Cimorelli@TalenEnergy.com SUSQUEHANNA STEAM ELECTRIC STATION LICENSEE EVENT REPORT 50-387 /2021-003-01 UNIT 1 LICENSE NO. NPF-14 PLA-7971 TALEN~,
ENERGY 10 CFR 50.73 Docket No. 50-387 Attached is Licensee Event Report (LER) 50-3 87/2021-003-01. The LER supplement reports an event involving an automatic scram due to a main turbine trip. The condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in an automatic actuation of the Reactor Protection System (including a reactor scram).
There were no actual consequences to the health and safety of the public as a result of this event.
This letter contains no new or revised regulatory commitments.
K. Cimorelli Attachment: LER 50-387/2021-003-0l Copy:
NRC Region I Ms. A. Klett, NRC Project Manager Mr. C. Highley, NRC Senior Resident Inspector Mr. M. Shields, PA DEP/BRP
Abstract
On July 21, 2021, at approximately 18:26, Susquehanna Steam Electric Station Unit 1 reactor automatically scrammed due to a main turbine trip. Both divisions of the Reactor Protection System (RPS) actuated and all control rods inserted. This event was reported by Event Notification 55370 in accordance with 10 CFR 50.72(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in automatic actuation of a system listed in 10 CFR 50.73(a)(2)(iv)(B).
The cause of the event was a dislodged de-ionizing baffle plate that contacted the C isophase bus causing a ground fault. The root cause was determined to be accelerated wear of the de-ionizing baffle tack welds due to increased isophase bus forced air. Corrective actions included replacement and tack welding the dislodged and loose baffle plates and implementing an Engineering Change to modify the Unit 1 Main Generator-end A and C isophase bus de-ionizer baffle plates to install an aluminum retaining rod to secure the baffle plates together.
There were no actual consequences to the health and safety of the public as a result of this event.
NRC FORM 366 (08-2020)
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CONDITIONS PRIOR TO EVENT
Unit 1 - Mode 1, approximately 100 percent Rated Thermal Power Unit 2 - Mode 1, approximately 100 percent Rated Thermal Power There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
EVENT DESCRIPTION
On July 21, 2021, at approximately 18:26, Susquehanna Steam Electric Station Unit 1 reactor automatically scrammed due to a main turbine [EIIS System/Component Code: TA/TRB] trip. The Unit 1 Control Room received indication of a main turbine trip with both divisions of the Reactor Protection System (RPS) [EIIS System Code: JC] actuated and all control rods inserted. The Reactor Recirculation Pumps [EIIS System/Component Code: AD/P] tripped on End of Cycle Recirculation Pump Trip.
Operations subsequently maintained reactor water level at the normal operating band using the Reactor Feedwater system [EIIS System Code: SJ].
This event was reported by Event Notification 55370 in accordance with 10 CFR 50.72(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in automatic actuation of a system listed in 10 CFR 50.73(a)(2)(iv)(B).
CAUSE OF EVENT
The cause of the event was a dislodged de-ionizing baffle plate that contacted the C isophase bus [EIIS System/Component Code: EL/IPBU] causing a ground fault. The root cause was determined to be accelerated wear of the de-ionizing baffle tack welds due to increased isophase bus forced air.
Contributing causal factors included: 1) industry isophase bus events were not effectively reviewed to identify and implement station learnings for prevention of isophase bus failures and 2) visual inspection guidelines used to inspect the isophase de-ionizer grids for fatigue were inadequate.
ANALYSIS/SAFETY SIGNIFICANCE
The scram did not require or result in the actuation of any Emergency Core Cooling System or the Reactor Core Isolation Cooling system [EIIS System Code: BN] and no main steam relief valves [EIIS System/ Component Code: SB/RV] opened. All safety systems responded properly during the event.
There were no actual consequences to the health and safety of the public as a result of this event.
CORRECTIVE ACTIONS
Key corrective actions included the following:
- 1. Replaced and tack welded the dislodged and loose baffle plates.
- 2. Implemented an Engineering Change to modify the Unit 1 Main Generator-end A and C isophase bus de-ionizer baffle plates to install an aluminum retaining rod to secure the baffle plates together.
Actions will be taken to address the extent of condition on Unit 2 as required.
COMPONENT FAILURE INFORMATION
Component Identification - 1G105 Component Name - Generator Bus, Isolated Phase Bus and Auxiliary Manufacturer - General Electric
PREVIOUS OCCURRENCES
None.