05000387/LER-2025-001-01, (SSES) Unit 1, Manual Reactor Scram Following Loss of Cooling to Main Transformer
| ML25317A743 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 11/13/2025 |
| From: | Casulli E Susquehanna, Talen Energy |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| PLA-8198 LER 2025-001-01 | |
| Download: ML25317A743 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3872025001R01 - NRC Website | |
text
Edward Casulli Site Vice President November 13, 2025 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 Edward.Casuili@TalenEnergy.com SUSQUEHANNA STEAM ELECTRIC STATION LICENSEE EVENT REPORT 50-387/2025-001-01 UNIT 1 LICENSE NO. NPF-14 PLA-8198 TALEN~
ENERGY 10 CFR 50.73 Docket No. 50-387 Attached is Licensee Event Report (LER) 50-387/2025-001-01. The LER supplements the earlier report of an event involving a manual scram following the partial loss of Main Transformer cooling that was determined to be reportable in accordance with 10 CFR 50.72(b)(2)(iv)(A), 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A). This event was also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in a manual actuation of the Reactor Protection System (including a reactor scram) and actuation of an emergency core cooling system.
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.
E. Casulli Attachment: LER 50-387/2025-001-0l Copy:
NRC Region I Mr. R. Wehrmann, NRC Senior Resident Inspector Mr. R. Guzman, NRC Project Manager Mr. M. Shields, PA DEP/BRP
Attachment to PLA-8198 LER 50-387/2025-001-01
Abstract
On July 22, 2025, at approximately 15:10, Susquehanna Steam Electric Station (SSES) Unit 1 was manually scrammed following a partial loss of cooling to the main transformer (one of two cooling fan groups were found tripped). The event was reported by Event Notification 57822 in accordance with 10 CFR 50. 72(b )(2)(iv)(A), 10 CFR 50. 72(b )(2)(iv)(B) and 10 CFR 50. 72(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 a manual actuation of the Reactor Protection System (including a reactor scram) and actuation of the emergency core cooling system. Following restoration of both cooling fan groups, SSES Unit 1 was restored to Mode 1 operation on July 25, 2025. The root cause of the event was determined to be that Operations leadership did not ensure the Operators were prepared to respond to a loss of cooling event on the main transformer to minimize the potential for a required manual scram. There were no actual consequences to the health and safety of the public as a result of this event.
CONDITIONS PRIOR TO THE EVENT 050 052 Unit 1: Mode 1, approximately 96% Rated Thermal Power Unit 2: Mode 1, approximately 100% Rated Thermal Power
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00387
[::7 NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 001 1-0 There were no inoperable structures, systems, or components at the start of the event that contributed to the event
EVENT DESCRIPTION
On July 22, 2025, a Main Transformer 1X101 [EIIS System Code/ Component Code - EL/ XFMR] trouble alarm was received in the Control Room and Operators entered a 10-minute manual scram requirement in accordance with the Alarm Response procedure.
The non-licensed operator (NLO) responding in the field to the Main Transformer correctly determined that only a partial loss of cooling had occurred with Group 1 cooling fans [IEEE Component Code - FAN] not running and Group 2 cooling fans in service. However, imprecise communications resulted in operators in the Control Room and Work Control Center (WCC) incorrectly believing that a total loss of cooling had occurred. As a result, a second NLO, dispatched to the transformer by the WCC, attempted to restore transformer cooling. Failing to recognize that partial cooling for the transformer was in fact in service, the second NLO secured the running Group 2 cooling fans while attempting to switch to the alternate power supply resulting in the total loss of transformer cooling.
At approximately 15: 10, control room operators manually scrammed SSES Unit 1 following a loss of cooling to the Main Transformer.
All systems responded normally following the manual scram. High Pressure Coolant Injection [EIIS System Code - BJ]
actuated briefly to assist in restoring reactor water level. The 1 B Reactor Feedwater Pump [El IS System Code/
Component Code - SJ/ P] stabilized and maintained reactor water level. Decay heat was removed by discharging steam to the main condenser using the turbine bypass valves.
The event was reported by Event Notification 57822 in accordance with 10 CFR 50.72(b)(2)(iv)(A), 10 CFR 50.72(b)(2)(iv)
(B) and 1 O CFR 50. 72(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 a manual actuation of the Reactor Protection System (including a reactor scram) and actuation of the emergency core cooling system.
Following restoration of both cooling fan groups, SSES Unit 1 was returned to Mode 1 operation on July 25, 2025.
CAUSE OF THE EVENT
The direct cause of the event was ineffective communication and direction between control room and field personnel responding to a partial loss of main transformer cooling (Group 1 Cooling). This resulted in incorrect operator actions being taken that caused a complete loss of transformer cooling.
CAUSE OF THE EVENT cont.
The root cause of the event was determined to be that Operations leadership did not ensure the Operators were prepared to respond to a loss of cooling event on the main transformers to minimize the potential for a required manual scram. This included not ensuring that procedures, training, and readiness were aligned to restore cooling in a total loss of cooling event and to protect the in-service cooling group in a partial loss of cooling event ANALYSIS I SAFETY SIGNIFIGANCE The actual consequence of this event was a manual reactor scram. All cooling to Main Transformer 1X101 was actually lost for approximately six minutes. No damage occurred to the transformer as confirmed by a review of data from the installed Dissolved Gas Analyzer and transformer winding temperatures.
Plant response to the scram was proper and per design. There were no Diesel Generator starts nor Safety Relief Valve actuations during the event. Fuel integrity was not challenged during the transient as evidenced by Main Steam Line and Offgas radiation levels lowering as expected following the scram. In-plant radiation levels and vent release rates were all normal based on plant conditions and within acceptable limits.
The scram was reviewed with regard to Final Safety Analysis Report Chapter 15 and determined to be fully bounded by the analysis. Section 15.2.3 specifically addresses main turbine trip with main turbine bypass operable and classifies this as a non-limiting event. The Main Turbine Bypass System operated as designed during this event to control reactor pressure. All safety systems were observed to operate as designed There were no equipment consequences due to this event and no impact to the safety and health of the public. This event will not be counted as a Safety System Functional Failure under the NRC Reactor Oversight Process Performance Indicator program.
CORRECTIVE ACTIONS
Key corrective actions taken in response to the event included repairing the breaker that caused the loss of Main Transformer 1X101 Group 1 cooling fans, conducting stand-downs with all of the operating crews to communicate lessons learned and reinforce Operations standards. Additionally, formal training on the use of 3-part communication will be completed followed by observations of Operations personnel in the field and control room focused on 3-part communication, teamwork and resource management. Lastly creation of a new Off-Normal procedure to address partial or total loss of transformer cooling will be completed.
COMPONENT FAILURE INFORMATION
NONE
PREVIOUS OCCURRENCES
NONE Page 3
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