05000387/LER-2024-002, B Diesel Generator Inoperable Due to Failed Excitation System Linear Reactor

B Diesel Generator Inoperable Due to Failed Excitation System Linear Reactor

ML24260A227
Person / Time
Site:	Susquehanna
Issue date:	09/16/2024
From:	Casulli E Susquehanna, Talen Energy
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
PLA-8131 LER 2024-002-00
Download: ML24260A227 (1)

LER-2024-002, B Diesel Generator Inoperable Due to Failed Excitation System Linear Reactor

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3872024002R00 - NRC Website

text

Edward Casulli Site Vice President September 16, 2024 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.Casulli@TalenEnergy.com SUSQUEHANNA STEAM ELECTRIC STATION LICENSEE EVENT REPORT 50-387(388)/2024-002-00 UNIT 1 LICENSE NO. NPF-14 UNIT 2 LICENSE NO. NPF-22 PLA-8131 TALEN~

ENERGY 10 CFR 50.73 Docket No. 50-387 50-388 Attached is Licensee Event Rep01i (LER) 50-387(388)/2024-002-00. The LER reports an event in which the B diesel generator (DG) tripped on generator differential due to the failure of an excitation system linear reactor during a monthly surveillance test. The condition was determined to be rep01iable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications and 10 CFR 50.73(a)(2)(v) as an event or condition that could have prevented the fulfillment of a safety function.

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(388)/2024-002-00 Copy:

NRC Region I Ms. J. England, NRC Senior Resident Inspector Ms. A. Klett, NRC Project Manager Mr. M. Shields, PA DEP/BRP

Abstract

On April 8, 2024, during the performance of surveillance testing, the B Diesel Generator (DG) tripped on generator differential current approximately 80 minutes into the full load period of the test. Operators declared Surveillance Requirement 3.8.1.3 failed and entered Unit 2 Technical Specification (TS) Limiting Condition of Operation 3.8.1, Condition B for the inoperable DG. The generator differential current trip was caused by a failure of the DG excitation system C-phase linear reactor. The cause of the C-phase linear reactor failure was a turn-to-turn short from a localized failure of the linear reactor insulation due to less than optimal original vendor design which, coupled with an increase in current from a system imbalance, caused thermally accelerated degradation. This condition was determined to be reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS and 10 CFR 50.73(a)(2)(v) as an event or condition which could have prevented the fulfillment of a safety function. Key corrective action included replacing all three linear reactors in the B DG. There were no actual consequences to the health and safety of the public as a result of this event.

CONDITIONS PRIOR TO EVENT

2. DOCKET NUMBER

3. LER NUMBER I

00387

~ NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-0 Unit 1 - Mode 5, approximately O percent Rated Thermal Power (RTP)

Unit 2 - Mode 1, approximately 100 percent RTP There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.

EVENT DESCRIPTION On April 8, 2024, at approximately 0715, during the performance of surveillance testing, the B Diesel Generator (DG) [EIIS Code/Component Code: EK/DG] tripped on generator differential current approximately 80 minutes into the full load period of the test. Operators declared Surveillance Requirement (SR) 3.8.1.3 failed and entered Unit 2 Technical Specification (TS) Limiting Condition of Operation (LCO) 3.8.1, Condition B for the inoperable DG. Unit 1 was in Mode 5 at the time with the requirement for two operable DGs met by the A and C DGs. At approximately 2115, the E DG was substituted in for the B DG, allowing LCO 3.8.1, Condition B to be exited. Following troubleshooting, all three linear reactors and the voltage regulator in the B DG were replaced. On April 27, 2024, the B DG was substituted back in for the E DG and declared operable.

Based on cause, there is evidence that the condition existed for longer than allowed by LCO 3.8.1 and therefore, in accordance with 10 CFR 50.73(a)(2)(i)(B), is reportable as a condition prohibited by TS. Additionally, since redundant DGs were concurrently inoperable following the last B DG 24-hour endurance run in January 2024, the condition is also reportable in accordance with 10 CFR 50. 73(a)(2)(v) as an event or condition which could have prevented the fulfillment of a safety function.

CAUSE OF EVENT The generator differential current trip was caused by a failure of the DG excitation system C-phase linear reactor. The cause of the C-phase linear reactor failure was a turn-to-turn short from a localized failure of the linear reactor insulation.

Failure of the linear reactor insulation was due to less than optimal original vendor design which, coupled with an increase in current from a system imbalance, caused thermally accelerated degradation.

ANALYSIS/SAFETY SIGNIFICANCE The safety function of the DGs is to provide power to equipment as assumed in the safety analyses described in Chapter 15 of the Susquehanna Updated Final Safety Analysis Report. Since the B DG was last able to demonstrate the ability to meet the 24-hour mission time in January 2024, an exposure time of January 2024 through April 2024 was assumed. A Probabilistic Risk Assessment evaluation for this duration was completed, concluding a low to moderate safety significance. However, there were no actual consequences to the health and safety of the public as a result of this event.

A Safety System Functional Failure (SSFF) evaluation was performed using guidance contained in NEI 99-02. As required by 10 CFR 50 Appendix A, General Design Criterion 17, Susquehanna is designed to accommodate a single failure of any diesel generator and still be able to maintain the plant within its design basis. The SSFF evaluation verified that during the period between January 2024 and April 2024, at least three DGs were always available to perform their safety function. Therefore, the failure of the B DG's C-phase linear reactor did not result in an actual loss of safety function and will not be counted as a Safety System Functional Failure under the NRC Performance Indicator program.

CORRECTIVE ACTIONS I

2. DOCKET NUMBER

3. LER NUMBER I

YEAR SEQUENTIAL REV I 2024 I NUMBER NO.

00387

- I I -0 002 Immediate corrective action included replacing all three linear reactors and the voltage regulator in the B DG. Additionally, the design of the DG excitation system linear reactors will be updated to enhance the temperature capabilities and to reduce heat generation. All linear reactors in the A, B, C, and D DGs will be replaced with the newly designed linear reactors.

COMPONENT FAILURE INFORMATION Component Identification - 0G501 B C-phase linear reactor Component Name - B DG C-phase linear reactor Manufacturer - Portee, Inc.

Part Number - CD-1296 EIIS Code - Since no code for linear reactors exists, a code of "Linear RX" was used.

PREVIOUS OCCURENCES None. Page 3

of 3