05000346/LER-2021-001-02, Emergency Diesel Generator Speed Switch Failure Due to Direct Current System Ground

Emergency Diesel Generator Speed Switch Failure Due to Direct Current System Ground

ML24344A257
Person / Time
Site:	Davis Besse
Issue date:	12/09/2024
From:	Tony Brown Vistra Operations Company
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
L-24-244 LER 2021-001-02
Download: ML24344A257 (1)

LER-2021-001, Emergency Diesel Generator Speed Switch Failure Due to Direct Current System Ground

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3462021001R02 - NRC Website

text

L-24-244 December 9, 2024 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject:

Davis-Besse Nuclear Power Station, Unit 1 Docket Number 50-346, License Number NPF-3 Licensee Event Report 2021-001-02 Davis-Besse Nuclear Power Station Terry J. Brown Site Vice President 5501 N. State Route 2 Oak Harbor, OH 43449 Office: 419.321. 7676 10CFR50.73 Enclosed is Revision 02 to Licensee Event Report (LER) 2021-001, Emergency Diesel Generator Speed Switch Failure due to Direct Current System Ground. This event was initially reported pursuant to 10 CFR 50.73(a)(2)(i)(B). The LER revision is being submitted to designate this event is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B).

There are no regulatory commitments contained in this letter or its enclosure. The actions described represent intended or planned actions and are described for information only. If there are any questions or if additional information is required, please contact Mr. Robert W. Oesterle, Manager, Site Regulatory Compliance, at (419) 321-7462.

Sincerely, GMW Enclosure: LER 2021-001-02 cc: NRC Region III Administrator NRC Resident Inspector NRR Project Manager Utility Radiological Safety Board

Abstract

On September 4, 2020, with the Davis-Besse Nuclear Power Station operating at approximately 100 percent power, Emergency Diesel Generator (EDG) 2 failed to start during scheduled monthly surveillance testing. A failed speed switch was replaced, restoring the EDG to Operable status on September 7, 2020. Based on vendor analysis and review of applicable plant data, it was determined on February 12, 2021 that the speed switch was failed on September 1, 2020, due to a shorted component within the switch in conjunction with a ground on the station DC system.

Since the EDG was determined to be failed prior to discovery during routine monthly testing and the necessary Technical Specification actions for an inoperable EDG were not performed at the time of failure, this event is being reported per 10 CFR 50.73(a)(2)(i)(B) as operation of the plant in a condition prohibited by the Technical Specifications. On October 10, 2024, it was determined this issue is also reportable per 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition because postulated events such as a fire or high energy line break may cause a DC system ground that could have adversely affected both EDG speed switches. The cause of this event was less than adequate use of operating experience for the station DC system, resulting in installation of EDG speed switches with components not rated for the maximum possible voltage. A new design of switch capable of withstanding the full voltage of the DC system was installed in both EDGs in January 2022.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

System Description

The Davis-Besse Nuclear Power Station (DBNPS) Class 1E Alternating Current (AC) Electrical Power Distribution System sources consist of the offsite power sources (preferred power sources, normal and alternate) and the two onsite standby Emergency Diesel Generators (EDGs). An EDG [EK-DG] starts automatically on a Safety Features Actuation System (SFAS) [JE] actuation or on an essential bus degraded voltage or loss of voltage signal. After an EDG has started, it will automatically tie to its respective bus after offsite power is tripped. A separate non-class 1E Station Blackout Diesel Generator (SBODG) [EK-DG] of the same capacity as each EDG can be manually started from the control room and aligned to either essential bus to cope with a Station Blackout as defined in 10 CFR 50.2.

The DBNPS Direct Current (DC) system [EJ] consists of two 250/125 Volt DC (VDC) motor control centers, four batteries, six battery chargers, four essential distribution panels, four 480V AC/125V DC rectifiers, and four nonessential distribution panels.The system is arranged to form two completely independent load groups. The DC system is designed as an ungrounded system, so a single ground will not have an adverse impact on the DC system or individual components receiving power from the DC System. Each DC motor control center has a positive, a negative, and a neutral bus arranged to form a 250/125 VDC system; each bus is equipped with ground detection instrumentation that allows for identification and removal of a ground to prevent more significant issues.

Technical Specifications:

Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1.b requires two EDGs be Operable in Modes 1 through 4. With one EDG inoperable LCO 3.8.1.b requires the following:

1. Surveillance Requirement 3.8.1.1 must be performed within one hour and at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter to verify correct breaker alignment and indicated power availability for each offsite circuit,

2. Any required feature(s) supported by the inoperable EDG must be declared inoperable when its redundant required feature(s) is inoperable within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> from discovery,

3. The Operable EDG must be determined to be not inoperable due to common mode cause failure or perform Surveillance Requirement 3.8.1.2 to verify the Operable EDG starts from standby conditions and achieves steady state voltage and frequency within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, and

4. Two EDGs must be restored to Operable status within 7 days or the plant must be in Hot Standby (Mode 3) within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Cold Shutdown (Mode 5) within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.

DESCRIPTION OF EVENT

On September 4, 2020, EDG 2 was declared inoperable and unavailable at 1112 hours0.0129 days <br />0.309 hours <br />0.00184 weeks <br />4.23116e-4 months <br /> for routine monthly testing to satisfy TS Surveillance Requirement 3.8.1.2. During the first idle start at 1134 hours0.0131 days <br />0.315 hours <br />0.00187 weeks <br />4.31487e-4 months <br />, EDG 2 locked out on a failure to start. Due to work having been performed on the air start side that was being used to start the EDG, a second start attempt was performed on the opposite air start side, which also resulted in an EDG lock out on failure to start. Based on the observed indications from the second start attempt, the likely cause of the failure to start was suspected to be either the EDG speed switch [EK-ST] or magnetic pickup. Subsequent inspection of the EDG 2 speed switch identified internal overheating damage. The failed speed switch was replaced with the same model switch and the EDG tested satisfactorily, allowing the EDG to be declared Operable on September 7, 2020, at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />.

CAUSE OF EVENT

The failed speed switch was installed in October 2019 as a ten-year preventive maintenance activity. The switch installed was a different part from a different manufacturer as the previous switch was obsolete and no longer available. Due to the short period of time between installation and failure, the failed speed switch was shipped to an external vendor, whose inspection revealed excessive damage at the input power section of the speed switch.

Based on the vendors conclusion that the failure may have been related to the power supplied to the speed switch, a review of the DBNPS 125 VDC system history was performed. This review, completed on February 12, 2021, identified a hard ground had been received on Direct Current Motor Control Center (DCMCC) 2 [EJ-MCC] on August 25, 2020. Troubleshooting of this ground commenced later that day. On September 1, 2020, the breaker [BQ-BKR] for the High Pressure Injection Pump 2 DC oil pump motor was opened at approximately 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> as part of these troubleshooting efforts, resulting in the ground switching from a hard ground to an oscillating ground.

This oscillating ground condition cleared when the EDG 2 speed switch was removed on September 5, 2020, following the failure during testing.

The cause of the failed EDG 2 speed switch was initially determined to be a shorted component within the speed switch concurrent with a ground on the 125 VDC power supply to the speed switch, resulting in failure of the switch.

With two competing grounds it was postulated there was sufficient direct current flow to cause the damage identified in the speed switch. Neither of the grounds on the 125 VDC system led plant personnel to conclude that any plant equipment had failed as a result of the grounds, and the speed switch was not known to be failed until regularly scheduled testing was performed.

In July 2021 it was identified that the EDG speed switch design installed in October 2019 utilized an auxiliary circuit board that contains electromagnetic and radio frequency interference (EMI/RFI) filtering and surge suppression devices at the DC input. This circuit contained a metal oxide varistor (MOV) that connected to the 125 VDC positive inlet terminal and station ground. This MOV had a maximum continuous rating of 170 VDC. Since the DC System includes a continuous ground monitor that connects the neutral and negative rails to station ground through resistors and an ammeter, the arrangement of the ground detector resulted in the positive 125 VDC terminal of the speed switch to see +200 VDC to ground. This overvoltage condition likely resulted in a decreased life of the MOV.

The speed switches for both EDGs were replaced in January 2022 with a model using an external power supply and capable of withstanding full voltage within the DBNPS DC Ground Detection System to correct this deficiency.

The Root Cause of this event was less than adequate incorporation, internalization, and anchoring of operating and in-house experiences as it pertains to the unique design vulnerabilities in the DBNPS DC distribution system. A similar issue occurred in 2010 when a component was installed onto the DC system but was not rated for the conditions present within the system. Because learnings from the previous incident were not anchored into any design documents or specifications, when replacing the EDG speed switches the station failed to select equipment that was suitable to function in the DC system.

ANALYSIS OF EVENT

A qualitative risk assessment was performed for the approximately six days EDG 2 was unavailable, starting with the initiation of the oscillating ground on September 1, 2020, at approximately 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br />, and ending with EDG 2 becoming available following completion of testing on September 7 at approximately 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br />. During this time period, all redundant train 1 equipment remained Operable, and no risk-significant equipment in train 2 was

unavailable or inoperable. The plant risk associated with the extended unavailability of EDG 2 for less than the 7-day Completion Time of TS LCO 3.8.1 Required Action B.4 is considered to be of very low safety significance.

Reportability Discussion:

When the EDG 2 speed switch was initially discovered failed following the unsuccessful test start on September 4, 2020, there was no firm evidence at that time that the switch had failed prior to the test, so the failure was assumed to be at the time of discovery in accordance with NRC reporting guidance contained in NUREG-1022, Event Reporting Guidelines for 10 CFR 50.72 and 50.73, Revision 3. Following receipt of the vendors evaluation of the failed switch and further review of the DBNPS 125 VDC System history, it was determined EDG 2 was inoperable prior to the start of the September 4, 2020, monthly test. EDG 2 was determined to be inoperable for approximately three days with the plant operating in Mode 1 prior to it being declared inoperable and unavailable on September 4, 2020, at 1112 hours0.0129 days <br />0.309 hours <br />0.00184 weeks <br />4.23116e-4 months <br /> for monthly testing. Because Technical Specification 3.8.1 requires actions to be taken within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> for an inoperable EDG, this issue represents operation of the plant in a condition prohibited by the plants Technical Specifications, which is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B). Revision 00 of this Licensee Event Report was submitted 60 days from the discovery of the reportable issue on February 12, 2021, in accordance with 10 CFR 50.73(a)(1). Because all redundant train 1 equipment remained Operable during the time EDG 2 was inoperable, no loss of Safety Function per 10 CFR 50.73(a)(2)(v) occurred.

When Surveillance Requirement 3.8.1.1 was performed within one hour of declaring EDG 2 inoperable on September 4, no issues were identified with the offsite circuit breaker alignments and power availability. EDG 1 was successfully idle started on September 5 at 0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br /> in accordance with Surveillance Requirement 3.8.1.2 to satisfy LCO 3.8.1 Action B.3.1 to verify it was not inoperable due to a potential common cause failure. EDG 2 was subsequently restored to Operable status within the seven days allowed by LCO 3.8.1 Action B.4.

On October 10, 2024, an additional failure mechanism was identified for the speed switch design installed in EDG 1 in October 2019 and installed in EDG 2 in November 2020. For some postulated initiating events such as a fire or high energy line break that could potentially cause a ground on the negative bus on one or both trains of the DC system, the EDG speed switches could have been adversely affected regardless of the existence of other DC system grounds, resulting in a failure of one or both EDGs to function. The postulated event resulting in the potential failure of both EDGs was reported to the NRC Operations Center at 1045 hours0.0121 days <br />0.29 hours <br />0.00173 weeks <br />3.976225e-4 months <br /> per 10 CFR 72(b)(3)(ii)(B) as an unanalyzed condition that significantly degrades plant safety (Event Number 57373). This LER revision is being submitted in accordance with 10 CFR 50.73(a)(2)(ii)(B). The SBODG utilizes a 125 VDC System separate from the Station DC system, and therefore was unaffected by this issue. Therefore, this postulated ground on the DC system due to a fire or high energy line break remained within the stations licensing basis as described in Section 15.2.9 of the Updated Final Safety Analysis Report.

CORRECTIVE ACTIONS

Completed Actions:

The ground on the High Pressure Injection Pump 2 DC oil pump motor was repaired on September 3, 2020, by repairing the damaged wire to the motor.

A replacement speed switch of the same model was installed and post maintenance testing was completed satisfactorily at 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on September 7, 2020, allowing EDG 2 to be declared Operable at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />.

Upon identification of the vulnerability of the installed speed switches to a higher than design voltage, a Standing Order was issued to the operators with guidance to more readily identify grounds that could affect the EDG speed switches.

The EDG 1 and 2 speed switches were replaced in January 2022 with a model using an external power supply and capable of withstanding full voltage within the DBNPS DC Ground Detection System.

The DC System voltage to ground configuration was added to the DC electrical distribution system of the station Design Criteria Manual as well as appropriate DC system specifications and DC system descriptions.

Scheduled Actions:

No further actions are planned.

PREVIOUS SIMILAR EVENTS

LER 2010-003 dated October 15, 2010, documents an event where a train of Auxiliary Feedwater was inoperable for longer than allowed by the TS LCO due to a ground in the DC system that was not adequately prioritized for identification and repair. The Auxiliary Feedwater Pump discharge control solenoid valve position controller board was designed for up to 140 VDC.

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