05000296/LER-2017-002

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LER-2017-002, 4kV Shutdown Board Potential Transformer Primary Fuses Do Not Coordinate with Secondary Fuses
Browns Ferry Nuclear Plant, Unit 3
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
ENS 53049 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition
2962017002R00 - NRC Website
LER 17-002-00 for Browns Ferry Nuclear Plant, Unit 3 Regarding 4kV Shutdown Board Potential Transformer Primary Fuses Do Not Coordinate with Secondary Fuses
ML17363A295
Person / Time
Site: Browns Ferry Tennessee Valley Authority icon.png
Issue date: 12/29/2017
From: Bono S M
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML17363A295 (7)


comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor. and a person is not required to respond to, the information collection.

05000-296

3. LER NUMBER

NO

- 00 Browns I. Plant At A.

C.

D.

Plant (BFN), Unit 3, was in Central Daylight Time was discovered. Four Shutdown Boards (SD primary fuses for fault currents could cause an affected a spurious, maintained motor load operation on the [DG] power [EK]. If a fire be affected. In fire area CDT, Event Notification that were inoperable inoperable at the 2017 - 002 100 II. Description B.

Operating Conditions Before the Event the time of discovery, Browns Ferry Nuclear percent rated thermal power.

of Event Event Summary On November 1, 2017, at approximately 1425 condition review, a fuse [FU] coordination issue [XPT] 10 amp secondary fuses on Unit 3 4kV selectively coordinate with upstream 0.5 amp on the 120 V secondary. Cable [CBL] fire damage disconnect from off-site power, and could cause The under-voltage trip signal would prevent power or Emergency Diesel Generator (EDG) 03-03 or 16, all four Unit 3 4kV SD BDs could 3EB could be affected.

On November 1, 2017, at approximately 2126 the NRC.

Status of structures, components, or systems and that contributed to the event.

There were no systems, structures, or components contributed to this condition.

Dates and approximate times of occurrences

Mode 1 at approximately (CDT), during an extent of Potential Transformer (PT) BD) [ECBD] do not greater than 30 amps SD BD to spuriously under-voltage trip signal.

board whether on off-site were to occur in fire areas 21, 4kV SD BDs 3EA and (EN) 53049 was made to at the start of the event time of discovery which November 1, 2017, 1425 CDT Unanalyzed condition discovered during an extent of condition review.

November 1, 2017, 2126 CDT Event Notification 53049 made to the NRC.

Manufacturer and model number of each component that failed during the event There were no failed components associated with this condition.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov. and to the Desk Officer. Office of Information and Regulatory Affairs, NEOB-10202. (3150-0104). Office of Management and Budget. Washington. DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER

- 002 - 00 Browns Ferry Nuclear Plant, Unit 3 05000-296 2017

E. Other systems or secondary functions affected

There were no other systems or secondary functions affected.

F. Method of discovery of each component or system failure or procedural error There were no component failures, system failures, or procedural errors associated with this condition.

G. The failure mode, mechanism, and effect of each failed component There were no failed components associated with this condition.

H. Operator actions

There were no operator actions associated with this condition.

I. Automatically and manually initiated safety system responses

There were no automatic or manual safety system responses associated with this condition.

III. Cause of the event

The cause of this event was the failure to perform a PT fuse coordination study to confirm that the primary and secondary fuses could coordinate. This condition was determined to be a legacy issue dating to the original design of the plant. The most likely cause is lack of rigorous oversight of the vendor during the preparation and subsequent issuance of the fuse evaluation for 4kV SD BDs A, B, C, D, 3EA, 3EB, 3EC, and 3ED, which was first issued by EBASCO in 1988. This may have resulted in failure of the vendor to perform a coordination study for the primary and secondary PT fuses.

A. Cause of each component or system failure or personnel error There were no component failures, system failures, or known personnel errors associated with this condition as it was determined to be a legacy issue.

B. Cause(s) and circumstances for each human performance related root cause The most likely cause is lack of rigorous oversight of the vendor during the preparation and subsequent issuance of the fuse evaluation for 4kV SD BDs A, B, C, D, 3EA, 3EB, 3EC, and 3ED, first issued by the vendor in 1988. This may have resulted in failure of the vendor to perform a coordination study for the primary and secondary PT fuses.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington. DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov. and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202. (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor. and a person is not required to respond to. the information collection.

IV. Analysis of the event

The Tennessee Valley Authority (TVA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(ii)(B), as any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.

The basic function of the normal auxiliary electrical power system is to provide power for plant auxiliaries during startup, operation, and shutdown, and to provide highly reliable power sources for plant loads which are important to its safety.

For 4kV SD BD 3EA, PT secondary fuses are 10 amp fuses and do not selectively coordinate with upstream primary fuses, which are 0.5 amp fuses, for fault currents greater than approximately 30 amps on the 120V secondary. Cables tapping off from the PT secondary fuse route external to the board to the Main Control Room. A fire induced fault on an affected cable could clear two of the PT primary fuses, resulting in de-energizing of the A-phase secondary leg. Clearing of the two PT primary fuses would result in de-energization of undervoltage and degraded voltage relays [RLY].

The combination of de-energized undervoltage and degraded voltage relays would result in spurious energization of degraded voltage auxiliary relays. This would result in 4kV motor load sheds, tripping of normal and off-site power breakers [BKR], starting of the DG, and closing of the DG breaker (provided these functions did not have separate unrelated fire damage). If the DG successfully connected to the 4kV SD BD, due to the cleared PT fuse condition, the degraded voltage signal would not reset. This condition could result in a loss of power to plant loads which are important to plant safety. Similar conditions exist on 4kV SD BDs 3EB, 3EC, and 3ED.

Assessment of Safety Consequences

This condition, if left uncorrected, could result in a loss of power to plant loads which are important to plant safety during postulated fire damage. However, a probabilistic risk analysis performed by WA concluded that the risk significance of this condition is very low, with a maximum change in Core Damage Frequency and Large Early Release Frequency of less than 1E-06 for the affected unit.

Based on this analysis, there was no significant increase in risk to the health and safety of the public or to plant personnel resulting from this condition.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event No components or systems failed due to this event.

2017 comments regarding burden estimate to the Information Services Branch (T-2 F43). U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001. or by e-mail to used to impose an information collection does not display a currently valid OMB control number: the NRC may not conduct or sponsor. and a person is not required to respond to: the information collection.

05000-296 2017

3. LER NUMBER

- 002

NO

- 00 B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident This event did not occur when the reactor was shut down.

C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from discovery of the failure until the train was returned to service This condition did not result in the inoperability of any safety systems.

VI. Corrective Actions

Corrective Actions are being managed by TVA's Corrective Action Program under Condition Report 1354129. The required fuse coordination studies have since been performed, and a vendor oversight process has been added to TVA procedures since this legacy event.

A. Immediate Corrective Actions

  • Hourly fire watches were put in place for the affected fire areas (FPIP# 17-332).

B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future

  • Issue an Engineering Change Package to replace Unit 3 4KV SD BD primary 0.5 amp PT fuses with 1 amp fuses of the same type.

VII. Previous Similar Events at the Same Site A review of the BFN CAP and Licensee Event Reports (LERs) for Units 1, 2, and 3 found no instances within the past five years of degraded or unanalyzed conditions related to coordination of PT fuses.

VIII. Additional Information

There is no additional information.

IX. Commitments There are no new commitments.