05000296/LER-2016-005

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LER-2016-005, Automatic Depressurization System Valve Inoperability Exceeded Technical Specification Limits
Browns Ferry Nuclear Plant, Unit 3
Event date: 04-18-2016
Report date: 06-17-2016
2962016005R00 - NRC Website
LER 16-005-00 for Browns Ferry, Unit 3, Regarding Automatic Depressurization System Valve Inoperability Exceeded Technical Specification Limits
ML16169A066
Person / Time
Site: Browns Ferry Tennessee Valley Authority icon.png
Issue date: 06/17/2016
From: Bono S M
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML16169A066 (9)


comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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.

I. Plant Operating Conditions Before the Event

At the time of discovery, Browns Ferry Nuclear Plant (BFN), Unit 3, was in Mode 1 at 100 percent power.

II. Description of Events

A. Event:

On April 18, 2016, during a scheduled surveillance of the Automatic Depressurization System (ADS) [SB] Logic System, the power to the Main Steam Line (MSL)[SB] B Relief Valve [RV] (3-PCV-001-0022) failed to transfer to the alternate feeder breaker when the normal feeder breaker was opened. Operations subsequently declared the ADS function of the MSL B Relief Valve inoperable, and entered Technical Specification (TS) Limiting Condition of Operability (LCO) 3.5.1 Condition E, a 14 day shutdown LCO action statement. Troubleshooting determined that the bus stab on the back of the alternate feeder Molded Case Circuit (MCC) breaker [BKR] (3-BKR-001-0022A) was not properly engaged with the bus. The stab was adjusted, the MCC breaker was returned to service, and MSL B Relief Valve was verified to have proper power. The valve's ADS function was declared operable on April 19, 2016 at 1400 Central Daylight Time (CDT).

Since the failure was discovered during surveillance testing, a past operability evaluation (POE) was performed to determine the duration of inoperability. The POE determined that the breaker was working properly until it was racked out for preventative maintenance (PM) performed during the previous Unit 3 refueling outage in March 2016. This failure went undetected due to improper post-maintenance testing (PMT). Prior to the outage, the associated breaker passed all required surveillances. The POE concluded that the time of inoperability began on March 26, 2016 at 2300 CDT and ended on April 19, 2016 at 1400 CDT.

Since the BFN, Unit 3 ADS system only requires four of the six ADS valves to remain operable, the ADS system was operable throughout this entire event. MSL B Relief Valve actuation through the ADS or Main Steam Relief Valve (MSRV) Automatic Actuation Logics were not affected under normal power. The valve's mechanical setpoints were unaffected by this event, and they remained capable of lifting.

B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event:

There were no structures, systems, or components (SSCs) whose inoperability contributed to this event.

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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.

3. LER NUMBER

2016 - 00

C. Dates and approximate times of occurrences:

Dates and Approximate Times Occurrence April 22, 2014 Last successful performance of 3-SR-3.3.5.1.6(ADS A), ADS Logic System Functional Test.

March 9, 2016 Performed breaker inspection and PM on 250V DC Motor- Operated Valve Board 3A.

March 26, 2016 at 2300 CDT Plant mode and conditions require ADS valve operability.

April 18, 2016 April 19, 2016 at 1400 CDT During scheduled performance of 3-SR-3.3.5.1.6(ADS A), Power to the MSL B Relief Valve failed to transfer to its alternate feeder breaker when the normal feeder breaker was opened. The ADS valve was declared inoperable.

Troubleshooting found a stab on the back of the MCC breaker which required adjustment. Following adjustment, the ADS valve was returned to operable status.

D. Manufacturer and model number (or other identification) of each component that failed during the event:

The inoperable valve was a Target Rock Corporation two-stage pressure control valve, model number 7567F. The breaker was manufactured by General Electric, model number THEF136015.

E. Other systems or secondary functions affected:

No other systems or secondary functions were affected by this event.

F. Method of discovery of each component or system failure or procedural error:

Valve inoperability was discovered during the scheduled performance of 3-SR-3.3.5.1.6(ADS A), ADS Logic System Functional Test, when Operations opened the normal feeder breaker for the MSL B Relief Valve on 250V DC Motor-Operated Valve Board 3A, and its associated green light did not remain illuminated. Breaker inoperability was discovered during troubleshooting for the inoperable ADS valve.

G. The failure mode, mechanism, and effect of each failed component, if known:

The MSL B Relief Valve failed when its associated MCC breaker, which supplied alternate power from 250V DC Motor-Operated Valve Board 3A, became unavailable. The MCC breaker failed because the bus stabs were not properly engaged with the bus.

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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.

3. LER NUMBER

2016 - 00

H. Operator actions:

There were no operator actions associated with this event.

I. Automatically and manually initiated safety system responses:

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

Ill. Cause of the event A. The cause of each component or system failure or personnel error, if known:

The direct cause for the ADS valve failure was the unavailability of its alternate power source due to a bus stab on the back of the MCC breaker not fully engaging with the bus. The most apparent cause for the valve failure was improper implementation of previous PMT.

B. The cause(s) and circumstances for each human performance related root cause:

Previous PMT was not properly performed, and failed to check if load operation was transferred upon breaker closure.

IV. Analysis of the event:

The Tennessee Valley Authority (TVA) is submitting this report in accordance with Title 10 of the Code of Federal Regulations 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's TS. It was determined that the ADS valve was inoperable from March 26, 2016 to April 19, 2016, due the failure of the valve's associated MCC breaker to supply alternate power because of a misaligned bus stab.

BFN, Unit 3, TS LCO 3.5.1 requires six Operable ADS Safety/Relief Valves (S/RVs) during Modes 1, and in Modes 2 and 3 when reactor steam dome pressure exceeds 150 psig. If one or more required ADS valve becomes inoperable, Required Action E.1 requires that valve to be restored to operability within 14 days. Based on this evaluation, BFN, Unit 3, operated with an inoperable ADS valve for longer than allowed by TS.

The condition was assumed to have begun on March 9, 2016, during routine breaker inspection, as part of refueling outage maintenance activities. ADS valve operability was not required until March 26, 2016, when BFN, Unit 3 entered Mode 2 and reactor dome pressure exceeded 150 psig.

Therefore, the ADS valve was considered inoperable from March 26, 2016 to April 19, 2016, when the breaker stabs were readjusted. The duration of ADS system inoperability was longer than allowed by plant TS LCO 3.5.1.

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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.

3. LER NUMBER

2016 - 00

V. Assessment of Safety Consequences

System availability was not impacted by this event. While one ADS valve was inoperable, the remaining five ADS valves remained operable for the duration of this event. Only four ADS valves are required to meet the ADS function in the Loss of Coolant Accident analysis described in the Final Safety Accident Report. This condition did not affect the Appendix R Function. However, due to the failure of its alternate power source, the MSL B Relief Valve was not able to perform its specified safety function under all conditions.

The MSL B Relief Valve remained operable for its required "Overpressure safety and relief protection for the nuclear system" function. The lack of reliable alternate power, from the MCC breaker had no effect on the mechanical setpoint of the valve, and it would have opened if reactor pressure exceeded 1145 psig.

The MSL B Relief Valve was inoperable for its "Automatic nuclear system depressurization" function.

All ADS valves are equipped with a logic circuit to automatically open the valve on concurrent high drywell pressure and low reactor water level or sustained reactor low water level when one of the Residual Heat Removal pumps is available in the Low Pressure Coolant Injection mode or two of the appropriate core spray pumps are available. Additionally, four of the six ADS valves have alternate power sources for manual actuation. The MSL B Relief Valve normal power source was aligned and operating as designed. However, during testing it was found that the alternate power source was not working as designed. The fuel vendor analyses of record assume between four and six available ADS valves, depending upon the applicable assumed single failure. A single failure of 250V DC Motor- Operated Valve Board 3A, which feeds the MSL B Relief Valve normal power supply would still have resulted in all six S/RVs remaining available for their ADS function.

The Main Steam system is required by TS 3.5.1 to have six operating S/RVs for ADS. TS Bases 3.5.1 states that each of the ADS S/RVs must be equipped with one air accumulator and associated inlet check valves, in order to provide pneumatic actuation. TS Bases 3.5.1 cites NEDC-32484P, "Browns Ferry Nuclear Plant Units 1, 2, and 3, SAFER/GESTR-LOCA Loss-of-Coolant Accident Analysis," which evaluated the effect of one ADS valve being out of service. Based on this analysis, five operable ADS valves will provide the required depressurization. However, the overall reliability of the ADS system is reduced, because any further valve failure could result in a reduction of depressurization capability. Therefore, operation is only allowed for a limited time. The 14 day completion time was based on a reliability study cited in "Recommended Interim Revisions to LCOs for ECCS Components" and has been found to be acceptable through operating experience.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:

The ADS system consists of six S/RVs as a defense-in-depth measure; only four of the six ADS valves are required for system operability. The five remaining S/RVs were unaffected by this event and were operable throughout the duration of this event. MSL B Relief Valve actuation comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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 through the ADS or MSRV Automatic Actuation Logics were not affected under normal power.

The valve's mechanical setpoints were unaffected by this event, and they remained capable of lifting.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shut down 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 shutdown.

C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:

While one ADS valve was inoperable for longer than permitted by TS, the ADS system itself remained operable throughout the duration of this event. Approximately one day elapsed between the time of discovery and returning the valve to service.

VI. Corrective Actions:

Corrective Actions are being managed by TVA's corrective action program under Condition Report (CR) 1161911.

A. Immediate Corrective Actions

A troubleshooting work order was initiated, which found a stab on the back of the MCC breaker cubicle that required adjustment. After adjusting the MCC bucket stab, the green light operated as designed.

B. Corrective Actions to Prevent Recurrence

Corrective actions to prevent recurrence include determining the scope of MCC breakers that feed loads that have a normal and alternate power source. Preventative Maintenance Change Requests were initiated for these MCC Breakers to revise their maintenance PMs to verify PMT includes power source isolation prior to closing the breaker under load. Additionally, a work order will be generated to replace the breaker bus stabs. Finally, a Performance Analysis and Training Needs Analysis will be prepared to develop targeted needs that will close the apparent knowledge gap associated with the implementation of breaker PMT.

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet 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.

3. LER NUMBER

2016 - 00

VII. Additional Information:

A. Previous Similar Events:

There were no previous similar events.

B. Additional Information:

There is no additional information.

C. Safety System Functional Failure Consideration:

In accordance with NUREG-1022, this event is not considered a safety system functional failure.

While one ADS valve was inoperable for longer than permitted by TS, the remaining five ADS valves remained operable throughout this event.

D. Scram with Complications Consideration:

This event did not result in a reactor scram.

VIII. COMMITMENTS

There are no new commitments.