05000259/LER-2015-003
| Browns Ferry Nuclear Plant (Bfn), Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| Initial Reporting | |
| ENS 51231 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 2592015003R00 - NRC Website | |
I. Plant Operating Conditions Before the Event
At the time of discovery, Browns Ferry Nuclear Plant (BFN) Units 1, 2, and 3 were operating in Mode 1 at approximately 100 percent rated thermal power.
II. Description of Events
A. Event:
On July 14, 2015, at 1715 Central Daylight Time (CDT), Operations personnel observed Control Bay (CB) Chiller (CHLR)[KM][CHU] A outlet water temperature to be higher than specified limits. CB CHLR A was declared inoperable.
CB CHLR B had been declared inoperable for planned maintenance on July 13, 2015.
The concurrent inoperability of A and B CB CHLR required BFN, Unit 1 and 2 to enter Technical Requirements Manual (TRM) LCO 3.7.6, Electric Board Room Air and 2 electric board room A/C subsystems inoperable, to declare the electrical equipment in the electric board rooms inoperable immediately. For BFN, Unit 1 and 2, all four 4160 V (4kV) Shutdown Boards (SDBs)[ECBD] were declared inoperable.
With 4kV SBDs A, B, C, and D inoperable, BFN, Unit 1 and 2, entered TS LCO 3.8.7.A, which requires that, with one Unit 1 and 2 4kV SBD inoperable, that the Unit 1 and 2 4kV SBD be restored to operable status within five days, and that the associated Emergency Diesel Generator (DG)[DG] be declared inoperable immediately. BFN, Unit 1 and 2, declared DG A, DG B, DG C, and DG D inoperable.
With 4kV SBDs A, B, C, and D inoperable, BFN, Unit 1, 2, and 3 entered TS LCO 3.8.7, Distribution Systems — Operating, Conditions F (Unit 1) and G (Units 2 and 3).
These LCOs require, with one or more required Unit 2 AC boards inoperable, that the affected Standby Gas Treatment (SGT)[BH] or Control Room Emergency Ventilation (CREV) subsystem be declared inoperable. BFN, Units 1 and 2, entered TS LCO 3.8.7, Conditions H (Unit 1) and I (Unit 2). These LCOs require, with two or more electrical power distribution subsystems inoperable that result in a loss of function, that the unit enter TS LCO 3.0.3 immediately. BFN, Unit 1 and 2, declared SGT A, SGT B, and CREV A inoperable.
With SGT A and B inoperable, BFN, Unit 1, 2, and 3, entered TS LCO 3.6.4.3, SGT System, Condition D. This LCO requires, with two or three SGT subsystems inoperable in MODE 1, 2, or 3, that the unit enter TS LCO 3.0.3 immediately.
BFN, Units 1, 2, and 3, entered TS LCO 3.0.3 with required action within one hour to initiate action to place Unit 1, 2, and 3 in MODE 2 within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />, in MODE 3 in 13 Hours, and in MODE 4 in 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.
On July 14, 2015, at 1810 CDT, BFN, Units 1, 2, and 3, initiated actions to commence shutdown as required by TS LCO 3.0.3. A 4-hour notification was submitted to the NRC as required by Title 10 of the Code of Federal Regulations (10 CFR) 50.72(b)(2)(i), the initiation of any nuclear plant shutdown required by the plant's TS. An inspection of the A CB CHLR was performed, and it was identified that the condenser [CDU] coils had a heavy coating of dirt and insects. No other issues were noted. The condenser coils were cleaned, and the outlet water temperature stabilized within operable limits.
On July 14, 2015, at 2352 CDT, CB CHLR A was declared operable, and BFN, Units 1, 2, and 3 exited all applicable TS LCOs.
B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event:
At the start of this event, CB CHLR B was inoperable for scheduled maintenance. This resulted in the concurrent inoperability of CB CHLR A and B.
C. Dates and approximate times of occurrences:
July 13, 2015, at 0315 CDT CB CHLR B tagged out for scheduled maintenance.
July 14, 2015, at 1715 CDT High chilled water temperature discovered on CB CHLR A.
CB CHLR A, CREV A, SGT A and B, and DG A, B, C, and D declared inoperable in accordance with BFN TSs.
Entered TS LCO 3.0.3.
July 14, 2015, at 1810 CDT Initiated actions to commence required shutdown of BFN, Unit 1, 2, and 3.
July 14, 2015, at 2159 CDT Provided Four-hour Event Notification 51231 to the NRC.
July 14, 2015, at 2352 CDT CB CHLR A declared operable following cleaning of condenser coil.
CREV A, SGT A and B, and DG A, B, C, and D declared operable.
Exited TS LCO 3.0.3.
D. Manufacturer and model number (or other identification) of each component that failed during the event:
No component failures were identified that occurred during the event.
E. Other systems or secondary functions affected:
Loss of the CB CHLRs resulted in a loss of the Unit 1 and 2 Control Room A/C System.
The following systems were rendered inoperable by the inoperability of all BFN, Unit 1 and 2, 4kV SDBs:
4kV SDB A 4kV SDB B 4kV SDB C 4kV SDB D
DGA
Residual Heat Removal (RHR)[BO] Pump [P] lA RHR Pump 2A Core Spray (CS)[BM] Pump lA CS Pump 2A RHR Service Water (RHRSW)[CC] Pump Al RHRSW Pump A2 Standby Liquid Control (SLC)[BR] Pump lA
SGT A
CREV A
DG B
RHR Pump 1C RHR Pump 2C CS Pump 1C CS Pump 2C RHRSW Pump Cl RHRSW Pump C2 SLC Pump 2A
CB CHLRA
CB CHLR 3A
DG C
RHR Pump 1B RHR Pump 2B CS Pump 1B CS Pump 2B RHRSW Pump B2 SLC Pump 1B RHRSW Pump B3
CB CHLR B
DG D
RHR Pump 1D RHR Pump 2D CS Pump 1D CS Pump 2D RHRSW Pump D2 SLC Pump 2B
SBGT B
RHRSW Pump D3 F. Method of discovery of each component or system failure or procedural error:
The failure of CB CHLR A was discovered when the outlet water temperature was observed by Operations personnel to be 47 degrees Fahrenheit (F), which is greater than the maximum allowable chilled water outlet temperature of 44 degrees F.
G. The failure mode, mechanism, and effect of each failed component, if known:
There were no failed components related to this event.
H. Operator actions:
- Identified chilled water outlet temperature was out of band, declared CB CHLR A, CREVA, SGT A and B, and DGA, B, C, and D inoperable.
- Entered TS LCO 3.0.3, initiated plant shutdown.
- Verified outlet water temperature returned to operable band following inspection and cleaning, declared CB CHLR operable, exited TS LCO 3.0.3.
I. Automatically and manually initiated safety system responses:
There were no automatic or manual safety responses associated with this event.
III. Cause of the event
A. The cause of each component or system failure or personnel error, if known:
The apparent cause of this event was the failure of plant processes to address increased monitoring following debris/dust-producing events, which allowed fouling of the CB CHLR A condenser coils with dirt (from onsite construction and excavation activities), and insects (from a large willow fly hatch).
The direct cause of this event was a lack of existing actions to address natural phenomena affecting plant equipment. There are no routine actions that System Engineers, Operators, or Maintenance personnel take to respond to a natural occurrence, such as a willow fly hatch, which can negatively impact operation of critical plant equipment.
B. The cause(s) and circumstances for each human performance related root cause:
There were no human performance root causes associated with this event.
IV. Analysis of the event:
The Tennessee Valley Authority (WA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(v)(A), (B), (C), and (D), as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor and maintain it in a safe shutdown condition, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident.
This event resulted from fouling of the CB CHLR A condenser coils by dirt from onsite construction and excavation activities, and by insects from a seasonal willow fly hatch.
This fouling caused the inoperability of the chiller, concurrent with a second Unit 1 and 2 chiller which was already inoperable for maintenance. Equipment in electric board rooms rely on chilled water from this system for proper functioning in all conditions, including accident conditions. Therefore, all Unit 1 and 2 4kV SDBs and associated diesels were declared inoperable along with the chiller.
The safety function of the 4kV SDBs, as part of the Standby AC Power System, is to provide a self-contained, highly reliable source of power, as required for the Engineered Safeguards System, so that no single credible event can disable the core standby cooling functions or their supporting auxiliaries. Inoperability of the Unit 1 and 2 4kV SDBs and diesels caused the loss of several safety systems and the required initiation of plant shutdown for BFN Units 1 and 2. The loss of two trains of SGT also caused the required initiation of a Unit 3 shutdown.
V. Assessment of Safety Consequences
This event resulted in inoperability of all four Unit 1 and 2 4kV SDBs and their associated DGs; however the equipment remained available during the event. This equipment was declared inoperable because it was unable to perform its safety function under all design basis conditions for supplying reliable power to safety-related equipment in order to shut down the reactor and maintain it in a safe shutdown condition, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident.
The 4kV SDBs are located in the Unit 1 and 2 Electric Board Rooms (EBR). Total system failure could result if the equipment operating temperature in the EBRs exceeded the 104 degree F operability limit. During this event non-qualified temporary cooling was available to ensure the temperature in the EBR would not exceed the 104 degree F operability limit of the associated safety-related components. Therefore, during the time period that the 4kV SDBs were inoperable, sufficient systems were available to provide the required safety functions to protect the health and safety of the public.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
During this event non-qualified temporary cooling was available to ensure the temperature in the EBR would not exceed the 104 degree F operability limit of the associated safety-related components.
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, an estimate of the elapsed time from discovery of the failure until the train was returned to service:
This event resulted in inoperability of multiple plant safety systems for approximately seven hours, from 1715 CDT when the CB CHLR A chilled water high temperature was discovered until 2352 CDT when the CB CHLR A condenser coils were cleaned and the unit was returned to normal operation.
VI. Corrective Actions:
Corrective Actions are being managed by TVA's Corrective Action Program under Condition Report (CR) 1056829. The following corrective actions are in progress:
1. Add guidance for maintenance walkdown crews to inspect and evaluate the condition of redundant equipment when walking down equipment to be worked.
2. Develop a list of plant equipment susceptible to natural phenomena and revise procedures to enhance equipment inspection during operator rounds.
3. Create conditional walkdown requirements on walkdown sheets to check for insect infestation.
4. Re-evaluate the most recent Probabilistic Risk Assessment calculation revision involving CB CHLRs in order to determine if the current CB CHLR Equipment Reliability Classification is appropriate.
VII. Additional Information:
A. Previous Similar Events at the same plant:
A review of the BFN CAP and Licensee Event Reports (LERs) for Units 1, 2, and 3 revealed no failures BFN CB CHLR systems for the last five years which were attributable to condenser fouling.
B. Additional Information:
There is no additional information.
C. Safety System Functional Failure Consideration:
This event resulted in the concurrent inoperability of all four BFN, Unit 1 and 2, 4kV SBDs. These electrical boards supply power to multiple safety-related systems including, but not limited to:
a. SLC b. RHR c. DGs d. CS These and other systems powered by BFN, Unit 1 and 2, 4kV SDBs are required in order to shut down the reactor and maintain it in a safe shutdown condition, remove residual heat, or mitigate the consequences of an accident.
This event also resulted in inoperability of two SGT trains. The SGT provides a means for minimizing the release of radioactive material from the containment to the environs by filtering and exhausting the air from any or all zones of the Reactor Building and maintaining the building at a negative pressure during containment isolation conditions. Two operable trains of SGT are required for minimum acceptable flow, under these conditions, in order to control the release of radioactive material. Therefore, in accordance with NUREG-1022, this event is considered a safety system functional failure.
D. Scram with Complications Consideration:
This event did not result in a reactor scram.
VIII. COMMITMENTS
There are no new commitments.