05000259/LER-2015-002

LER-2015-002, High Pressure Coolant Injection System Inoperable Due to Slow Containment Isolation Valve Closing Time

Browns Ferry Nuclear Plant, Unit 1
Event date:	7-22-2015
Report date:	09-21-2015
2592015002R00 - NRC Website

I. Plant Operating Conditions Before the Event

At the time of discovery, Browns Ferry Nuclear Plant (BFN), Unit 1, was in Mode 1 at approximately 100 percent power. BFN, Units 2 and 3, were unaffected by this event.

II. Description of Event

A. Event:

On July 22, 2015, at 0205 Central Daylight Time (CDT), during the performance of the BFN, Unit 1, quarterly surveillance 1-SR-3.6.1.3.5(HPCI), High Pressure Coolant Injection System Motor Operated Valve Operability, the inboard steam isolation valve [ISV] 1-FCV-073-0002 closing times were in the high alert range. Due to the inability to analyze the stroke time deviation within the required Technical Specification (TS) limit of four hours, the valve was subsequently declared inoperable and TS Limiting Condition for Operation (LCO) 3.6.1.3, Primary Containment Isolation Valves (PCIVs), was entered on July 22, 2015, at 0445 CDT. TS LCO 3.6.1.3 Condition A required the HPCI [BJ] steam line to be closed and de-activated; the outboard steam isolation valve was closed and its breaker opened. As a result, BFN, Unit 1, HPCI was declared inoperable.

On July 22, 2015, at 1138 CDT, BFN made an event notification to the NRC in accordance with 10 CFR 50.72(b)(3)(v)(D) for an event or condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.

On July 25, 2015, at 0035 CDT, 1-FCV-073-0002 was declared operable, but degraded. On July 25, 2015, at 0119 CDT, BFN, Unit 1, HPCI was declared operable.

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) which were inoperable and contributed to this event.

C. Dates and approximate times of occurrences:

Dates & Approximate Times July 22, 2015 at 0015 CDT July 22, 2015 at 0425 CDT Occurrence 1-SR-3.6.1.3.5(HPCI) was performed. Closing stroke times were measured to be 13.08, 12.99, and 13.10 seconds, exceeding the stroke time code criteria of between 9.54 and 12.9 seconds.

Inservice Test (IST) Program Engineers evaluated stroke times as unacceptable. A complete evaluation could not be completed in the 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> LCO for TS 3.6.1.3.

July 22, 2015 July 22, 2015 July 25, 2015 at 0445 CDT at 1138 CDT at 0035 CDT BFN, Unit 1, HPCI declared inoperable after isolating the outboard steam isolation valve.

Event notification provided to the NRC in accordance with 10 CFR 50.72(b)(3)(v)(D).

HPCI steam line inboard isolation valve declared operable after Operations review of the Prompt Determination of Operability (PDO).

July 25, 2015 at 0119 CDT HPCI declared operable.

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

There were no failed components associated with this event; 1-FCV-073-0002 was declared inoperable for slow closure. This valve is an Anchor Darling double disc HPCI inboard steam line isolation valve, model number 900C WEOS.

E. Other systems or secondary functions affected:

No other systems were affected by this event.

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

The failure of 1-FCV-073-0002 to meet the test criteria was self-revealing during the performance of 1-SR-3.6.1.3.5 (HPCI).

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

There were no failed components associated with this event. HPCI Steam Line Inboard Isolation Valve 1-FCV-073-0002, was declared inoperable due to exceeding its closing time limits resulting in the inoperability of the BFN, Unit 1, HPCI system.

H. Operator actions:

BFN, Unit 1, Operations declared HPCI inoperable after the isolation of the outboard steam isolation valve. Operators later restored the flow path through the main steam isolation valve after the inboard isolation valve was determined to be operable but degraded.

I. Automatically and manually initiated safety system responses:

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

Ill. Cause of the Event / Problem Statement A. The cause of each component or system failure or personnel error, if known:

The apparent cause of this event was the closing stroke time of the HPCI inboard steam isolation valve increased due to the valve coasting further open after motor cutoff resulting in a greater total distance of travel during valve closure, which is likely due to decrease in stem factor in packing loads.

Engineering analysis has shown that when the valve strokes open, the limit switch turns the motor off when the valve is 85 percent open, but the stem continues to coast until it hits the backseat. This limit switch setting is insufficient to compensate for the improved stem factor due to the reduced packing load and premature wear.

Potentially contributing to the condition is that aluminum is denser than magnesium and consequently has more momentum than the lighter rotor would have had since both were operated at the same speed.

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

There were no human performance related root causes for this event.

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 (10 CFR) 50.73(a)(2)(v)(D), as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.

BFN, Unit 1, TS 3.6.1.3 requires each PCIV (excluding reactor building-to-suppression chamber vacuum breakers) to be Operable when in Modes 1, 2, or 3, or when associated instrumentation is required to be Operable per LCO 3.3.6.1, "Primary Containment Isolation Instrumentation." When a PCIV is inoperable in one or more penetration flow paths (except due to MSIV leakage not within limits), Required Action A.1 requires the isolation of the affected penetration flow path by use of at least one closed and de-activated automatic valve, closed manual valve, blind flange, or check valve with flow through the valve secured within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, and within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> for the main steam line. This condition was satisfied by closing the outboard steam isolation valve and opening its breaker.

The safety function of HPCI is to assure that the reactor is adequately cooled to limit fuel cladding temperature in the event of a small break in the nuclear system and loss of coolant which does not result in rapid depressurization of the reactor vessel. Due to the isolation of the HPCI steam line, the HPCI system was unable to perform its safety function.

The PDO evaluation determined that the valve and operator would have the capability to perform the safety function to close. The closure time is longer than acceptable per the current 1ST criteria but does not exceed the limiting value. All aspects associated with the valve and actuator operation have been evaluated and found to be functioning as designed with no degradation. Since the valve's ability to close is not hindered by the changes to stem factor or packing loads, it was concluded that the valve is operating acceptably and all conditions which could be identified have been analyzed and determined to have a negligible effect on the valve's ability to close under design basis conditions. Additionally, this valve is limit switch controlled in the closing direction (torque switch is not in the circuit); therefore, full actuator capability is available to close the valve. Based on the PDO evaluation, the outboard steam isolation valve was subsequently declared operable.

V. Assessment of Safety Consequences

This event resulted in inoperability and unavailability of the single train of the BFN, Unit 1, HPCI system resulting in the inability of the HPCI system to perform its safety function. In the event of an emergency, the Reactor Core Isolation Cooling (RCIC) system remained operable, and all other Emergency Core Cooling Systems (ECCS) and Automatic Depressurization Systems (ADS) were available during this event to facilitate core cooling.

Based on the discussion above, during the time period that the HPCI system was 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:

In the event of an emergency, the RCIC system remained operable, and all other ECCS and ADS systems were available during this event to facilitate core cooling.

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:

The reactor was not shutdown during this event.

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:

The BFN, Unit 1, HPCI system was declared inoperable on July 22, 2015, at 0445 CDT when the outboard steam isolation valve was isolated. HPCI operability was restored on July 25, 2015, at 0119 CDT.

VI. Corrective Actions

Corrective Actions are being managed by TVA's corrective action program under Condition Reports 1061051 and 1061074.

The actions to reduce the probability of similar events from occurring in the future have been summarized in the list below:

• Perform online Motor Control Center Motor Operated Valve (MOV) testing of FCV-073-0002 on Units 2 and 3 to determine limit switch settings for comparison to the previously attained online Unit 1 data.

• Verification of complete diagnostic testing of FCV-073-0002 in accordance with ECI-0-000-MOV009, Testing of Motor Operated Valves Using Viper 20, on Units 1, 2, and 3, to ensure the limit switch setting is adjusted to compensate for that valve's tendency to drift open to the backseat following motor shutoff.

• Verification that packing friction for 1-FCV-073-0002 did not change significantly since 2014. If the change was significant, provide improved guidance to maintenance for future valve packing activities.

• Evaluation of performance of FCV-073-0002 on Units 1, 2, and 3 following adjustment of limit switch settings and MOV diagnostic testing during the first quarterly test with the respective unit in Mode 1.

VII. Additional Information:

A. Previous similar events at the same plant:

A review of plant corrective action program reports and previous Licensee Event Reports did not identify any previous similar events.

B. Additional Information:

There is no additional information.

C. Safety System Functional Failure Consideration:

This event resulted in the inability of the BFN, Unit 1, HPCI system to perform its safety function for mitigation of the consequences of an accident. 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.