05000327/LER-2015-004

LER-2015-004, Manual Reactor Trip due to Main Steam Isolation Valve Drifting in the Closed Direction

Sequoyah Nuclear Plant Unit 1
Event date:	11-23-2015
Report date:	03-09-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3272015004R01 - NRC Website

LER 15-004-00 for Sequoyah, Unit 1, Regarding Manual Reactor Trip Due to Main Steam Isolation Valve Drifting in the Closed Direction

ML16028A100
Person / Time
Site:	Sequoyah
Issue date:	01/22/2016
From:	Schwarz C J Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 15-004-00
Download: ML16028A100 (8)

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 intemet e-mail to Infocollects.Resource©nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, 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 the event, Sequoyah Nuclear Plant (SQN) Unit 1 reactor was operating at 100 percent rated thermal power (RTP). The condition described in this LER did not impact SQN Unit 2.

II. Description of Events

A. Event:

On November 23, 2015, at 0844 Eastern Standard Time (EST), SQN Unit 1 reactor was manually tripped due to plant parameters indicating that Loop 3 Main Steam Isolation Valve (MSIV) [EIIS Code SB] [EIIS Code ISV] had started drifting in the closed direction. Prior to the reactor trip, the open light indication [EIIS Code IL], on the main control room (MCR) panel for the MSIV was noted to be extinguished. The light bulb was replaced with no change in indication. At the same time, the Post Accident Monitoring (PAM) indicator for the MSIV displayed full open; however, within two to three minutes dual indication (mid-position) was provided. Subsequently, operators noted that the reactor coolant system (RCS) [EIIS Code AB] temperature and Loop 3 Steam Generator (SG) [EIIS Code SG] pressure were both slowly rising, and the Loop 3 SG flow was slowly lowering. These indications confirmed the Loop 3 MSIV was slowly drifting closed. Operators placed the handswitch [EIIS Code HS] for the MSIV in the open position for approximately 5 seconds. This resulted in no apparent affect. Operators manually tripped the reactor per procedure.

After the reactor trip, it was noted that all three lights on the MCR panel for the MSIV (closed, 10 percent closed, and open) illuminated followed by an immediate return to full open indication.

Additionally, PAM indication confirmed the MSIV was full open.

Troubleshooting identified a loose nut on a termination for the handswitch associated with the Loop 3 MSIV. The loose nut on the terminal could cause intermittent power through the circuit, which could cause flickering indicator lights and intermittent power to the solenoid. The loss of a single source of power to the solenoid could cut off the air supply to the MSIV, but not completely open the vent. This could result in a slow loss of air pressure and cause the Loop 3 MSIV to slowly drift in the closed position.

All plant safety related equipment operated as designed, all control rods fully inserted, and auxiliary feedwater (AFVV) [EIIS Code BA] automatically initiated from the feedwater isolation signal, as expected. No complications were experienced during the reactor trip.

This event is' reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in a manual or automatic actuation of the Reactor Protection System and the Auxiliary Feedwater System.

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

There were no inoperable structures, components, or systems that contributed to this event.

C. Dates and approximate times of occurrences:

On November 23, 2015, at 0815 EST, operators noted the open light indicator on the MCR panel for the Loop 3 MSIV was extinguished while the PAM panel indicator for the MSIV indicated the valve was full open. Within minutes, the PAM panel indicated the MSIV was in mid-position. Operators noted the RCS temperature and Loop 3 SG pressure were both slowly rising, and the Loop 3 SG flow was slowly lowering. These indications confirmed the Loop 3 MSIV was slowly drifting closed. At 0844, the Unit 1 reactor was manually tripped.

November 23, 2015 at I All three light indicators for the MSIV on the MCR panel were 10845 EST illuminated followed by an immediate return to only full open indication. Coincidently, the indicator for the MSIV on the PAM panel indicated the valve was full open.

D. Manufacturer and model number of each component that failed during the event:

There were no components that failed during this event.

E. Other systems or secondary functions affected:

There were no other systems or functions affected by this event.

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

Operators observed open light indication for the Loop 3 MSIV on the MCR panel was extinguished while PAM indication initially showed full open. Approximately two to three minutes later, the PAM panel displayed dual indication. Subsequently, operators noted that the RCS temperature and Loop 3 SG pressure were both slowly rising, and the Loop 3 SG flow was slowly lowering. These indications confirmed the Loop 3 MSIV was slowly drifting closed.

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

There were no failed components associated with this event.

H. Operator actions:

After the Loop 3 MSIV was verified to be drifting closed by diverse indications, the operators established trigger values for Loop 3 SG pressure and RCS Tave-Tref mismatch. Once the Loop 3 MSIV showed dual indication on the PAM instrumentation, operators briefed for a potential manual reactor trip. After it was apparent that the Loop 3 MSIV was continuing to close, the operators made the decision to manually trip the reactor. Following the reactor trip, operators entered Emergency Procedure E-0, "Reactor Trip or Safety Injection," and then transitioned from E-0 to Emergency Subprocedure ES-0.1, "Reactor Trip Response." No human performance issues were identified.

I. Automatically and manually initiated safety system responses:

All plant safety related equipment operated as designed, all control rods fully inserted, and AFW automatically initiated from the feedwater isolation signal, as expected.

III. Cause of the event

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

The direct cause of the MSIV drifting in the closed direction was a loose connection (terminal lug and nut assembly) on the MSIV handswitch located in the MCR.

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

The root cause for this event was determined to be inadequate work practices during MSIV handswitch replacement in 1994. In 1994 during replacement of the handswitch, technicians utilized less than adequate work practices and human performance tools (i.e., fastener tightness, situational awareness, self-check, verification and procedure use) resulting in the assembly of the handswitch with a loose connection.

The root cause analysis is documented in Condition Report 1107656.

IV. Analysis of the event:

Prior to the event, SQN Unit 1 was operating at approximately 100 percent RTP with the RCS pressure and temperature near the nominal value of approximately 2235 pounds per square inch gauge (psig) and approximately 578 degrees Fahrenheit. Both the motor driven and the turbine driven AFW pumps and steam dump valves and the atmospheric relief valves were available.

The plant transient response including reactor power, RCS pressure, RCS temperature, pressurizer level, RCS secondary side pressure, and AFW flow remained within technical specification limits and were bounded by the Updated Final Safety Analysis Report (UFSAR) analysis. Containment pressure, temperature, and radiation levels were unaffected by this transient. SG level changes experienced during this event were bounded by UFSAR analysis. The plant responded as expected for the conditions of the trip.

V. Assessment of Safety Consequences

There were no safety consequences as a result of the event. All safety systems functioned as designed and no complications were experienced. Subsequent investigation determined that the Loop 3 MSIV remained capable of closing during the event and able to perform its safety function.

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

There were no components that failed during this event. There were no other components that could have performed the same function as the Loop 3 MSIV.

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. Safety-related systems that were needed to shut down the reactor, maintain safe shutdown conditions, remove residual heat or mitigate the consequences of an accident remained available throughout the 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:

There was no failure that rendered a train of a safety system inoperable during this event.

VI. Corrective Actions

Corrective Actions are being managed by TVA's corrective action program under Condition Report 1107656.

A. Immediate Corrective Actions:

Troubleshooting of the Loop 3 MSIV handswitch was conducted. The cause of the intermittent electrical signal to the MSIV handswitch was identified and corrected.

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

The corrective action to prevent recurrence is revision of the work control planning procedure to ensure specific connection fastener torque values are utilized during work order planning.

VII. Additional Information

A. Previous similar events at the same plant:

A review of previous reportable events for the past three years at SQN identified standards for multi-wire terminations and verifications associated with work performed in the mid-1990s.

B. Additional Information:

None.

C. Safety System Functional Failure Consideration:

This event did not result in a safety system functional failure.

D. Scrams with Complications Consideration:

This event did not result in an unplanned scram with complications.

VIII. Commitments:

None.