05000390/LER-2016-004

LER-2016-004, Automatic Reactor Trip Due to Actuation of Over Temperature Delta Temperature Bistables

Watts Bar Nuclear Plant, Unit 1
Event date:	03-22-2016
Report date:	05-23-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 51815	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
3902016004R00 - NRC Website

LER 16-004-00 for Watts Bar, Unit 1, Regarding Automatic Reactor Trip Due to Actuation of Over Temperature Delta Temperature Bistables

ML16144A377
Person / Time
Site:	Watts Bar
Issue date:	05/23/2016
From:	Simmons P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 16-004-00
Download: ML16144A377 (6)

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 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

Watts Bar Nuclear Plant (WBN) Unit 1 was in Mode 1 at 100 percent rated thermal power (RTP).

II. DESCRIPTION OF EVENT

A. Event

(WBN1) reactor tripped due to the actuation of the Over Temperature Delta Temperature bistables. Concurrent with the reactor trip, the Auxiliary Feedwater (AFW) system actuated. All control rods inserted upon the reactor trip and safety systems functioned as expected.

An investigation into the cause of the trip determined that a failure of a Valve Position Limit (VPL) up/down circuit card {EIIS:CBD} in the Analog Electro-Hydraulic (AEH) turbine controls {EIIS:TG} resulted the closure of the turbine high pressure governor valves {EIIS:XCV}, resulting in an automatic reactor trip and subsequent turbine trip on WBN1. The failed card was replaced and WBN Unit 1 was returned to service.

This event was reported to the Nuclear Regulatory Commission (NRC) on March 22, 2016 via Event Notification 51815 pursuant to 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A).

B. Inoperable Structures, Components, or Systems that Contributed to the Event A DC to DC converter failed on the VPL Up/Down counter card in the AEH controls, which resulted in the plant trip.

C. Dates and Approximate Times of Occurrences On March 22, 2016, the main turbine control system VPL Up/Down circuit card failed, which resulted in the four main turbine governor valves going closed. Eight seconds after valve closure the reactor tripped on over temperature delta temperature.

D. Manufacturer and Model Number of Components that Failed The up/down counter circuit card was supplied by Siemens, Part number 1651049-101.

E. Other Systems or Secondary Functions Affected

The failure of the Valve Position Limiter did not directly affect the function of any other systems or other secondary functions.

F. Method of discovery of each Component or System Failure or Procedural Error The component failure was discovered by performance of the maintenance procedure that checks/calibrates the main turbine control system. This was only a component failure, no system failures or procedural errors caused this event.

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G. Failure Mode and Effect of Each Failed Component The VPL circuit card's output failed to zero percent, which as designed, caused the main turbine control system to limit the position of the four governor valve to zero percent. This constitutes a complete loss of external electrical load.

H. Operator Actions

This was an uncomplicated reactor trip. No special operator actions were required.

I. Automatically and Manually Initiated Safety System Responses All automatic and manual safety systems responded as expected.

III. CAUSE OF THE EVENT

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

A DC to DC converter failed on the VPL Up/Down counter card in the AEH controls, which caused the card's output to fail to zero percent.

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

There were no human performance causes associated with this plant trip.

IV. ANALYSIS OF THE EVENT

On March 22, 2016 at 1131 EDT, the main turbine control system VPL circuit card failed. This failure caused all four main turbine governor valve to go fully closed. This constituted a complete loss of external electrical load for the unit. The reactor continued to operate for approximately 8 seconds after the governor valves closed until the reactor protection system tripped the reactor on over temperature delta temperature. All safety systems operated as expected, and operations personnel were able to promptly stabilize the plant in Mode 3.

V. ASSESSMENT OF SAFETY CONSEQUENCES

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The turbine controls system is not safety related. The failure of the VPL Up/Down card resulted in a plant trip, but otherwise did not impact any plant safety functions .

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 Not applicable.

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C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service Not applicable.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority Corrective Action Program and is being tracked under condition report 1152462.

A. Immediate Corrective Actions

The failed VPL Up/Down card that resulted in the plant trip was replaced.

B. Corrective Actions to Prevent Recurrence

The existing turbine control system has a number of identified single point vulnerabilities. The long term plan is to replace the existing AEH turbine control system with a modern fault tolerant turbine digital control system.

VII. ADDITIONAL INFORMATION

A. Previous similar events at the same plant

On May 21, 2010, WBN Unit 1 experienced a reactor trip on turbine trip from full 100 percent rated thermal power caused by closure of the Main Turbine Throttle/Stop valves as reported in LER 2010-001 dated July 20, 2010. The probable cause was determined to be an intermittent failure of a circuit card in the AEH turbine control system. The existing AEH circuit cards that were in the signal path that resulted in the trip were replaced.

B. Additional Information

None.

C. Safety System Functional Failure Consideration

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

D. Scrams with Complications Consideration There were no complications as a result of this reactor trip.

VIII. COMMITMENTS

None.