05000443/LER-2012-003

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LER-2012-003, Reactor Trip Due to Ci cuit Board Failure that Closes Feed Regulating Valve
Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No.
Event date: 09-14-2012
Report date: 11-08-2012
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4432012003R00 - NRC Website

Description of Event

On September 14, 2012 at 8:25 pm while operating at approximately 85% power, Seabrook Station experienced an automatic reactor trip on low steam generator (SG)[AB, SG} water level. The failure of a printed circuit board [JB, CBD} in the 7300 process control system caused the feedwater regulating valve [SJ, FCV} for SG-C to close. As a result, the water level in SG-C decreased to the low level trip setpoint and initiated an automatic reactor trip. The emergency feedwater system [BA] actuated on low SG level, and plant equipment functioned as expected.

Cause of Event

The root cause of the event was the failure of an internal component on a printed circuit board (manual nuclear tracking driver board) in the 7300 control system.

Analysis of the Event

The 7300 control system instrument loops for the feedwater regulating valves include three circuit boards: a nuclear controller board (NCB), an automatic (auto) nuclear tracking driver (NTD) board, and a manual NTD board. During operation in auto, the NCB receives inputs from the SG level controller and the difference between feed and steam flow. The NCB then provides an input to the auto NTD board, which provides an output to control the position of the feedwater regulating valve. If the manual NTD board fails when the instrument loop is in auto, the loop will revert to a manual mode that is controlled by the auto NTD board.

During operation in the manual mode, the output of the manual NTD board controls the position of the feedwater regulating valve. In this mode, the NCB and auto NTD board track the output of the manual NTD to permit a bump less transfer to the auto mode.

A failure of the manual NTD board initiated this event. A failure of a circuit board is detected when the fuse [JB, FLJj on the circuit board opens. The internal failure on the manual NTD board transferred control to the manual mode and also caused the manual NTD board output to decrease (which provided a close signal to the feedwater regulating valve) prior to the fuse opening. At the same time, the auto NTD board tracked the decreasing output from the manual NTD board. Subsequently, when the fuse on the manual NTD opened, the instrument loop reverted to the manual mode with control by the auto NTD board. However, since the output of the auto NTD had decreased, the feedwater regulating valve was nearly closed, and SG water level decreased to the low level reactor trip setpoint.

This event resulted in a valid actuation of the reactor protection system and met the reporting criteria of 10 CFR 50.72(b)(3)(iv)(A). A four hour report was made to the NRC at approximately 2205 on September 14, 2012 (event number 48310). The operators responded to the plant trip in accordance with approved procedures, and safety systems functioned as expected. No adverse consequences resulted from this event, and this incident had no adverse impact on the health and safety of the public or the plant and its personnel. This event did not involve a safety system functional failure. No inoperable structures, systems, or components contributed to this event.

Corrective Actions

The corrective actions included replacing the failed circuit card and three additional cards in the same instrument loop.

Similar Events Seabrook has experienced no similar events within the last five years involving a plant trip due to failure of a 7300 control system card.

Additional Information

The Energy Industry Identification System (EllS) codes are included in this LER in the following format: [ENS system identifier, El IS component identifier].