05000336/LER-2016-002

LER-2016-002, Manual Reactor Trip Due to Loss of Two Circulating Water Pumps

Millstone Power Station Unit 2
Event date:	08-11-2016
Report date:	09-20-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3362016002R00 - NRC Website

LER 16-002-00 for Millstone Power Station Unit 2 Regarding Manual Reactor Trip Due Loss Of Two Circulating Water Pumps

ML16271A409
Person / Time
Site:	Millstone
Issue date:	09/20/2016
From:	Daugherty J R Dominion Nuclear Connecticut
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 16-002-00
Download: ML16271A409 (6)

Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send 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.Reeource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.

1. EVENT DESCRIPTION:

On August 11, 2016 at 09:59 while operating in MODE 1 at 100% power, operators manually tripped the Millstone Power Station Unit 2 reactor due to degraded condenser vacuum caused by the loss of two out of four circulating water pumps (CWPs) ('A' and 'C' CWPs). The reactor trip was uncomplicated and decay heat removal was via the steam dumps to the condenser. All control rods inserted on the reactor trip. All safety systems functioned as required.

Following the reactor trip, main feedwater continued to be available. The main feedwater regulating valves closed as expected and the main feedwater regulating bypass valves opened as designed. Steam generator level was restored to the desired post trip band using main feedwater.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).

2. CAUSE:

The 'A' and 'C' CWPs stopped due to a loss of normal control power in the associated variable frequency drive (VFD) system. This coincided with a grid disturbance that occurred during a storm. A design feature of the VFD is to swap to a battery back-up redundant uninterruptible power supply (UPS) in the associated VFD system when power is lost or becomes unreliable. Both backup UPSs associated with the 'A' and 'C' CWPs failed to function resulting in the 'A' and 'C' CWPs stopping. The loss of two out of four CWPs caused a reduction in condenser vacuum. The operators manually tripped the reactor due to degrading condenser vacuum.

3. ASSESSMENT OF SAFETY CONSEQUENCES:

The operating crew responded to the reactor trip by completing EOP 2525, Standard Post Trip Actions, and entering EOP 2526 Reactor Trip Recovery. All control rods inserted on the reactor trip. With the 'B' and 'D' CWPs still running, condenser vacuum remained adequate for operation of the condenser dump valves following the reactor trip. Steam generator level was restored to the desired post trip band using main feedwater. No safety functions were challenged and plant operation was maintained within the bounds of the FSAR Chapter 14 Safety Analysis. Based on the above discussion, there were no safety consequences for the event.

4. CORRECTIVE ACTION:

The associated UPSs were replaced. Additional corrective actions are being taken in accordance with the station's corrective action program.

5. PREVIOUS OCCURRENCES:

condenser vacuum. There are no previous occurrences with the same underlying reason or consequences.

Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send 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, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.