05000336/LER-2015-003

LER-2015-003, Valid Actuation of the Reactor Protection System

Millstone Power Station Unit 2
Event date:	11-08-2015
Report date:	01-07-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
LER closed by
IR 05000245/2016003 (4 November 2016)
3362015003R00 - NRC Website

LER 15-003-00 for Millstone, Unit 2, Regarding Valid Actuation of the Reactor Protection System

ML16020A068
Person / Time
Site:	Millstone
Issue date:	01/07/2016
From:	Daugherty J R Dominion Nuclear Connecticut
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
16-002 LER 15-003-00
Download: ML16020A068 (8)

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1. EVENT DESCRIPTION:

On November 8, 2015 during Millstone Power Station Unit 2 (MPS2) reactor start-up from refueling outage 2R23 with the reactor in mode 1 at approximately 57.5 % power, the operators received indication of an oil leak on the 'C' reactor coolant pump (RCP) motor lower oil reservoir.

The operators monitored the 'C' RCP oil level and bearing temperatures. Upon noting the RCP oil level dropping at a rate of approximately 1.7% per hour and the lower RCP guide bearing temperature rising, the operators entered abnormal operating procedure (AOP) 2575, Rapid Downpower, and commenced a rapid down-power to return MPS2 to mode 3. The operators manually tripped the reactor at approximately 19% power in accordance with AOP 2575 and entered emergency operating procedures. The reactor trip was uncomplicated. All safety systems operated as designed.

The operators manually tripped the reactor, Dominion reported this event pursuant to 10 CFR 50.72(b)(2)(iv)(B) as any event or condition that results in actuation of the reactor protection system (RPS) when the reactor is critical except when the actuation results from and is part of a pre-planned sequence during testing or reactor operation. (NRC Event Number: 51521).

The reactor trip was in response to degrading plant equipment, not preplanned, and the operator actions were directed by an AOP. Dominion is reporting this event pursuant to 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in manual or automatic actuation of any of the systems (RPS) listed in paragraph 10 CFR 50.73 (a)(2)(iv)(B).

BACKGROUND:

The reactor trip pushbutton switches on the control panel causes interruption of the AC power to the control element drive mechanism (CEDM) power supplies. De-energizing the CEDM coils allows the shutdown and regulating Control Element Assemblies (CEAs) to drop into the core by gravity.

2. CAUSE:

The cause of the valid actuation of the RPS was the operators manually tripping the reactor via the manual reactor trip push button. The AOP was entered due to an oil leak from the tubing to the level transmitter on the 'C' RCP motor lower oil reservoir. The tubing failure mechanism was determined to be high cyclic fatigue.

3. ASSESSMENT OF SAFETY CONSEQUENCES:

The shutdown of MPS2 was conducted in accordance with plant procedure AOP 2575 and proceeded in an orderly controlled manner. As directed in the procedure, the reactor was manually tripped. The reactor trip was uncomplicated. All safety equipment operated as designed and as expected.

4. CORRECTIVE ACTION:

Repairs were completed, the unit was restarted, and returned to service. Additional corrective actions are being taken in accordance with the station's corrective action program.

5. PREVIOUS OCCURRENCES:

• MPS2, & MPS3 LER 2014-006-00, Millstone Power Station Dual Unit Reactor Trip on Loss of Offsite Power

• MPS2, LER 2013-004-00, Reactor Trip While Backwashing 'D' Waterbox

• MPS3 LER 2013-007-00, Reactor Trip on Low-Low Steam Generator Level 6. Energy Industry Identification System (EIIS) codes:

• Reactor Coolant System — AB

• Control Rod Drive System — AA

• Pump — P

• Motor — MO

• Transmitter, indicating level — LIT

• Tubing — TBG