05000423/LER-2016-002

LER-2016-002, Feedwater Isolation Signal Defeated Due to Wiring Error

Millstone Power Station Unit 3
Event date:	01-25-2016
Report date:	03-23-2016
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
LER closed by
IR 05000245/2016002 (25 July 2016)
4232016002R00 - NRC Website

LER 16-002-00 for Millstone Power Station Unit 3 Regarding Feedwater Isolation Signal Defeated Due to Wiring Error

ML16092A068
Person / Time
Site:	Millstone
Issue date:	03/23/2016
From:	Daugherty J R Dominion Nuclear Connecticut
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
16-067 LER 16-002-00
Download: ML16092A068 (6)

1. EVENT DESCRIPTION:

On January 25, 2016 an automatic reactor trip occurred on Millstone Power Station Unit 3 (MPS3) [LER 2016-001-00]. Following the MPS3 reactor trip, (Mode 3, 0% power), while operators were performing feedwater isolation (FWI) actuation verification steps in ES-0.1 (Reactor Trip Response), it was identified that the 'C' feedwater isolation valve (FWIV), 3FWS*CTV41C, did not auto-close as expected. Operators subsequently closed 3FWS*CTV41C from the control room.

Troubleshooting on the 3FWS*CTV41C FWIV identified that a temporary jumper, associated with a maintenance activity completed in November 2014, was inadvertently left installed. Technical Specification (TS) 3.3.2 requires two independent channels of Solid State Protection System (SSPS) be available for FWI upon an actuation signal as a result of a safety injection signal or a reactor trip coincident with low average reactor coolant temperature, TAVE. This jumper bypassed the Train 'B' of the SSPS input to isolate 3FWS*CTV41C upon receipt of an actuation signal, rendering the channel inoperable.

The safety function to isolate feedwater was met due to redundant valves in series ('C' feed regulating valve (FRV) and the 'C' feed regulating bypass valve (FRBV)) closing and the main feed pump tripping. These valves received their actuation signal from the Train 'A' of the SSPS. These components were not affected by the installed jumper. The operators remotely closed 3FWS*CTV41C from the control room, thus maintaining the containment isolation safety function.

This condition is reportable in accordance with 10 CFR 50.73 (a)(2)(i)(B) as any operation or condition that is prohibited by the plant's TS.

2. CAUSE:

This was a human performance error that occurred during refueling outage 3R16 as part of a plant modification (completed in November 2014) which replaced all four Unit 3 FWIV actuators. Site personnel failed to follow the requirements of the work control process and station procedures when installing and removing the temporary jumper in the feedwater control circuit for 3FWS*CTV41C.

3. ASSESSMENT OF SAFETY CONSEQUENCES:

The main feedwater isolation function prevents steam generator overfill following steam generator tube rupture and excess feedwater events and is also credited with mitigating excess cooldown following steam line break events. Isolation of main feedwater is also credited in containment response analyses, limiting mass and energy releases to the containment for inside containment secondary pipe breaks.

The feedwater isolation function can be accomplished by closing the FWIVs or the FRVs and FRBVs, which occurs automatically following a safety injection signal or reactor trip coincident with low average reactor coolant temperature, TAVE. In addition, the main feedwater pumps are automatically tripped following a safety injection signal or a reactor trip.

While being able to fulfill the safety function, the diverse methods of isolating main feedwater would not act as rapidly as closure of the main feedwater isolation valves. There is margin between the safety analysis results Final Safety Analysis Report (FSAR Chapter 15) and containment analysis results (FSAR Chapter 6) and the ultimate event acceptance criteria of the events to accommodate the slightly longer isolation function.

With three redundant methods capable of isolating/stopping main feedwater flow, the unavailability of one individual method (i.e., the automatic closure of 'C' FWIV) results in an insignificant increase in the likelihood of failing the safety function. Consequently, the safety significance associated with exceeding the 'C' FWIV TS allowed outage time is very low.

4. CORRECTIVE ACTION:

The jumper was removed and the appropriate surveillance for FWIV 3FWS*CTV41C was performed and completed satisfactorily, verifying the valve is OPERABLE. Other feedwater system components were inspected and no additional installed jumpers were identified. Workers involved were coached concerning the need for compliance with work control procedures and standards. This event will be used as operating experience (OE) for activities associated with installation and removal of jumpers.

5. PREVIOUS OCCURRENCES:

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

6. ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES:

• Reactor Coolant System — AB

• Containment- NH

• Feedwater System — JB

• Isolation Valve — ISV

• Pump — P

• Solid State Protection System — JG

• Steam Generator- SG