05000458/LER-2016-002

LER-2016-002, Automatic Reactor Scram and Division 2 Primary Containment Isolation Due to Offsite Grid Electrical Transient

River Bend Station - Unit 1
Event date:	1-9-2016
Report date:	03-07-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4582016002R00 - NRC Website

LER 16-002-00 for River Bend, Unit 1, Regarding Automatic Reactor Scram and Division 2 Primary Containment Isolation Due to Offsite Grid Electrical Transient

ML16084A143
Person / Time
Site:	River Bend
Issue date:	03/07/2016
From:	Vazquez S Entergy Operations
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RBG-47661 LER 16-002-00
Download: ML16084A143 (5)

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REPORTED CONDITION

On January 9, 2016, at approximately 2:37 a.m. CST, with the plant operating at 100 percent power, an automatic reactor scram occurred concurrent with the closure of all main steam isolation valves (MSIVs). That action was the result of an electrical transient caused by a phase-to-phase fault on a nearby 230kV transmission line. The transient caused a momentary decrease in the voltage on both reactor protection system busses, which also power the MSIV control solenoids. The Division 2 primary containment isolation logic was also actuated, causing the Division 2 valves in balance-of-plant systems to close. Both divisions of the standby gas treatment system [BH] automatically started due to the shutdown of the normal annulus pressure control system. Both reactor recirculation [AD] pumps downshifted to slow speed.

The initial upward swell of reactor water level caused all three reactor feedwater pumps to trip. Reactor feedwater pump "C" was restarted approximately eight minutes after the scram.

Following the first automatic actuations of the reactor safety-relief valves (SRVs), operators controlled reactor pressure with intermittent manual opening of selected SRVs. After reactor parameters were stabilized, the MSIVs on the "D" main steam line were opened at approximately 4:29 a.m. to re-establish automatic pressure control.

The Division 1 and 2 reactor protection system (RPS) busses were on their alternate power supplies (i.e., offsite power) at the time of the event. No safety-related systems were out of service at the time. No plant parameter limits requiring the automatic actuation of any of the emergency core cooling systems or the emergency diesel generators were exceeded.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as the actuation of the reactor protection system [JC] and the primary containment isolation logic [JM].

INVESTIGATION and CAUSAL ANALYSIS The company's transmission department investigated the event. Although no definite source of the fault was found, it was concluded that a lightning strike likely caused the transient.. The fault occurred on a 230kV transmission line approximately three miles from the station. The fault lasted for 5.4 cycles before it was-isolated by automatic breaker action, and caused the voltage on the switchgear , supplying the RPS busses to decrease to approximately 34 percent of normal. This transient was sufficient to trip the scram solenoids and the MSIV solenoids.

CORRECTIVE ACTIONS TO PREVENT RECURRENCE

As of January 17, both divisions of the RPS system were running on their normal power sources (i.e., in-plant switchgear).

PREVIOUS OCCURRENCE'EVALUATION No reactor scrams resulting from grid transients have occurred at River Bend Station in the last three years.

SAFETY SIGNFICANCE

The response of the plant was bounded by the corresponding section of the Updated Safety Analysis Report. No safety-related systems were out of service at the time of the event. No plant parameter limits requiring the automatic actuation of any of the emergency core cooling systems or the emergency diesel generators were exceeded. This event, thus, was of minimal significance to the health and safety of the public.

(NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)