05000458/LER-2015-009

LER-2015-009, 1 OF 3

River Bend Station - Unit 1 ,
Event date:	11-27-2015
Report date:	05-18-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
4582015009R01 - NRC Website

LER 15-009-00 for River Bend Station, Unit 1 Regarding Automatic Reactor Scram Due to Partial Loss of Offsite Power Caused by Fault in Local 230K Switchyard

ML16053A095
Person / Time
Site:	River Bend
Issue date:	01/26/2016
From:	Olson E W Entergy Operations
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RBG-47643 LER 15-009-00
Download: ML16053A095 (5)

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CONTINUATION SHEET

05000- River Bend Station - Unit 1 458

REPORTED CONDITION

On November 27, 2015, at 4:31 a.m. CST, with the plant operating at 100 percent power, an automatic reactor scram occurred following the loss of power to both divisions of the reactor protection system (RPS)[JC]. This condition resulted from a single-phase fault in the local 230kV switchyard. The initial response of the protective relays for the switchyard caused the breakers connected to the north 230kV btis in the switchyard to trip. The fault caused a voltage transient on the in-plant switchgear sufficient to trip the scram relays in the Division 2 RPS, resulting in a half-scram. The action of the protective relays continued, eventually causing the de-energization of reserve station service line no. 1. This lead to the loss of Division 1 RPS and a full reactor scram.

The main generator remained online until it was tripped, as designed, by the reverse-power relays when reactor steam pressure was insufficient to drive the main turbine. The Division 1 and 3 emergency diesel generators started as designed to restore power to their respective safety-related onsite electrical distribution subsystems. Both trains of the standby gas treatment system [BH] started, and the primary containment isolation system logic responded as designed. No safety- related systems were out of service at the time of the scram, and reactor pressure and water level were promptly stabilized. All reactor control rods inserted properly. The "B" reactor recirculation pump should have automatically downshifted to slow speed, but instead tripped off.

Multiple actuations of the main steam safety-relief valves (SRV5)(**RV**) occurred during the event. The nuclear steam supply system vendor reported this action was likely due to a localized pressure transient in the SRV instrumentation lines.

SRV tailpipe temperature recorders indicated that all valves re-seated correctly following the initial transient.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an automatic actuation of the reactor protection system, the primary containment isolation logic, and the Division 1 and 3 emergency diesel generators.

INVESTIGATION and IMMEDIATE CORRECTIVE ACTION.

When power was restored to both divisions of RPS, the primary containment isolation signal was reset, and the affected systems were restored to service. The isolation had caused a partial loss of the normal service water system, resulting in the automatic actuation of the standby service water system. The isolation had also caused the spent fuel pool cooling pump to trip, and operators aligned the alternate pump for service. The plant was taken to cold shutdown in a controlled manner.

CAUSAL ANALYSIS

The cause of the event was an animal-induced fault in the 230kV switchyard that resulted in the automatic trip of the north bus feeder breaker to the RSS No. 1. The fault also caused the south bus feeder breaker to trip, de-energizing RSS No. 1. The associated voltage transient on the in-plant switchgear also caused the trip of relays on both RPS buses for the scram and containment isolation logic circuits.

Contributing to this event was the insufficient recognition of risk in power operations with both RPS buses on the alternate power sources.

• The transmission department is evaluating the appropriate means of minimizing animal intrusion to the 230kV switchyard.

• An engineering study was performed to evaluate the transient limitations of the RPS motor-generator sets and alternate power sources, and the associated risk of the impact to the RPS scram logic. The study also provided recommendation for operational restrictions on aligning the RPS buses to the alternate power sources. The RPS system design criteria document was updated to identify the risk associated with aligning both RPS buses to the alternate power source during power operations.

• A design change was implemented to correct a legacy error in the wiring configuration of the Division 2 RPS bus discovered during this investigation. This modification will significantly reduce the risk of tripping bOth RPS buses for a given fault if both buses are aligned to the alternate power sources.

SAFETY SIGNIFICANCE

Other than the response of the reactor safety-relief valves and the "B" reactor recirculation pump, the plant responded as designed to the reactor scram. The emergency diesel generators responded as designed, and no conditions requiring the actuation of the emergency core cooling systems occurred. The operators were able to quickly stabilize RPV parameters without complication. This event was of minimal significance with regard to the 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.)