05000416/LER-2016-001

LER-2016-001, Valid Engineered Safety Feature Actuation and Temporary Loss of Residual Heat Removal

Grand Gulf Nuclear Station (Ggns), Unit 1
Event date:	03-17-2016
Report date:	05-16-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4162016001R00 - NRC Website

LER 16-001-00 for Grand Gulf, Unit 1, Regarding Valid Engineered Safety Feature Actuation and Temporary Loss of Residual Heat Removal

ML16138A291
Person / Time
Site:	Grand Gulf
Issue date:	05/16/2016
From:	Nadeau J Entergy Operations
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
GNRO-2016/00025 LER 16-001-00
Download: ML16138A291 (6)

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2016 416

3. LER NUMBER

001 0 05000- A. Initial Conditions:

At the time of the event, Grand Gulf Nuclear Station (GGNS) was in Mode 5 for Refueling Outage RF20. The station was in a shutdown condition with the reactor cavity flooded and the spent fuel pool gates not installed. System lineups were as follows:

- Engineered Safety Feature (ESF) Transformers [XFMR], ESF 11 and ESF 12, were available from offsite electrical feeders - Division 1 bus was being supplied by ESF 11 - Division 2 and 3 buses were being supplied by ESF 12 - Division 1 and Division 2 Standby Diesel Generators (SDGs) [EK] were available for onsite electrical feeders - Residual Heat Removal (RHR) [BO] 'B' was in Shutdown Cooling mode supplied by the Division 2 bus - Alternate Decay Heat Removal (ADHR) was available

B. Description of Events:

At approximately 15:15, on March 17, 2016, an electrical fault occurred on the 115kV feeder supplying the GGNS ESF 12 transformer. The fault caused an undervoltage condition in the GGNS switchyard which was present long enough to generate a valid actuation signal to the Division 2 Load Shedding and Sequencing (LSS) [JE] System and the Division 2 Standby Diesel Generator (SDG). RHR 'B' was load shed and the Division 2 SDG started. The SDG automatically sequenced to the Division 2 bus, restoring power as designed, within 7 seconds. Core alterations, in progress at the time, were suspended and fuel bundles were placed in their proper positions. RHR 'B' was restored by SDG 2 within 3 minutes and 13 seconds. The ESF 11 transformer was then paralleled with SDG 2. The Division 2 bus was then placed back to the ESF 11 offsite electrical feed and the Division 2 SDG was secured. ADHR remained available throughout the event and no changes in Spent Fuel Pool or Reactor Cavity temperature were observed.

Severe weather was present at the time of the event and high winds were suspected to have initiated the transient. Transmission personnel performed a walkdown and investigation of the Baxter Wilson to Port Gibson 115kV transmission line. In an area where the 115kV transmission line crosses an 8kV distribution line, burn marks were identified on two of the three phase conductors ('A' and 'B' phases). The lines either came into contact with one another or came within close proximity to one another. A subsequent review of the data recorded during the event revealed that there was initially a single phase fault ('B' to phase-to-ground) and evolved into a phase-to-phase fault ('A' phase-to-'B' phase-to-ground). This was determined to be the direct cause of the event.

The Port Gibson switchyard is fed from both the Baxter Wilson and Natchez transmission lines. Due to the location of the fault on the Baxter Wilson line, 2.1 miles from Baxter Wilson, and the current protective relaying design, the Baxter Wilson breaker opened instantaneously (zone 1 fault). Opening of the Port Gibson breaker is delayed 30 cycles (-0.5 seconds), per design, for the given fault distance (zone 2). After the fault was present for the required 30 cycles, the breaker opened within 5-6 cycles. In total, the fault was present on the line for .58 seconds. With the Natchez line feeding the Port Gibson switchyard, the fault caused the yard voltage to drop approximately 21 kV phase-to-ground (-31% of the nominal 67kV phase-to-ground) for the duration of the fault until the Port Gibson breaker opened. Although the feeder from the Port Gibson yard to GGNS remained intact, this degraded voltage condition was detected by the Division 2 LSSS which load shed and, in turn, initiated the ESF actuation .46 seconds later. All onsite and offsite equipment operated as expected for the current design. GGNS personnel responded in accordance with Off Normal Event Procedures and appropriate actions were taken.

The apparent cause was determined to be that the Baxter Wilson to Port Gibson 115kV line does not have pilot scheme protection. The phase-to-phase fault would have cleared sooner with protective relaying. A project is planned, in 2017, to upgrade the Baxter Wilson to Port Gibson transmission line with a fiber optic pilot scheme.

Event Notification No. 51800 was made to the NRC Operations Center for both the declaration of a valid ESF actuation and a loss of Shutdown Cooling in accordance with 10CFR50.72(b)(3)(iv)(A) and 10CFR50.72(b)(3)(v)(B), respectively.

C. Cause of Event:

The apparent cause was determined to be that the Baxter Wilson to Port Gibson line does not have pilot scheme protection. The phase- to-phase fault would have cleared sooner with protective relaying.

D. Extent of Condition:

A ground fault similar to the one described here could potentially occur on any of the four qualified GGNS electrical feeder lines.

Investigation revealed that the Baxter Wilson line is the only line that experiences a delayed breaker clearing upon receipt of a ground fault condition. The other three lines are all protected by pilot scheme protective relaying which allows for nearly instantaneous clearing which would prevent actuation of load shedding on a perceived loss of offsite power. It was also determined that, in order for the event described here to occur, the ground fault would have had to be at a distance greater than 80% of the the way from Port Gibson to Baxter Wilson, which was the case. Implementation of pilot scheme protective relaying would also prevent actuation of load shedding anywhere that a ground fault occurs on the line.

E. Corrective Actions:

The immediate action was to clear the fault and re-energize the Baxter Wilson to Port Gibson 115kV line.

A longer term corrective action is to implement pilot scheme protective relaying. Work is planned and expected to be completed in July of 2017.

F. Previous Similar Events:

CR-GGN-2003-1128 documented an event in April of 2003 where a fault occurred on the 115kV Natchez to Port Gibson line during severe weather. The fault occurred between the Port Gibson substation and GGNS, at a distance that would have allowed nearly instantaneous clearing of the Port Gibson breaker. At the time of the fault, no plant equipment was being supplied by the Natchez line.

The line was re-energized and no further corrections were initiated. Because of the location of the fault, the fact that the plant was not being supplied by the Natchez line and the fact that there was no plant response, this event would not have been a learning for GGNS.

Also, of note, the Natchez line now has pilot scheme protection.

F. Safety Significance:

At the time of the event, GGNS Unit 1 was in Mode 5 for a Refueling Outage. The Division 2 LSSS and SDG automatically load shed and re-energized the Division 2 bus. Shutdown Cooling was temporarily lost but was returned within 3 minutes and 13 seconds.

ADHR was available for the duration of the event. All systems operated as designed and GGNS personnel took appropriate actions.

There was no change in Spent Fuel Pool or Reactor Cavity temperature. The Unit remained in Mode 5 and offsite power was restored.

The safety significance is considered to be low and there were no actual nuclear safety consequences.

G. Basis of Reportability:

This LER is being submitted pursuant to Title 10 Code of Federal Regulations 10 CFR 50.73(a)(2)(iv)(A) for the actuation of an Engineered Safety Feature (SDG) and 10 CFR 50.73(a)(2)(v)(B) temporary loss of RHR (Shutdown Cooling).