05000368/LER-2017-002

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LER-2017-002, Automatic Start of an Emergency Diesel Generator Due to the Momentary Loss of Offsite Power due to Severe Weather
Arkansas Nuclear One – Unit 2
Event date: 04-26-2017
Report date: 06-26-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3682017002R00 - NRC Website
LER 17-002-00 for Arkansas Nuclear One, Unit 2, Regarding Automatic Start of an Emergency Diesel Generator Due to the Momentary Loss of Offsite Power due to Severe Weather
ML17177A348
Person / Time
Site: Arkansas Nuclear One – Unit 2, Arkansas Nuclear Entergy icon.png
Issue date: 06/26/2017
From: Anderson R L
Entergy Operations
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
2CAN061704
Download: ML17177A348 (6)


Another source of power to ESF bus 2A3 / 2A4 cross-tie is the alternate AC diesel generator (AACDG). The AACDG is an independent, non-safety related power source intended to be used in the event of a station blackout (SBO). The AACDG is started using a touch screen in the ANO-2 Control Room or the AACDG building.

A. PLANT STATUS

Arkansas Nuclear One, Unit 2 (ANO-2) was operating at 0% rated thermal power with the core completely offloaded to the Spent Fuel Pool (SFP) when the condition was discovered. Startup Transformer No. 3 (SU3) (one normal off-site power source) and the 4160-volt bus 2A2 were out of service for planned maintenance. The 500kV transmission line to the substation at Pleasant Hill was out of service for planned maintenance. The area around the plant was experiencing severe weather from thunderstorms and tornado warnings had been issued from the National Weather Service for the four county area. There were no other structures, systems, or components (SSCs) that were inoperable at the time that contributed in the event.

The switchyard 500kV bus is a ring bus design, which allows transmission of 500kV power through three transmission lines to substations in Mabelvale, Ft. Smith, and Pleasant Hill, Arkansas. A fourth line supplies power to the bus tie auto-transformer. The autotransformer interconnects the 500kV and 161kV busses. It has two tertiary windings to provide 22kV power from the 500kV or 161kV buses to SU1, which supplies power to ANO-1, and to SU3, which supplies power to ANO-2. The 161kV bus is also a ring bus design, and includes two transmission lines to substations at Russellville East and Pleasant Hill, Arkansas, and a distribution line to the London, Arkansas, substation. It also supplies power to the SU2 which can supply power to both ANO-1 and ANO-2.

During normal operation of ANO-2, station equipment is supplied from the main generator through the Unit Auxiliary Transformer (UAT). During startup and shutdown conditions, the switchyard is used as a means of supplying station equipment from the utility grid through one of the startup transformers.

There are four 4160-volt buses. The main buses, 2A1 and 2A2 [EA], provide power to non-Engineering Safeguard Features (ESF) motors and supply transformers that feed 480-volt non-ESF load centers. The 4160-volt ESF buses, 2A3 and 2A4 [EB], are powered through 2A1 and 2A2, and supply equipment essential for the safe shutdown of the plant. Two 6.9kV buses, 2H1 and 2H2 [EA], supply the reactor coolant pumps. During shutdown, all of these buses are supplied from either SU3 or SU2.

SU3, which only supplies ANO-2, can support buses 2A1, 2A2, 2H1, and 2H2 simultaneously. Since SU2 has limited capacity and is shared by both units, it must be protected from overload. Procedures administratively limit automatic transfer of loads to SU2 only to ANO-1 buses A1 and A3, and ANO-2 buses 2A1 and 2A3. This is assured by normally maintaining the supply breakers from SU2 to ANO-1 buses A2, H1, and H2, and ANO-2 buses 2A2, 2H1, and 2H2 in pull-to-lock. In this condition, the associated supply breakers from SU2 to these buses will not close automatically after a loss of power from another power source.

B. BACKGROUND

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On April 26, 2017, ANO-2 was in day 28 of a refueling outage with a complete core off load that had moved all fuel to the SFP. Power to ANO-2 plant equipment was supplied from SU2 while SU3 was out of service for planned maintenance. 500kV lines to Fort Smith and Mabelvale, Arkansas, were in service. 161kV transmission lines to Russellville and Pleasant Hill, Arkansas, were also in service.

ANO- 1 was operating normally at full power. The area around the plant was experiencing severe weather from thunderstorms, and tornado warnings had been issued from the National Weather Service for the four county area. Switchyard work was ceased.

At approximately 1002 CST switchyard breakers for 500kV lines to Fort Smith and Mabelvale, Arkansas, opened on fault current. These two transmission lines run offsite in the same right-of-way. High winds had damaged the transmission towers approximately 16 miles away from ANO and caused phase to ground faults. This resulted in a loss of all offsite power lines to the 500 kV bus. The autotransformer also locked out, as designed, when the 500kV transmission lines faulted. This also resulted in an ANO-1 unplanned scram because the ANO-1 main generator was connected to the 500 kV bus with no transmission lines available (addressed in LER 50- 313/2017-001-00).

When the 500kV bus tripped, 4160-volt bus 2A1, which feeds 2A3 vital 480 volt bus, was subjected to a voltage transient and the #1 EDG [EK] auto started. The EDG output breaker never closed due to the fact that 2A3 bus voltage was restored to normal almost immediately from SU2. This EDG was secured due to running unloaded.

Bus 2A2 was out of service for maintenance. Both SFP cooling pumps were out of service after the transient (pumps required manual re-start following loss of power). A SFP cooling pump was restarted at 1020 CST. The temperature of the SFP did not change during the time no forced cooling was available. SFP temperature remained at approximately 91 °F.

The identified condition occurred due to a very short voltage transient on SU2. The voltage lowered to ~149.8kV and immediately restored to ~ 160kV. The transformer remained operable and capable of performing its required function. The #1 EDG performed as designed and remained capable of performing its 30-day mission time.

C. DESCRIPTION OF EVENT

E. CORRECTIVE ACTIONS

The following corrective actions were completed upon identification

  • Entered the Spent Fuel Pool Emergencies procedure and restarted both SFP cooling pumps.
  • Secured the #1 EDG due to running unloaded.

D. EVENT CAUSES

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

This event had no actual safety consequences impacting plant or public safety. In general, when the unit is shut down, the Technical Specifications requirements ensure that the unit has the capability to mitigate the consequences of postulated accidents that are assumed to potentially occur during shutdown conditions.

However, assuming a single failure and concurrent loss of all offsite or all onsite power is not required. The rationale for this is based on the fact that many Design Basis Accidents (DBAs) that are analyzed in Modes 1, 2, 3, and 4 have no specific analyses in Modes 5 and 6 or when the fuel is completely offloaded from the reactor vessel and placed in the SFP. Worst case bounding events are deemed not credible in Modes 5 and 6 or when the vessel is defueled because the energy contained with the reactor pressure boundary, reactor coolant temperature and pressure, and the corresponding stresses result in the probabilities of occurrence being significantly reduced or eliminated, and have minimal consequences.

A review of the ANO corrective action program and LERs for the previous three years was performed. There were no similar events identified at ANO during this time period.

The #1 EDG automatically started properly when a loss of AC power was detected, as designed. All other systems functioned normally.

This event is reportable pursuant to the following criteria:

The guidance provided in NUREG 1022 states under 10 CFR 50.73(a)(1):

10 CFR 50.73(b)(5) states that this report shall contain reference to “any previous similar events at the same plant that are known to the licensee.” NUREG 1022 reporting guidance states that term “previous occurrences” should include previous events or conditions that involved the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.

Energy Industry Identification System (EIIS) codes and component codes are identified in the text of this report as [XX].

F. SAFETY CONSEQUENCES

G. BASIS FOR REPORTIBILITY

H. ADDITIONAL INFORMATION

The holder of an operating license for a nuclear power plant (licensee) shall submit a Licensee Event Report (LER) for any event of the type described in this paragraph within 60 days after the discovery of the event.

10 CFR 50.73(a)(2)(iv)(A) Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section.

Paragraph (B)(8) of 10 CFR 50.73 lists “Emergency ac electrical power systems, including: emergency diesel generators (EDGs)”.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.