05000423/LER-2007-002

LER-2007-002, Loss of Offsite Power Caused by Transmission System Operator While Defueled

Docket Number
Event date:	04-25-2007
Report date:	06-11-2007
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4232007002R00 - NRC Website

1. Event Description:

At 1047 on April 25, 2007 with the plant shutdown (0% power) and in a defueled condition, a loss of offsite power (LOP) caused by a switching error on the part of the offsite transmission system operator (TSO) occurred at Millstone Power Station Unit 3 (MPS3). The MPS3 "A" emergency diesel generator (EDG) [DG] started as designed (the "B" EDG was out of service for planned maintenance) and supplied power to the "A" train 4160 volt AC emergency bus [EK]. At that time, the diesel sequencer closed the breakers for service water [BI] and reactor plant component cooling water [CC]. Operators restored power to spent fuel pool cooling at 1054 in accordance with plant procedures. The shift manager declared an Unusual Event (UE) at 1101 based on the Emergency Action Level criteria for a loss of offsite power. The UE was terminated at 1427.

At the time of the event, the applicable plant technical specifications (TS) such as Reactor Coolant System Pressure / Temperature Limits (TS 3.4.9.1), Spent Fuel Pool Reactivity (TS 3.9.13) and Water Level — Storage Pool (TS 3.4.9.11) were not challenged by the event.

When the LOP occurred, the plant response was as expected with the following exceptions or challenges to the operators:

The "A" train control building chiller was identified in the trip condition approximately 10 minutes after the LOP. The loss of power apparently caused the bearing oil temperature to reach the trip set point. Operators successfully restarted the chiller. (The set point has subsequently been raised to address this situation.) Firewatches were suspended in some locations until normal power and plant lighting were restored (normal power was restored at approximately 1424).

Since the plant was in a refueling outage, numerous personnel were working throughout the plant, including within confined spaces not normally accessible. The LOP resulted in the loss of all normal and temporary lighting. A controlled evacuation of workspaces was carried out. There were no injuries or radioactive releases attributable to this event. Additionally, Millstone Power Station Unit 2 remained at 100% power and connected to the grid throughout the event.

This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv) as an event or condition that resulted in manual or automatic actuation of emergency AC electric power systems including emergency diesel generators.

2. Cause:

The loss of offsite power was caused by a human error on the part of the offsite transmission system operator (TSO). At the time of the event, offsite power to MPS3 was being back-fed via the normal station service transformer (15G-3SA-NSST). The MPS3 reserve station service transformer (15G-23SA-RSST) was removed from service for maintenance. Due to an emergent fault in another portion of the transmission system, one of the offsite supply lines (#348) needed to be removed from service by operations at the offsite TSO at the Connecticut Valley Electric Exchange (CONVEX). This required operation of the Millstone Power Station 15G-14T-2 and 15G-15T-2 switchyard breakers. However, operations at the offsite TSO facility resulted in the 15G-13T-2 breaker being opened instead of the 15G-15T-2 breaker. This error resulted in a LOP to MPS3 (see attached sketch).

(During MPS3 operation at power, interlocks exist which would have prevented the offsite TSO from making this switching error.) Based upon a review by the offsite TSO, inadequate self-checking by the offsite TSO System Operations Supervisor prior to breaker manipulation was the apparent cause of this human error.

3. Assessment of Safety Consequences:

The event had no safety consequences because of its duration and the defueled condition of the plant. The LOP had no impact on the parameters addressed in the few applicable plant technical specifications (TS) (such as Reactor Coolant System Pressure / Temperature Limits (TS 3.4.9.1), Spent Fuel Pool Reactivity (TS 3.9.13) and Water Level — Storage Pool (TS 3.4.9.11)). There was no measurable rise in Spent Fuel Pool temperature.

Emergency equipment functioned as designed, operator actions were performed consistent with applicaNe procedures, and there were no radioactive releases and no injuries.

4. Corrective Action:

The offsite TSO has initiated corrective actions to address the human performance aspects of this event.

Additional corrective actions at Millstone are being taken in accordance with the station's corrective action program.

5. Previous Occurrences:

There have been no previous occurrences of a loss of offsite power at MPS3.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

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43.15G-22S1-415G - 15G -15G - 15G - 3XA1-4 3XB1-423SB1-i 93SA1-4 5G-2X1-4 AMIN ■ MIi MMI NM i IMIO MOW MI= "run" MM 15G-3 5G-3XB15G-i 15G- GSU GSU23SB 23SA fv-wv-% MM 15G-15G-22S 15G­ 2X5G-3U-2 RSST 2S ~arr.rv

• trynk 15G-3SB 156-3SA

NSST NSST

Millstone 15G one-line diagram