05000282/LER-2007-004

LER-2007-004, Technical Specification Required Shutdown Due to Both Emergency Diesel Generators Being Inoperable

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function
2822007004R02 - NRC Website

On December 21, 2007, with the Unit 1 Train B emergency diesel generator' (D2) out of service for planned maintenance, Nuclear Management Company, LLC (NMC)2 staff were conducting a required surveillance test on the Unit 1 Train A 4kV3 Bus4 (Bus 15) load sequencers. The load sequencer failed the surveillance and the Unit 1 Train A emergency diesel generator (D1) was declared inoperable as a result. With both D1 and D2 inoperable, Technical Specifications requires the affected Unit to be shutdown. Shutdown to Mode 3 was completed at approximately 0700 on December 21, 2007.

EVENT ANALYSIS

The event resulted in Unit 1 being brought to Mode 3, thus, this event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(A) as a Technical Specification required shutdown.

Impact on Safety System Functional Failure Performance Indicator Unit 1 Train A 4kV Bus 15 could not be automatically powered from D1 because its load sequencer was inoperable, which resulted in D1 being declared inoperable. Unit 1 Train B 4kV Bus 16 could not be powered from D2 because D2 was inoperable while maintenance was being performed. D1 was considered available, but would not have started automatically on a loss of offsite power (LOOP) concurrent with a loss of coolant accident. Since the Unit 1 onsite power supplies could not have provided power to systems required to mitigate an accident in the event of a concurrent LOOP, this represents a loss of safety function. Consequently, this event is reportable per 10CFR 50.73(a)(2)(v).

SAFETY SIGNIFICANCE

Unit 1 emergency AC buses remained energized throughout the event. D1 was available for powering Bus 15 using manual operator actions had offsite sources to the Unit 1 emergency AC buses been lost during the event. Additionally, the cross-ties between Unit 1 and Unit 2 emergency AC buses were available to repower Unit 1 emergency AC buses from the Unit 2 emergency diesel generators. Thus, this event did not affect the health and safety of the public and the safety significance of this event is considered minimal.

1 EIIS System Identifier: EK; EIIS Component Identifier: DG 2 On September 22, 2008, NMC transferred its operating authority to Northern States Power Company, a Minnesota Corporation (NSPM), a wholly owned subsidiary of Xcel Energy. By letter dated September 3, 2008, NSPM assumed responsibility for actions and commitments previously submitted by NMC.

3 EIIS System Identifier: EB 4 EIIS Component Identifier: BU 5 EIIS Component Identifier: PMC

• Age related degradation of the input/output cards of the load sequencer (Equipment Root Cause), and

• No preventive maintenance strategy established for the load sequencers and their subcomponents (Organizational Root Cause).

CORRECTIVE ACTION

NMC staff conducted troubleshooting and replaced an input/output card on the Bus 15 load sequencer, after which the Bus 15 load sequencer successfully passed the surveillance. As a result, the Bus 15 load sequencer and D1 were returned to operable status. D2 was returned to operable status after completion of the planned maintenance.

Corrective actions to prevent recurrence include:

1. Replace input/output cards for Bus 16, 25, and 26 load sequencers.

2. Establish the appropriate preventive maintenance strategy for the load sequencers and their subcomponents.

3. Develop a spare parts list for the load sequencers and their subcomponents, and issue stocking analysis worksheets for the load sequencers. Procure and stock the identified spare parts.

PREVIOUS SIMILAR EVENTS

Review of Licensee Event Reports for Unit 1 and Unit 2 since 2005 found three previous Technical Specification required shutdowns. However, none of these were the result of a failed bus sequencer (two were for Unit 2 emergency diesel generator issues and one was for containment fan coil unit leakage.)