05000285/LER-2007-003

LER-2007-003, Inoperability of a Diesel Generator with an Inoperable Containment Cooling Fan from the Opposite Bus

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2852007003R01 - NRC Website

BACKGROUND

The Fort Calhoun Station (FCS) has two emergency diesel generators (DG-1 and DG-2). They arc designed to furnish a reliable source of 4160 volt AC power for safe plant shutdown and operation of engineered safeguards when the normal sources of power are lost. The diesel generators are normally aligned in a standby mode ready to automatically start, come up to rated speed and voltage, and energize the engineered safeguard buses when an undervoltage condition is sensed on the bus. The two diesel generators will supply the two vital buses. The non-vital busses are normally fed from either the main generator or off-site power.

Component Cooling Water is supplied to the two containment air cooling and filtering units. The function of the containment air cooling and filtering fans is to circulate air throughout the containment to equalize the temperature and prevent the formation of hydrogen pockets.

Technical Specification (TS) 2.4(1)(a) provides the containment cooling minimum requirements to assure operability of equipment required to remove heat from the containment during normal operation. There is one containment air cooling and filtering unit associated with each of the diesel generators.

TS 2.4(1)(b) reads as follows:

"During power operation one of the components listed in (1)a.i. and ii. may be inoperable. If the inoperable component is not restored to operability within seven days, the reactor shall be placed in hot shutdown condition within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

EVENT DESCRIPTION

On February 16, 2007 at approximately 1010, during the monthly surveillance testing of Diesel Generator (DG) 1 speed was increased from approximately 500 RPM to 900 RPM. At approximately 750 RPM the generator field is expected to self-flash. Field flashing of the diesel generator did not occur. The diesel was shutdown and declared inoperable.

Investigation determined that the DG-1 had been inoperable since February 14, 2007 when a problem with a relay was repaired.

During the time DG-1 was inoperable, the larger containment cooling fan (VA-3B) powered from DG-2 was made inoperable for approximately 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> on February 15, 2007 to perform routine surveillance testing. Since DG-1 was inoperable during the time the containment cooling fan was inoperable, only the smaller containment cooler (VA-7D) was operable. This event is reportable per 10CFR50.73(a)(2)(i)(B) and 10 CFR 50.73(a)(2)(v)(D).

CONCLUSION

Work Order 263153 was written to disassemble, clean and repair the GE 2CR relay and replace the auxiliary contacts.

Troubleshooting determined that the GE 2CR relay auxiliary contact over travel stop tang was hung up on the main relay phenolic base, preventing the auxiliary contacts from returning to their normally closed position. Binding appears to have resulted from a slight misalignment of the relative positions of the auxiliary contact bracket and the main relay phenolic base. Due to the design of the GE 2CR relay, the relative positions need only change by as little as approximately 1/64 of an inch to result in binding due to the over travel tang bypassing the stop. The resulting misalignment causes the normally closed auxiliary contacts to stick in the open state when the GE 2CR relay energizes.

Fort Calhoun Nuclear Station 05000285 2 � OF � 3 The auxiliary contacts were replaced with new contacts from the warehouse. The GE 2CR contactor was tested 12 times by energizing the main contactor coil using an external power supply. Resistance readings were taken on the contacts during the testing to ensure good contact. The diesel was started and flashed twice to ensure proper operation of the field flashing circuit. The monthly performance of the DG-1 surveillance test was completed satisfactorily on Saturday, February 17, 2007.

The root cause for this event was determined to be a PMT process that did not follow industry standards. This process was over-reliant on the knowledge base in work management and engineering. Specifically, this involved failure to verify that the normally closed auxiliary contacts had returned to the closed state following the surveillance test of the diesel.

The field flash circuitry is common to both emergency DG-1 and DG-2. During follow up to the DG-1 failure, the identical GE 2CR relay auxiliary contacts in DG-2 were replaced and successfully tested.

In addition, auxiliary contacts are utilized on other relays in the plant. Routine surveillance testing of associated equipment provides assurance that the safety function will be met, as evidenced by identification of misoperation of the contacts associated with DG-1 on February 16, 2007. However, actions are planned to address weaknesses in post maintenance testing practices as a result of this event.

CORRECTIVE ACTIONS

As previously indicated, the auxiliary contacts associated with DG-1 and DG-2 were replaced with new contacts.

Procedural guidance for post maintenance testing has been enhanced to improve the quality of PMT to ensure no additional problems are created by the work activities.

Training has been provided on this event and on the changes to procedural guidance. This training was provided to personnel who specify PMT as well as to personnel reviewing PMT requirements.

SAFETY SIGNIFICANCE

At the time of discovery, both trains of the containment spray system were available from its normal power supply to provide cooling to the containment should it be required.

The amount of time that the VA-3B containment cooling fan was inoperable was a short, approximately 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> to perform surveillance testing. The amount of time required to restore the system to operability when being tested is short. Although the system was inoperable due to the surveillance testing, it was quickly restorable if needed. The probability of an accident occurring during the time of inoperability is very low. The DG-1 was inoperable for approximately 3 days.

Due to the short amount of time of inoperability of VA-3B, the ability to quickly restore VA-3B, and the low probability of an accident during that time, there was very small impact on the health and safety of the public.

SAFETY SYSTEM FUNCTIONAL FAILURE

This event does result in a safety system functional failure in accordance with NEI-99-02.

PREVIOUS SIMILAR EVENTS

There have not been any other instances of a similar nature resulting in the diesels not being able to field flash due to post maintenance testing at FCS in the last three years.