05000285/LER-2006-006
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 11-08-2006 |
|---|---|
| Report date: | 01-08-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2852006006R00 - NRC Website | |
BACKGROUND
The Fort Calhoun Station (FCS) has two emergency diesel generators (DG-1 and DG-2). They are 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 fed from either the main generator or the grid.
The 4160V system consists of four electrically separated buses (1A1, 1A2, 1A3 and 1A4). Buses 1A1 and 1A2 carry only 4160V non-vital loads and are normally connected to unit auxiliary transformers via the main generator. Buses 1A3 and 1A4 supply all engineered safeguard and essential support systems either directly or through the 480V distribution system. The bus supply breakers' control switches have three positions, trip/close/pull-to-lock. When the switch is in the pull-to-lock position, it trips the breaker and prevents breaker closure.
EVENT DESCRIPTION
On November 8, 2006, Fort Calhoun Station was in a refueling outage with the core offloaded. At 1140 Central Standard Time (CST), Emergency Diesel Generator 2 (DG-2) inadvertently started while de-energizing vital 4160 VAC bus during functional test, EM-FT-EX-1403 "4160 VAC Bus 1A4 Ground Fault Locator Functional Test". The DG-2 did not load onto the bus because the output breaker switch was in the pull-to-lock position for the test. No vital load or safety related 480 VAC bus were lost. Loads were transferred to vital 4160 VAC bus 1A3 prior to commencing the functional test. An operator was dispatched to verify proper operation of DG- 2. DG-2 responded as expected. The first time the operator attempted to shutdown DG-2, it was not taken out of auto. This resulted in DG-2 restarting. DG-2 was then properly secured. No other safeguards systems or equipment were actuated.
At 1430 CST on November 8, 2006, an eight (8) hour notification was made to the NRC Headquarters Operation Office (H00) per 10 CFR 50.72 (b)(3)(iv)(A). Condition Report 200605235 was written to document this event. This report is being made per 10 CFR 50.73 (a)(2)(iv)(A).
CONCLUSION
A root cause analysis of this event was performed. During the root cause analysis, a historical review of procedure EM-FT-EX-1403 was completed. This procedure was revised and issued in September 2003. This revision changed the initial conditions for performing the test. Prior to that revision, the procedure was performed with bus 1A4 de-energized, the primary and alternate feeder breakers tagged opened, and a ground truck installed on the bus. The new revision removed all requirements for bus 1A4 to be removed from service and added a step to have bus 1A4 in service with beaker 1A44 closed. The new revision added additional information regarding the need to cycle off loads as needed and listed all breakers associated with bus 1A4 by cubicle but not associated load. The changes were made to allow testing of the ground fault circuitry with the bus energized. However, as the procedure is currently written, bus 1A4 gets de-energized when breaker 1A44 is opened. (It is necessary to open breaker 1A44 in order to de-energize bus 1A4 to functional test the ground fault circuitry.) The initial conditions were revised, without regard to the actual performance of the procedure or effect on system interrelationships. The revision, which was issued in September 2003, was in place at the time of this event. The root cause analysis concluded the cause of this event was inadequate procedural guidance due to the lack of verification/validation and cross functional review of EM-FT-EX-1403.
The operating crew was aware that ground fault testing was to occur on bus 1A4 and that during the testing the bus was going to be de-energized. All loads and equipment, with the exception of the low pressure safety injection (LPSI) pump (SI-1B) providing shutdown cooling, had been shifted and supplied by bus 1A3. The crew successfully rotated to the LPSI pump (SI-1A) being supplied by bus 1A3, but never recognized the need to remove DG-2 from auto-standby status when the bus was de-energized by opening breaker 1A44. A contributing cause to this event was that the operators failed to recognize the interrelationship of de-energizing bus 1A4 and the affect on DG-2 with it aligned in auto-standby.
CORRECTIVE ACTIONS
1) A revision to EM-FT-EX-1403 will include steps to prevent an inadvertent diesel generator start. In addition, a review of procedures EM-FT-EX-1400, "4160 VAC Bus 1A1 Ground Fault Locator Functional Test" and EM-FT-EX-1402, "4160 VAC Bus 1A3 Ground Fault Locator Functional Test" will be completed to ensure adequate guidance exists in these procedures. These procedures are only performed during refueling outages. These changes will be completed by March 30, 2007.
2) Training outlining the details and cause of this event has been developed. This material emphasizes the need to acquire proper verification and validation and cross functional review. This training will be tracked by the corrective action system.
In addition, enhancements are being administered through the Corrective Action Program (CR 200603965).
SAFETY SIGNIFICANCE
DG-2 is a mitigating component that is designed to assist during an accident or transient condition. De- energizing bus 1A4 was a planned evolution and DG-2 responded as designed. DG-2 continued to be available.
Both diesel generators remained available. Minimal operator actions were needed to have DG-2 supply bus 1A4.
Fort Calhoun Station was defueled at the time of the event. The power for the spent fuel pooling cooling was not affected. Therefore this event had no impact on the health and safety of the public.
SAFETY SYSTEM FUNCTIONAL FAILURE
This event does not result in a safety system functional failure in accordance with Nuclear Energy Institute (NEI) 99-02, "Regulatory Assessment Performance Indicator Guideline".
PREVIOUS SIMILAR EVENTS
There have not been other events involving inadvertent starting of an emergency diesel generator at Fort Calhoun Station for the past three years.