05000338/LER-2013-001
| North Anna Power Station Unit 1 | |
| Event date: | 09-26-2013 |
|---|---|
| Report date: | 12-20-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3382013001R01 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On September 26, 2013, at 1717 hours0.0199 days <br />0.477 hours <br />0.00284 weeks <br />6.533185e-4 months <br /> with Unit 1 in Mode 6, zero percent power, a condition prohibited by Technical Specifications (TS) occurred when reactor core alterations began with one emergency diesel generator (1J EDG), in a two train system, out of service for maintenance with the second emergency diesel generator (1H EDG) (EllS System EK, Component DG) inoperable, but unknown at the time. A subsequent failure of the 1H EDG determined prior inoperability. Technical Specifications 3.8.2 requires one qualified circuit between the offsite transmission network and the onsite Class 1 E AC electrical power distribution subsystem(s) required by TS 3.8.10 and one OPERABLE EDG.
On September 24, 2013 the 1H EDG was returned to service and declared operable following post maintenance testing (PMT) for a design change modification of the governor control system. Parts installed by the Design Change included a 2301A Load Sharing and Speed Control, a Digital Reference Unit (DRU) and an EGB-13P Governor/Actuator.
On September 24, 2013 the 1J EDG was removed from service for maintenance. Reactor core reload commenced on September 26, 2013 and completed on September 28, 2013.
During this time the 1H EDG was relied upon as the OPERABLE EDG to meet TS.
Subsequently on October 3, 2013 at 1115 hours0.0129 days <br />0.31 hours <br />0.00184 weeks <br />4.242575e-4 months <br />, the 1J EDG was declared operable following repairs and PMT.
On October 5, 2015 at 0828 hours0.00958 days <br />0.23 hours <br />0.00137 weeks <br />3.15054e-4 months <br />, the 1H EDG was started for a twenty four hour periodic test run. On October 6, 2013 at 0006 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, 1H EDG experienced a loss of load with local indication showing the fuel racks moved to a lower fuel output. The 1H EDG was secured and declared inoperable.
The failure of the governor control system occurred with the 1H EDG parallel to the grid under 24-hour load testing. Assuming a complete and permanent loss of control power to the 2301A Electronic Governor and DRU, the mechanical governor would have operated at the high speed stop at 63 Hz per design.
However, a momentary loss of power to the Electronic Governor and DRU, due an internal issue in the DRU electronics, a loose wiring connection, or a loose fuse holder would have resulted in the DRU resetting to 900 RPM / 60 Hz set point. The 900 RPM / 60 Hz set point is the reference value used in isochronous mode of operation (design basis function for the EDG).
A complete loss of power or momentary power interruption to the 2301A and DRU would not have prevented the 1H EDG from supplying emergency electrical power during a loss of offsite power to the emergency bus in the isochronous mode. However, the 1H EDG was unable to fulfill its design function since the 1H EDG could not meet TS Surveillance Requirement 3.8.1.15. Synchronization with the offsite power source while loaded with emergency loads upon a simulated restoration of offsite power and the ability to transfer loads to offsite power could not be performed to satisfy the TS surveillance requirement.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS No significant safety consequences resulted from this event since one OPERABLE qualified circuit between the offsite transmission network and the onsite Class 1 E AC electrical power distribution subsystem(s), required by TS 3.8.10, was available and the station blackout EDG was also available to supply power if required. Additionally, had a loss of offsite power occurred, the 1H EDG would have been able to supply power to the emergency bus. As such, the health and safety of the public were not affected by the event. This event is reportable per 10 CFR 50.73(a)(2)(i)(B) for a condition prohibited by Technical Specifications.
3.0 CAUSE The direct cause for the complete loss of electrical load of the 1H EDG during 24-hour testing was the momentary loss of electrical control power to the DRU. The apparent cause for the momentary loss of electrical control power to the DRU was a loose fuse holder due to numerous removal and re-installation evolutions during design change implementation, testing, and troubleshooting. This lead to the spreading of the fuse clips. The final installation of the fuse prior to the 24-hour test run was performed by a supplemental electrician not familiar with normal expectations for verification of tightness.
4.0 IMMEDIATE CORRECTIVE ACTION(S) Following the loss of load the 1H EDG was tripped from the Control Room.
5.0 ADDITIONAL CORRECTIVE ACTIONS The 1H EDG mechanical actuator, DRU, and power supply fuse holder for fuse FU1NA were subsequently replaced. All wiring connections were verified tight. Post maintenance testing was performed under 1-PT-82.12H, 1H Diesel Generator Isochronous Mode (Start by ESF Actuation) and 1-PT-83.12H, 1H Diesel Generator Test (Start by ESF Actuation) followed by 24-hour run and hot restart test. Post maintenance testing was completed satisfactorily.
The mechanical governor and DRU removed from the 1H EDG were sent to a vendor for testing. The mechanical governor and DRU were found to operate properly following 24- hour bench testing. A loose power supply fuse holder (fuse holder for FU1NA) to the 2301A Load Sharing and Speed Control and DRU was also identified. Vendor testing has verified that a momentary loss of electrical power to the DRU would result in a loss of load event.
2. DOCKET 6. LER NUMBER 3. PAGE 6.0 ACTIONS TO PREVENT RECURRENCE Guidance for verification of fuse clip tightness and inspection, as part of fuse removal and replacement, will be documented in Maintenance Operating Procedures. Guidance for fuse and fuse clip inspection expectations for supplemental personnel will be documented in the governing administrative procedure. Training needs for electrical maintenance activities with regards to installing fuses, inspecting fuse holders and verification of adequately securing fuses is being evaluated. The results will be provided to the Maintenance Training Review Board for inclusion in Electrical Maintenance Training. A time based preventive maintenance replacement strategy for EDG system fuse holders is being evaluated. These actions have been entered in the Station's Corrective Action System.
7.0 SIMILAR EVENTS None.
8.0 ADDITIONAL INFORMATION Unit 2 was operating in Mode 1, 100 percent power, and was not affected by this event.