05000338/LER-2011-001
| North Anna Power Station , Unit 1 | |
| Event date: | 02-03-2011 |
|---|---|
| Report date: | 04-01-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation |
| LER closed by | |
| IR 05000338/2011003 (28 July 2011) | |
| 3382011001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT A voluntary report is being made to share information and lessons learned regarding a circuit card failure for two Main Control Room annunciators.
On February 3, 2011, at 0345 hours0.00399 days <br />0.0958 hours <br />5.704365e-4 weeks <br />1.312725e-4 months <br /> annunciator 1H-G4, Annunciator System (EllS System — IB) DC Ground was received in the Main Control Room (MCR). At 0348 hours0.00403 days <br />0.0967 hours <br />5.753968e-4 weeks <br />1.32414e-4 months <br /> annunciator 1B-D3, BAT 1B Hi-Lo Level CH I-II (El IS Component — ANN) was received which, when acknowledged, locked in and annunciator 1H-G4 cleared. While investigating, an acrid smell was noticeable in the MCR. At 0353, upon entry into the annunciator system cabinet room, adjacent to the MCR, the door of the cabinet, 1-EI-CB-21, (El IS Component — CAB) was opened and flames approximately 2 - 4 inches long were observed coming from an annunciator circuit card. Operations personnel entered fire contingency action procedure 0-FCA-0, Fire Protection — Operations Response. At 0354 hours0.0041 days <br />0.0983 hours <br />5.853175e-4 weeks <br />1.34697e-4 months <br /> a two second discharge of a portable CO2 fire extinguisher put out the fire and a re-flash fire watch was established. At 0437 hours0.00506 days <br />0.121 hours <br />7.225529e-4 weeks <br />1.662785e-4 months <br /> the fire contingency action procedure was exited.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS The small fire was extinguished quickly, did not pose an actual threat to the safety of the nuclear power plant and it did not affect equipment required for safe operation of the plant.
The function of the annunciator card in question is to provide two alarm circuits in the MCR (i.e., 1B-D3, BAT 1B Hi-Lo Level CH I-II and1B-H3, PZR Surge Line Lo Temp). The requirements for initiating a declaration of an emergency were never met. Interviews with the Operations crew on shift at the time of the incident noted their ability to perform duties necessary for the safe operation of the plant were never hampered. The health and safety of the public were not affected.
The failure probability of these resistors is based on their current drawing service over time.
The annunciator system is the only system that uses these resistors in an application where they are continuously drawing current. Other applications use these resistors, but their current drawing service is limited. When these resistors are exposed to current for extended periods of time, they heat up due to natural energy losses. The risk and consequence is not associated with the resistor alone, but rather the resistor being exposed to current draw for extended periods of time (typically months/years). When they are used in such a manner, there is a risk of combustion due to prolonged exposure to heat through current draw. The consequence of this application is that a resistor may combust.
However, after combustion, the resistor will typically create an open circuit within a very short time frame (due to their rapidly degrading state) removing the energy from the resistor limiting the combustion to a single card (which could possibly impact two annunciators).
The set up of the carbon resistors used on the annunciator card is particular to the Hathaway (annunciator) system.
3.0 CAUSE The cause of the annunciator card fire was age related degradation of the carbon resistor.
As the resistors age, they can either increase or decrease in resistance. In either case, the heat dissipation of the resistors increases causing the temperature to approach the high end of the specification for these resistors. This is defined as electrical fatigue.
4.0 IMMEDIATE CORRECTIVE ACTION(S) Fire Contingency Action procedure 0-FCA-0, Fire Protection — Operations Response was entered. The fire was promptly extinguished and a re-flash fire watch was established.
5.0 ADDITIONAL CORRECTIVE ACTIONS Engineering performed a thermography scan of both Unit 1 and Unit 2 Hathaway cabinets to determine if any of the other resistors were at risk for combustion. All scans showed that, while temperatures on some resistors were elevated, none of the temperatures were above the resistors specification. Resistors were replaced on eight cards that had elevated temperatures.
6.0 ACTIONS TO PREVENT RECURRENCE A plan has been developed to replace the carbon resistors in the Hathaway system annunciator cards with metal film resistors. Temperature monitoring for the Hathaway cabinets to trend component temperatures and drive component replacements is being established. Appropriate Preventive Maintenance strategies for Hathaway System will include a replacement frequency for the resistors.
7.0 SIMILAR EVENTS None 8.0 ADDITIONAL INFORMATION Unit 2 was operating at 100 percent power, Mode 1, and was not affected by this event.
Component Manufacturer: Hathaway Component Model Number: 744202 Component Type: Annunciator Point Card