05000458/LER-2014-001
| River Bend Station - Unit 1 | |
| Event date: | 01-09-2014 |
|---|---|
| Report date: | 03-10-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4582014001R00 - NRC Website | |
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River Bend Station - Unit 1 SEQUENTIAL
REPORTED CONDITION
On January 9, 2014, with the plant operating at 100 percent power, the final review of industry operating experience regarding the adverse effects of unfused direct current ammeter circuits in the main control room determined the described condition to be applicable to River Bend Station, resulting in a potentially unanalyzed condition with respect to 10 CFR 50 Appendix R analysis requirements.
River Bend Station does have unfused ammeter circuits. This normally is not a concern, since two failures are required to cause the problem described in the document referenced above; an ammeter lead must short to ground, and another lead of the opposite polarity must also short to ground. Either of these events by itself will result in an alarm annunciation, but not cause equipment damage. This allows for locating and repairing the problem before the second failure occurs. However, during a fire, there is a greater chance that these two failures could happen simultaneously, or before a single failure can be repaired. If an unfused ammeter lead shorts to ground while another conductor of the opposite polarity shorts to ground, the unfused path through the lead could allow enough current to flow that the lead will heat up and damage other wiring, with potentially adverse effects on the ability to safely shut down the plant in the event of a fire in the main control room.
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B). Interim compensatory measures have been implemented.
CAUSAL ANALYSIS
This condition has existed since the plant was constructed. Neither the original design criteria nor any industry standards in effect at the time accounted for the possibility of the multiple short-to-ground failure mode and therefore, did not require overcurrent protection for remote ammeter circuits.
CORRECTIVE ACTION
This condition was documented in the station's corrective action program. Compensatory measures were established to restore margin potentially reduced by postulated fire scenarios. The compensatory actions credit operator presence in the plant for maintaining heightened awareness and taking action to minimize fire hazards. Compensatory actions are incorporated in pertinent operating procedures. The compensatory actions remain in effect.
SAFETY SIGNIFICANCE
There have been no actual adverse safety consequences resulting from the reported condition. The administrative controls of the fire protection program, the availability of fire detection and suppression systems, and a trained on-site fire brigade all combine to make it highly unlikely that a fire could occur and progress in a manner that actually leads to the event postulated in this scenario.