05000397/LER-2014-002
| Columbia Generating Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49898 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3972014002R00 - NRC Website | |
Background Information Operational experience available from INPO on December 10, 2013 through report IER-L4-13- 54, "Unprotected Direct Current Ammeters Result in Unanalyzed Conditions," Involved Control Room direct current (DC) ammeters [II) lacking overcurrent protection In their original plant wiring designs, which significantly degraded plant safety. Specifically, wiring designs for the DC ammeter circuits In the Control Room often included a shunt [17) in the current flow path from each DC battery [BTRY] or charger [(rm. Overcurrent protective devices were not used to isolate fault current In the ammeter wiring attached to the shunts at stations that reported this condition. A postulated fire in the station cable raceway, cable spreading room, or Control Room could cause an ammeter wire short to ground. If another DC wire from the opposite polarity on the same DC source shorted to ground, fault current would flow through the unprotected ammeter cable with the potential for overheating and initiating a secondary fire.
Either the secondary fire or the ground condition could adversely affect safe shutdown equipment and hinder the ability of the plant to safely shut down as committed to 10 CFR 50 Appendix R.
Identification of Occurrence and Plant Conditions Prior to Occurrence The identification of specific applicability to Columbia of the unfused DC ammeters condition occurred on February 27, 2014 after issuance of an extent of condition report; however, analysis to determine impact to safety systems was not completed until March 11, 2014. The NRC was notified through Event Notification 49898 on the same date, in accordance with 10 CFR 50.72(b)(3)(ii)(B) reporting requirements for the unanalyzed condition. The plant was operating in Mode 1 at 100 percent power, within design temperature and pressure conditions, and there were no structures, systems, or components that were inoperable at the start of the event and that contributed to the event.
Event Analysis and Extent of Condition Plant wiring design drawings for the DC Electrical System ammeter circuits that utilize a shunt in the current flow path for each DC battery or charger were analyzed to determine affected components.
The extent of condition analysis identified the following Columbia components that provide remote monitoring or Control Room indication associated with 24 VDC, 125 VDC, or 250 VDC circuits as being vulnerable to the condition:
- Division 1: a total of six ammeters that provide operator indication for associated battery or battery charger current output, and remote monitoring of current flow.
- Division 2: a total of six ammeters that provide operator indication for associated battery or battery charper current output, and remote monitoring of current flow.
26168A R3 NRC FORM 368A (01-2014) Columbia Generating Station
- Division 3: a total of two ammeters that provide operator indication for associated battery or battery charger current output, and remote monitoring of current flow.
DC ammeter circuitry cables for two of the Division 2 ammeters are non-safety related feeding their corresponding non-safety related DC ammeter. All other DC ammeter circuitry divisional cables referenced above are safety-related in class lE raceways feeding their respective safety-related DC ammeters.
Failures and Errors There were no safety system or safety component failures or errors resulting from this condition.
Safety System Responses
The plant's safety systems that could have been affected by this condition have not experienced a condition-related abnormal response. The condition has not resulted in declarations of system inoperabilities at the plant.
Cause of Occwrence This condition occurred because the original plant wiring design did not include overcurrent protection features to isolate fault current in the current flow path from the shunts for each DC station battery and/or charger to the remote ammeter circuits in the Control Room. The postulated event requires two concurrent extremely low resistance (hard) grounds; one on the positive side of the battery through the ammeter circuit wiring routed to the Control Room and one on the negative side of the battery. Because this is a very low probability event, it was likely not considered in the original design.
Assessment of Safety Consequences
The discovered condition is being reported under criterion in 10 CFR 50.73(a)(2)(ii)(B) — Any event or condition that results in the nuclear plant being in an unanalyzed condition that significantly degraded plant safety.
There have been no actual adverse nuclear, radiological, or industrial safety consequences resulting from the reported condition. The administrative controls of the Fire Protection Program, the availability of fire detection and suppression systems, the established compensatory measures, and a trained on-site fire brigade all combine to mitigate the event postulated in the scenario until permanent corrective actions described below are implemented.
26158A R3
Corrective Actions
Due to the complexity of the engineering review that is being conducted towards mitigation of this condition, a conservative decision was made to establish hourly fire tours for the fire areas of concern. Barrier Impairment measures have been implemented per established station procedures.
In addition to the immediate compensatory action consisting of hourly fire watches and establishment of barrier impairments in the affected areas, an engineering change is being developed to modify the affected DC electrical circuits to comply with the commitments of 10 CFR 50 Appendix R. Specifically, the scope of the engineering change will modify the existing DC ammeter circuits to include fuses, credited as overcurrent protective devices, to ensure that the postulated fire scenario does not impact the plant safety systems. Ensuing corrective action implementation work orders will include design changes, analysis updates, and physical work orders to bring affected DC circuits in full compliance with Columbia commitments to10 CFR 50 Appendix R.
Previous Occurrences
Columbia has not experienced similar occurrences in the past, based on a review of LER historical records.
26158A R3 NRC FORM 388A (01-2014)