05000397/LER-2013-001
| Columbia Generating Station | |
| Event date: | 01-07-2013 |
|---|---|
| Report date: | 03-08-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 48656 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3972013001R00 - NRC Website | |
PLANT CONDITIONS
Columbia was operating at 100% power. Standby Gas Treatment (SGT) System "A" was in an operability run following corrective maintenance on an electric heater disconnect. This did not contribute to the event.
EVENT DESCRIPTION
On January 7, 2013, at 1427 PST both doors [EllS: DR] of the 501 ft. elevation airlock [EllS: AL] entrance of the Reactor Building [EllS: NG] were simultaneously opened for a short period of time. This was the result of the failure of the interlock [EllS: IMEC] between the outer door (R304), and the inner door (R305).
Maintenance personnel were moving scaffolding through the Reactor Building outer door when an equipment operator opened the inner door to exit the Reactor Building. This equipment operator immediately closed the inner door and contacted the Main Control Room. It was estimated that door was only open for a few seconds.
The outer door was key locked closed until corrective actions could be implemented.
Having a condition where both doors in a Reactor Building airlock were opened simultaneously while not undergoing a planned evolution for maintenance or surveillance testing, results in an unintended entry into Technical Specification (TS) 3.6.4.1 Secondary Containment Condition A due to a failure to satisfy Surveillance Requirement (SR) 3.6.4.1.3. SR 3.6.4.1.3 requires verification that each secondary containment access inner door or each secondary containment access outer door in each access opening is closed. It was determined that this condition resulted in a loss of safety function (Secondary Containment) and was reported to the NRC (Event Notification 48656) at 1629 PST on January 7, 2013.
The reactor building is maintained at a negative pressure to minimize potential leakage of radioactivity to the environment. During normal operations, the Reactor Building Ventilation System maintains this negative pressure. During a design basis accident, the Standby Gas Treatment System [EllS: BH] maintains this negative pressure. This brief opening of the doors had no measurable impact on the Reactor Building pressure.
IMMEDIATE CORRECTIVE ACTION
Access through the airlock was restricted by locking the outer door (R304) until Reactor Building airlock inner door R305 was repaired.
Access to and from the Reactor Building was restricted to two airlock doors on the 441 ft. elevation. Personnel were stationed at the doors to ensure Secondary Containment integrity was maintained.
A temporary modification was implemented installing warning lights on the outsides of both airlock doors and a buzzer to alert station personnel when an individual is transiting the airlock.
The surveillance procedure for SR 3.6.4.1.3 was updated to verify proper operation of airlock doors and interlocks without compromising Secondary Containment.
CAUSE
A root cause evaluation of the issue determined that there were two root causes.
26158 R5 First Root Cause: A legacy design weakness for the Secondary Containment airlock door interlocks allows conditions where both the inner and outer doors can be opened at the same time.
Second Root Cause: Implementation of TSTF-18-A, Rev 1, at Columbia changed the wording of TS Bases for SR 3.6.4.1.2 and SR 3.6.4.1.3, reducing the clarity of the description of conditions that define Secondary Containment operability.
FURTHER CORRECTIVE ACTION
A design change is being evaluated that includes replacing the existing mechanical locks on all Secondary Containment airlock inner doors with a more reliable door locking device, improving the interlock logic to eliminate current design imperfections, and installing warning lights on the outside of the airlock to warn personal when the airlock is in use.
The TS Bases for SR 3.6.4.1.2 and SR 3.6.4.1.3 is being revised to ensure that the wording clearly identifies that Secondary Containment operability requires that one door in each airlock be closed at all times when Secondary Containment is required.
ASSESSMENT OF SAFETY CONSEQUENCES
There was no equipment damage, injuries or dose exposure to station personnel. There was no change in plant status or operating condition and there was no actual risk to the public at any time. There was no impact to public health and safety as the Primary Containment was intact with Primary Containment Isolation Valves operable. The Reactor Building Ventilation System maintained the designed negative pressure for Secondary Containment during the event.
There was a potential significance if there had been an event where the Primary Containment failed to isolate concurrent with a loss of coolant accident such that radioactive materials were released to the Reactor Building. Having both Reactor Building airlock doors open would potentially provide a release path for radioactive materials to enter the external environment or would inhibit the ability to draw down Secondary Containment within the time frame assumed in the dose analysis.
SIMILAR EVENTS
A similar event occurred at Columbia on December 30, 2012 (Event Notification 48639) and reported in LER 2012-007-00.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) INFORMATION CODES ElIS codes are bracketed where applicable in the narrative.
26158 R5