05000397/LER-2013-007
| Columbia Generating Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3972013007R01 - NRC Website | |
Plant Conditions
At the time of event, the plant was operating in Mode 1 at 100% power. There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
Event Description
On August 25, 2013 at 1818 hours0.021 days <br />0.505 hours <br />0.00301 weeks <br />6.91749e-4 months <br />, during a suddenly occurring.thunderstorm, the secondary containment [NH] pressure was not maintained greater than or equal to 0.25 inches of vacuum water gauge. This condition existed for a period of no greater than 160 seconds. Because Reactor Building [NG) pressure increased above -0.25 inches of water gauge (inwg) secondary containment was logged as inoperable in the surveillance tog and Technical Specification 3.6.4.1 Action A was entered.
Extent of Condition This condition of Reactor Building differential pressure momentarily rising above the prescribed limit is specific to the Reactor Building Heating Ventilation and Air Cooling (HVAC) [VA] and Standby Gas Treatment (SGT) [BH] systems, and their capability to establish and maintain secondary containmentvacuum. No other systems are affected as a result of this condition.
Immediate Corrective Action Operations took manual control of the Reactor Exhaust Air (REA) differential pressure (dP) controller [PDC] and quickly adjusted Reactor Building Exhaust Air flow to restore secondary containment pressure to less than -0.25 inwg. Later, after the storm had passed, operators placed the controller back in automatic.
Cause
During normal plant operation, the non-safety related Reactor Building HVAC system maintains environmental conditions within the Reactor Building at a vacuum with respect to the outside. The cause of secondary containment not being maintained greater than or equal to -0.25 inwg was due to improper tuning of the. Reactor Building Outside Air (ROA) differential pressure controller. The transient response of the controller was not properly tuned to compensate for sudden pressure changes that are expected during operation of the pressure control system.
Operating Experience & Previous Occurrences A loss of the ability to maintain secondary containment below the required negative pressure has occurred at Columbia Generating Station (Columbia) twice in the past two years. On July 24, 2012, reported under LER 2012-003-00, secondary containment was compromised due to an inadvertent trip of one set of the redundant Reactor Building HVAC fans, during ongoing maintenance on the SGT system.
The second instance occurred on December 10, 2011, reported under LER 2011-004-00. Secondary containment was declared inoperable and the cause was determined to be ice buildup and subsequent release on exterior equipment supplying the Reactor Building HVAC system.
Each of these events have associated corrective actions and the causes are not applicable to the current cause of weather related pressure excursion, nor would corrective actions from these past events prevented occurrence of this event.
28158A R3 NRC FORM 368A (01-2014) S. LER NUMBER 3 PAGE L DOCKET 1. FACILITY NAME Columbia Generating Station YEAR 2013 - 007 - 01 NRC FORM MA (11.2014) LICENSEE EVENT REPORT (LER) U.S. NUCLEAR REGULATORY COMMISSION
CONTINUATION SHEET
Further Corrective Actions The potential for these momentary pressure excursions will continue to occur as there has been no change in the design of the equipment. To minimize recurrence actions are being taken to update the current calibration procedures to reflect the optimum control system tuning for the REA/ROA system, and increasing margin in secondary containment by lowering the differential pressure set point on the REA controllers. Columbia is also investigating methods to alert operations to a potential issue with secondary containment before the Technical Specification secondary containment pressure is reached.
Assessment of Safety Consequences
This event resulted in an unplanned entry into LCO 3.6.4.1.A. Secondary containment pressure was above -0.25 inwg for no more than 160 seconds. The peak pressure during this event was -0.03 inwg. While the actual pressure was beyond the range allowed by Technical Specifications, the purpose of maintaining a slight vacuum is to assist in drawdown of secondary containment to support accident response of the safety related SGT system. Existing engineering analysis demonstrates that for this event, the drawdown credited In accident response could have been attained using either of the two available trains of the SGT system, thus there were no potential safety consequences. There was no actual safety consequence associated with this event since no event involving radiological hazards were experienced during the event.
Energy industry Identification System (EIIS) Information Energy Industry Identification System (EIIS) Information codes from IEEE Standards 805-1984 and 803-1983 are represented in brackets as pog and [XXX) throughout the body of the narrative.
26158A R3