05000331/LER-2013-006
| Duane Arnold Energy Center | |
| Event date: | 7-18-2013 |
|---|---|
| Report date: | 07-14-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 49657 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3312013006R00 - NRC Website | |
I. Description of Event:
On December 18, 2013 at 0830 CST, while operating at 100% power, two doors in Secondary Containment Airlock 216 were opened concurrently. An individual was in the process of pushing open Door-225 (Access Control Airlock Door) when another individual opened Door-227 (Access Control to Stairwell #8). Both individuals realized that it was not allowed to have both doors opened and closed their respective door. The doors being open at the same time caused a failure to meet SR 3.6.4.1.2 to verify that either the outer door(s) or the inner door(s) in each Secondary Containment access opening are closed. The identified condition caused Secondary Containment to be considered inoperable per TS LCO 3.6.4.1. Upon discovery, immediate action was taken to close the doors. This action allowed SR 3.6.4.1.2 to be met, and restored Secondary Containment to an operable status.
This resulted in an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> reportable event. The Resident Inspector was notified, and Event Notification Number 49657 made pursuant to 10 CFR 50.72(b)(3)(v)(C) due to a condition at the time of discovery that prevented the fulfillment of the Secondary Containment safety function.
Secondary containment leak tightness is required to ensure that the release of radioactive materials from the primary containment is restricted to those leakage paths and associated leakage rates assumed in the accident analysis and that fission products entrapped within the secondary containment structure will be treated by the Standby Gas Treatment System prior to discharge to the environment.
The interlock devices use a steel plate and an adjustable permanent magnet (mounted on the door), and an electromagnet (on the door frame) arranged in an electrical circuit so that door(s) are held closed and/or are allowed to open. The permanent magnet interacts with a reed switch inside the electromagnet housing. It was determined that a door magnet on Door 225 allowed the airlock to sense Door 225 was closed prior to it latching closed. This resulted in a red indicating light turning off before the door was latched and before the electromagnet was close enough to hold the door closed. Because of this, the interlock no longer prevented Door 227 from opening, which ultimately led to both doors being opened concurrently. Subsequently, the magnet's strength was adjusted so that the red light turned off just as the door latched. On December 18, 2013 at 0906 CST, surveillance testing was performed satisfactorily per Surveillance Test Procedure (STP) 3.6.4.1-02, Secondary Containment Airlock Verification.
There were no radiological releases associated with this event. There were no other structures, systems or components inoperable at the start of this event that contributed to the event.
II. Assessment of Safety Consequences:
There were no actual safety consequences associated with this event; the potential safety consequences were minimal. Both doors on the airlock were open simultaneously for less than 10 seconds, and were able to close immediately upon discovery of the condition. According to the DAEC UFSAR, secondary containment integrity is not achieved until 10 seconds following an isolation signal.
The interlock is not credited as performing a safety function. It is used to prevent inadvertent openings and should be maintained to ensure reliable operation.
This event did not result in a safety system functional failure. There were no automatically or manually initiated safety system responses.
III. Cause of Event:
An Apparent Cause Evaluation was completed. The evaluation identified the apparent cause of Door- 225 and Door-227 being open at the same time as excess permanent magnet strength. After the permanent magnet was adjusted, the interlock was able to prevent both doors from opening.
IV. Corrective Actions:
An operational check of the Secondary Containment door interlocks is performed monthly via STP 3.6.4.1-02, Secondary Containment Airlock Verification. STP 3.6.4.1-02 will be revised to check the permanent magnet for proper adjustment by verifying that the red interlock light(s) turn OFF as close to when the door latches as possible. The permanent magnet will then be adjusted as needed.
V. Additional Information:
Previous Similar Occurrences:
A review of DAEC Licensee Event Reports from the past 5 years did not identify any similar occurrence.
A review of the corrective action program identified additional occurrences of airlock conditions causing momentary secondary containment inoperability - one additional occurrence in 2013 and eight occurrences in 2012.
EIIS System and Component Codes:
IEL Interlock Reporting Requirements:
This event is being reported as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material, 10CFR50.73(a)(2)(v)(C).