05000331/LER-2015-001
| Duane Arnold Energy Center | |
| Event date: | 03-21-2015 |
|---|---|
| Report date: | 06-25-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 50914 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3312015001R01 - NRC Website | |
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I. Description of Event:
On March 21, 2015 at 0030, while operating at 100% power, the Control Room Supervisor (CRS) received a call that Door 225 and Door 228, both in Secondary Containment Airlock 216 had been opened concurrently. 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. The individuals involved immediately closed their respective doors upon encountering this unexpected condition. 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 an Event Notification was 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 (Reference EN#50914). 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 Secondary Containment airlock utilizes an interlock device with 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. Immediately following the event, the doors were verified to be functioning properly and no deficiencies were noted on either door. On March 22, 2015 at 1619 hours0.0187 days <br />0.45 hours <br />0.00268 weeks <br />6.160295e-4 months <br />, 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.
This event will not be reported as a safety system functional failure since an engineering analysis (Corrective Action ACE1968923-01) determined that the system is capable of performing its safety function during events when the airlock is open for less than 10 seconds. The post-LOCA dose calculation does not credit secondary containment integrity for mitigation of on-site and off-site doses for the first 5 minutes of the event. Therefore, this event is bounded by the existing dose calculation.
Duane Arnold Energy Center This event did not result in a safety system functional failure. There were no automatically or manually initiated safety system responses.
Cause of Event:
Technical Specifications Surveillance Requirement SR 3.6.4.1.2 requires one inner or one outer secondary containment airlock door to be closed at all times. A Root Cause Evaluation was conducted and determined the root cause of this event is that the airlock door interlock is not designed to prevent more than one airlock door from opening under all possible conditions.
Specifically, the interlock may allow opening both doors in an airlock if both permissive buttons are depressed simultaneously.
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. Additionally, signs have been installed at each airlock door instructing personnel who are accessing or leaving the airlock to wait 2 seconds after activating the interlock before opening the door. This corrective action introduces a time delay, which allows additional time for the interlock mechanism to actuate and prevent the other door from being opened. TSTF-551, "Revise Secondary Containment Surveillance Requirements" will be implemented once approved.
V. Additional Information:
Previous Similar Occurrences:
A. review of DAEC Licensee Event Reports from the past 5 years identified three similar occurrences, reference LER 2013-006, LER 2014-002 and LER 2014-003.
A review of the corrective action program identified additional occurrences of airlock conditions causing momentary secondary containment inoperability - seven additional occurrences in the past two years, with five of those occurring in the last year.
EIIS System and Component Codes:
I EL 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).