05000423/LER-2013-002
| Millstone Power Station - Unit 3 | |
| 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)(i)(B), Prohibited by Technical Specifications |
| 4232013002R00 - NRC Website | |
1. EVENT DESCRIPTION
On November 21, 2012, a roll-up door in the auxiliary building at Millstone Power Station Unit 3 (MPS3) was found not fully closed after a security test. The door was open approximately 3/8 of an inch. Upon discovery, the operators entered Technical Specification Action Statement (TSAS) 3.6.6.2. The door was unlocked, fully closed and relocked. At the time the door was found open, the operators assessed this as a single opening and determined there was no loss of safety function. Therefore, this event was determined not reportable. This condition was entered into the station's corrective action program.
Engineering was assigned to perform a maintenance rule evaluation for this condition. On January 28, 2013, with MPS3 in MODE 1 at 100% power, engineering completed a maintenance rule evaluation of the condition where the roll-up door in the auxiliary building at MPS3 was found not fully closed after a security test. The total size of the opening under the door was determined to be approximately 27 square inches. In addition to being a security boundary, this door is also a secondary containment boundary, specifically a supplemental leak collection release system (SLCRS) boundary. Engineering concluded the partially open roll-up door exceeded any available margin from the previous surveillance drawdown test. This condition is being reported under 10 CFR 50.73(a)(2)(v)(C) 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.
During the investigation, it was determined the door had been in the partially open condition in excess of the plant's Technical Specification Action Statement (TSAS) allowed outage time, most likely the result of testing activities performed by security. It was determined the door had most likely been in the partially open condition from 0157 hours0.00182 days <br />0.0436 hours <br />2.595899e-4 weeks <br />5.97385e-5 months <br /> on November 17 through 1251 hours0.0145 days <br />0.348 hours <br />0.00207 weeks <br />4.760055e-4 months <br /> on November 21, 2012, fora total of 4 days, 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />, and 12 minutes.
Since the secondary containment boundary was not re-established within the requirements of the TSAS (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />), this condition is also being reported under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition which was prohibited by the plant's Technical Specifications.
Background Information:
The secondary containment is comprised of the containment enclosure building, engineered safety features building (partial), auxiliary building, main steam valve building (partial), and hydrogen recombiner building (partial). Together with the associated SLCRS, they mitigate the radiological consequences of postulated accidents for MPS3.
The SLCRS consists of two exhaust fans, each supplied from a separate emergency bus, two filter banks, and associated ductwork and dampers.
All SLCRS boundaries are established by use of low leakage doors (weather stripped), sealed building joints, sealed piping, conduit cable and ductwork penetrations, and boundary isolation dampers for ventilation systems. Therefore, containment leakage is contained in these areas until filtered by the SLCRS and the auxiliary building ventilation system filtration subsystem.
2. CAUSE
The direct cause of the event was the door had not been fully closed after completion of security testing.
Upon discovery the door was unlocked, fully closed and relocked. The testing procedure has been changed to test the door without requiring the door to be opened. Additional corrective actions are being taken in accordance with the station's corrective action program.
3. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences associated with the breach of secondary containment via the auxiliary building roll-up door is considered low. The purpose of secondary containment is to restrict leakage paths and associated leak rates of radioactive materials from the primary containment atmosphere.
The auxiliary building roll-up door not being fully closed resulted in a secondary containment breach.
The limiting scenario associated with an event is considered to be a design basis loss of coolant accident. The secondary containment in-leakage associated with this secondary containment breach was assessed. With the secondary containment breach, the ability of one train of SLCRS to draw a sufficient negative pressure in secondary containment could not be assured. However, since both trains of SLCRS were available, adequate SLCRS flow was available to compensate for the increased flow into the secondary containment supplied through the breach of secondary containment. This condition most likely existed for less than five days.
4. CORRECTIVE ACTION
Upon discovery the door was closed. Since this type of door requires a particular configuration that is beyond normal testing for a security door, the testing procedure has been changed to test the door without requiring the door to be opened. Additional corrective actions are being taken in accordance with the station's corrective action program.
5. PREVIOUS OCCURRENCES
MPS3 LER 2010-003-00, Secondary Containment Rendered Inoperable Due to Misaligned Dampers 6. Energy Industry Identification System (EllS) codes
- Auxiliary Building - NF
- Door - DR