05000336/LER-2014-003
| Millstone Power Station Unit 2 | |
| Event date: | 04-05-2014 |
|---|---|
| Report date: | 06-02-2014 |
| 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 |
| 3362014003R00 - NRC Website | |
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1. EVENT DESCRIPTION
On April 5, 2014, with Millstone Power Station Unit 2 (MPS2) at 100% power in Mode 1, the Enclosure Building was rendered inoperable during a maintenance activity. The condition was created at approximately 1025 and restored to operable status at 1300 on April 5, 2014. Plant Technical Specification (TS) 3.6.5.2 requires that the Enclosure Building shall be operable in Modes 1, 2, 3, and 4. The TS 3.6.5.2 Action is to restore the Enclosure Building to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or be in cold shutdown within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Therefore, the TS 3.6.5.2 Action requirements were met.
Since the Enclosure Building was inoperable from approximately 1025 until restored at 1300, the safety function of the Enclosure Building to limit radiological releases or mitigate the consequences in the event of a design basis accident could not be assured.
This condition is being reported pursuant to 10CFR50.73(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 and 10CFR50.73(a)(2)(v)(D), mitigate the consequences of an accident.
2. CAUSE
The condition was created as the result of a human performance error while performing a maintenance activity associated with the Enclosure Building boundary. Flexible boots are installed on the main steam safety valve vent pipes that extend through the Enclosure Building boundary. There are two sets of boots, upper and lower, on each main steam safety valve vent pipe. The upper boots are credited as the Enclosure Building boundary. Maintenance workers were sent out to remove the lower boots on eight main steam safety valve vent pipes to support a planned surveillance. The procedure also required that the upper boots be inspected to ensure they are installed. The workers were inattentive to the scope of the work activity and removed both the lower and upper boots.
Removal of the upper boots rendered the Enclosure Building inoperable.
3. ASSESSMENT OF SAFETY CONSEQUENCES
The purpose of the Enclosure Building is to contain, collect, and process potential containment leakage prior to its release to the environment to minimize radioactivity levels and resulting dose consequences from a design basis loss-of-coolant-accident. The Enclosure Building Filtration System (EBFS) is designed to establish and maintain the required negative pressure of -0.25 inches wg within the Enclosure Building (secondary containment).
The flexible boots were removed from eight main steam safety valve vent pipes. Each main steam safety valve also has a sliding bushing that can be credited for sealing the associated boundary, if the bushing is properly seated. Proper seating of the sliding bushings has not been completely reliable historically (as described in LER 2011-001-00). The pipes were inspected to assess the condition of the sliding bushings and five of the eight were seated.
Millstone Power Station Unit 2 05000336 Therefore, three of the main steam safety valve pipe boundaries were compromised. An Engineering Technical Evaluation was performed to estimate the system response based on the as-found condition of the main steam safety valve pipe boundaries. The Engineering Technical Evaluation concluded that the aggregate impact of the openings was too close to assure that the required negative pressure of -0.25 inches wg would be achieved. The condition existed for less than three hours.
NRC Branch Technical Position CSB 6-3 defines the secondary containment as "positive" for pressures less negative than -0.25 inch wg. This criterion accounts for wind loads and uncertainties in pressure measurements. With differential pressures less negative than -0.25 inch wg, a conservative assumption, consistent with design basis, is made that all primary containment leakage is released directly to the environment. Although NRC Branch Technical Position CSB 6-3 defines the secondary containment as "positive" for pressures less negative than -0.25 inch wg, it is estimated that a negative pressure close to -0.25 inches wg would have been achieved based on the as-found condition. Since this condition existed for less than three hours and estimated pressure would have been close to the -0.25 inches wg pressure acceptance criteria, the safety consequences are considered low.
4. CORRECTIVE ACTION
The Enclosure Building was restored to operable status. Since this was a human performance error, the event and lessons learned were communicated to the Millstone Site as a Station Human Performance Clock Reset.
Additional corrective actions are being taken in accordance with the station's corrective action program.
5. PREVIOUS OCCURRENCES
failed barriers. The corrective action associated with LER 2011-001-00 was to modify the barrier design to credit the upper flexible boots described above.
6. ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES Enclosure Building Filtration System (EBFS) [BD]