05000336/LER-2011-001
| Millstone Power Station - Unit 2 | |
| Event date: | 04-03-2011 |
|---|---|
| Report date: | 05-24-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3362011001R00 - NRC Website | |
1. Event Description On April 3, 2011, with Millstone Power Station Unit 2 (MPS2) in a refueling outage at 0% power in Mode 5, data taken during plant shutdown indicated that the Enclosure Building Filtration System (EBFS) [BD] had not met acceptance criteria rendering the Enclosure Building inoperable while MPS2 was in Mode 4. Plant Technical Specification (TS) 3.6.5.2 requires that the Enclosure Building shall be operable in Modes 1, 2, 3, and 4. 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 />. The-EBFS drawdown test was commenced at 04:49 on April 3, 2011 and Mode 5 (cold shutdown) was entered at 05:08 on April 3, 2011. Therefore, the TS 3.6.5.2 Action requirements were met.
Since the EBFS did not meet the acceptance, criteria for establishing 0.25" water gauge (wg) negative pressure, the safety function of the Enclosure Building to limit radiological releases 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.
2. Cause The direct cause for not meeting the Enclosure Building drawdown acceptance criteria was that sliding bushings on the main steam safety valves (MSSVs) exhaust piping had dislodged and not reseated. Review of the event did not identify any specific time of occurrence.
The apparent cause of this event was determined to be a design/application deficiency in the use of main steam safety valve (MSSV) exhaust piping sliding bushings as an Enclosure Building boundary.
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 (LOCA). The EBFS is designed to establish and maintain the required negative pressure of -0.25 inches wg within the Enclosure Building (secondary containment). The functional test is performed with one train of EBFS fans and conditions as close to design as possible. The Facility 1 test recorded a pressure differential of -0.22 inches wg. As such, the ability of this train of EBFS to draw sufficient negative pressure in secondary containment could not be assured.
NRC Branch Technical Position CSB 6-3 defines the secondary containment as "positive" for pressures greater 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. The Facility 1 test measurement of -0.22 inch wg is within 0.03 inches wg from the defined limit and represents some degree of negative pressure achieved in the secondary containment. Although leakage during 'positive pressure' periods cannot be determined, it is reasonable to assume that a fair amount of containment leakage that could leak from a design basis LOCA into the secondary containment under -0.22 inch wg would be captured and processed before release. The safety consequences associated with not achieving a negative pressure differential of -0.25 inches wg in the secondary containment is considered low.
4. Corrective Action A design change was implemented that no longer relies on the MSSV exhaust piping sliding bushings as Enclosure Building boundaries. Instead, improved boot seals located on the MSSV exhaust piping form the boundary for the MSSVs.
Additional corrective actions are being taken in accordance with the station's corrective action program.
5. Previous Occurrences Condition Report CR342330 describes a similar condition on July 19, 2009 in which the Enclosure Building did not meet acceptance criteria.
Energy Industry Identification System (EllS) codes are identified in the text as [XX].