05000336/LER-2012-001
| Millstone Power Station - Unit 2 | |
| Event date: | 06-07-2012 |
|---|---|
| Report date: | 12-19-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3362012001R01 - NRC Website | |
1. Event Description At 19:30 on June 7, 2012, with Millstone Power Station Unit 2 (MPS2) operating at 100 percent power in Mode 1, it was determined that a series of gaps in a high energy line break (HELB) barrier rendered equipment in the west 480VAC switchgear room inoperable. The Shift Manager entered Technical Specification (TS) 3.8.2.1 Action, TS 3.8.2.1A Action "c" and TS 3.3.3.5 Action "a" at 19:30 on June 7, 2012. The openings were sealed and the equipment restored to operable status at 16:05 on June 8, 2012.
The openings in the HELB barrier were identified during maintenance activities to support a plant modification, when maintenance personnel identified a series of gaps in a wall that separates the MPS2 turbine building from the west 480VAC switchgear room. The west 480VAC switchgear room contains 480 volt emergency load center 22E [EC]. TS 3.8.2.1 "A.C. Distribution — Operating" applies in Modes 1, 2, 3, and 4 and requires certain electrical busses to be operable. With less than the complement of A.C. busses operable, TS 3.8.2.1 Action requires restoration of the bus to operable status within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-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 gaps in the HELB barrier also affected the operability of Inverter 5 and Inverter 6 [EE]. TS 3.8.2.1A "A.C.
Distribution — Operating" applies in Modes 1, 2 and 3 and requires Inverters 5 and 6 to be operable. Action "c stipulates with Inverters 5 and 6 inoperable or unavailable for automatic transfer via static switches VS1 and VS2 to power busses VA-10 and VA-20, respectively, restore the inverters to operable status or restore their automatic transfer capability within 7 days.
The remote shutdown instrumentation panel (C-21) [PL] is also located in the west 480VAC switchgear room. TS 3.3.3.5 "Remote Shutdown Instrumentation" applies in Modes 1, 2 and 3 and requires remote shutdown " stipulates with the number of operable remote instrumentation channels to be operable. TS 3.3.3.5 Action "a shutdown monitoring instrumentation channels less than required by TS Table 3.3-9, restore the inoperable channel to operable status within 7 days.
This condition is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B) "any operation or condition prohibited by the plant's technical specifications". At the time of discovery, appropriate and timely actions were taken that met the TS action requirements. There is evidence that this condition has existed since initial construction. Since the HELB barrier required to maintain operability of bus 22E, Inverters 5 and 6, and remote shutdown panel C-21 was non-functional for an extended period, the Actions for TS 3.8.2.1, TS 3.7.2.1A and TS 3.3.3.5 were historically not met.
Upon further engineering analysis, it was determined in addition to the above, that for limited exposure time of 66 days when compensatory cooling was in place for the west 480VAC switchgear, if a failed steam line was not isolated, the operability of DC switchgear could have been impacted. Therefore, this condition did not meet the LCO requirements of TS 3.8.2.3 "D.C. Distribution". It was also determined that for limited exposure time of 2 days when compensatory cooling was in place for the east 480VAC switchgear, if a failed steam line was not isolated, the operability of 480 volt emergency load centers 22E and 22F could have been impacted. Therefore, this condition did not meet the LCO requirements of TS 3.8.2.1 A.C. Distribution - Operating" For limited exposure times of 66 days when compensatory cooling was in place for the west 480VAC switchgear and 2 days when compensatory cooling was in place for the east switchgear, completion of safety functions could have been prevented for certain postulated high energy line breaks. Therefore, this condition is also being reported pursuant to 10 CFR 50.73(a)(2)(v)(A),(D).
2. Cause This is a historical condition dating back to original construction. The gaps were located high in an overhead in the wall to ceiling interface of Q-decking at the north wall of the west 480VAC switchgear room, not visible during normal plant observation. The gaps were discovered because scaffolding had been erected to support an independent plant modification in an area not typically accessible. The apparent cause, dating back to original construction, is that no construction details were developed for sealing Q-decking when the ribbing is perpendicular to the top of the wall.
3. Assessment of Safety Consequences As a design feature, the HELB barrier is in place to limit the effects of a steam environment created by a HELB in the turbine building. The gaps in the barrier associated with the west 480VAC switchgear room area were located high in the overhead and partially obstructed by building steel. The specific HELB gaps were 91 unsealed openings in the Q-deck (ceiling structure), with a total gap area of approximately 3 square feet.
There were no adverse consequences to the health and safety of the public or the plant and its personnel resulting from the gaps in the HELB barrier. This report assumes the steam from a high energy line break in the turbine building would have affected the operability of safety related equipment in the west 480VAC switchgear room. When necessary, MPS2 has operated in the past with normal ventilation to either the west or east 480VAC switchgear rooms out of service and compensatory cooling established. A more detailed engineering analysis was conducted by Dominion and the NRC through the Significance Determination Process (SDP) (ADAMS ACCESSION NO:
ML13312A992). The gaps would allow high energy steam to enter the switchgear rooms, causing the electrical equipment inside to potentially fail. This was assessed as very low safety significance as determined by a detailed risk assessment using SAPHIRE 8 and a modified main steam line break outside of containment event tree from the Millstone 2 SPAR model.
4. Corrective Action The gaps were sealed. Plant walk-downs were conducted to assess the extent of condition and whether any similar conditions existed. Additional gaps in the same wall did not affect safety related equipment. No other gaps were identified.
Additional corrective actions are being taken in accordance with the station's corrective action program.
Previous Occurrences 5.
There are no previous occurrences identified.
Energy Industry Identification System (EMS) codes are identified in the text as [XX].