05000336/LER-2011-003
| Millstone Power Station - Unit 2 | |
| Event date: | 09-03-2011 |
|---|---|
| Report date: | 10-26-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation |
| 3362011003R00 - NRC Website | |
1. Event Description At 09:31 on September 3, 2011, with Millstone Power Station Unit 2 operating at 100 percent power in Mode 1, the "A" train service water [BI] loop was declared inoperable when leakage from a degraded service water spool piece degraded beyond pre-established limits. Plant Technical Specification 3.7.4.1 stipulates with one service water loop inoperable, restore the inoperable loop to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or be in cold shutdown (Mode 5) within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Since the leak was unisolable, operators commenced a plant shutdown. Cold Shutdown Mode 5 was entered at 17:03 on September 4, 2011.
The service water leak was from a degraded piping flange located in the "A" train 10-inch service water line to the emergency diesel generator heat exchangers. A defect in the coating lead to accelerated galvanic corrosion of the flange. Full characterization of the size of the flaw and rate of degradation was limited. Therefore, in regard to leakage integrity, DNC established a leakage limit and commenced increased monitoring of the affected flange and surrounding pipe. A plant equipment operator was observing the degraded spool piece in accordance with the additional administrative controls for this degraded flange and identified that the leak had increased.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(A) as completion of a nuclear plant shutdown required by the plant's Technical Specifications.
2. Cause The direct cause of the service water leak was a degraded coating on the piping flange located in the "A" train 10-inch service water line to the emergency diesel generator heat exchangers. The degradation mechanism of the flange is attributed to galvanic corrosion of the carbon steel material. The carbon steel is anodic to the adjacent alloy surfaces.
3. Assessment of Safety Consequences The safety consequences of this event were low. This was a controlled plant shutdown without complications. No safety systems actuated. One train of service water remained operable. There was no loss of decay heat removal capability, because main and auxiliary feedwater pumps were available to feed the steam generators. Neither departure from nucleate boiling nor fuel centerline melt design limits were challenged. As such, there were no challenges to the fuel, reactor coolant system or containment fission product barriers. An evaluation of the degraded piping flange showed that structural integrity of the system was not compromised.
4. Corrective Action The degraded pipe spool flange was replaced.
An extent of condition review was conducted and additional inspections of similar piping were performed.
Additional corrective actions are being taken in accordance with the station's corrective action program.
5. Previous Occurrences No previous similar events were identified in the last ten years in which the condition of the service water system resulted in a plant shutdown required by plant Technical Specifications.
Energy Industry Identification System (EllS) codes are identified in the text as [XX].