05000219/LER-2014-001
| Oyster Creek, Unit 1 | |
| Event date: | 07-11-2014 |
|---|---|
| Report date: | 09-08-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2192014001R00 - NRC Website | |
Plant Conditions Prior To Event
Event Date: July 11, 2014 Event Time: 0314 EDT Unit 1 Mode: Run Power Level: 56%
Description of Event
Reactor Startup from the 1F34 Forced Outage began at 2026 hours0.0234 days <br />0.563 hours <br />0.00335 weeks <br />7.70893e-4 months <br /> on 07/09/14 with criticality achieved at 0125 hours0.00145 days <br />0.0347 hours <br />2.066799e-4 weeks <br />4.75625e-5 months <br /> on 07/10/14. At 1830 on 7/10/14, with Reactor Power at approximately 10% of rated thermal power (RTP), condenser vacuum was fully established and stabilized at approximately 28.7” H2O. On July 11, 2014 at approximately 0312 EDT, during reactor power ascension, with reactor power at approximately 56% RTP, main condenser vacuum began to degrade. ABN 14, Loss of Condenser Vacuum, was entered due to the degrading condenser vacuum. Reactor power was lowered to approximately 44% in an attempt to stabilize plant conditions.
Vacuum continued to degrade and at 0314, a manual reactor SCRAM was inserted when condenser vacuum degraded below 23.5” H2O.
Following the reactor scram, operations and maintenance personnel identified two holes, one approximate 2"x6" and the other approximate 2”x3”, on the last convolute of the downstream side of Y-1-26 ('B' Condenser Steam Inlet Expansion Joint).
Analysis of Event
The basic function of the expansion joint, Y-1-26 (Sola Basic Industries, model 97-5516) is to provide a flexible pressure retaining connection to absorb motion in the system caused by thermal expansion and low levels of vibration. The need to be flexible requires the expansion joint to be fabricated from 1/32” (wall thickness) commercial grade stainless steel A240 type 304. The design of the expansion joint is such that an upstream and a downstream bellows in series work together to account for the required lateral movements.
On October 6, 2013, the upstream bellows was repaired due to a circumferential fracture. Between October 7, 2013 and July 09, 2014 the upstream fracture in the bellows was repaired with standard fiberglass wraps, high temperature carbon fiber wraps, and the application of Belzona. Repeated wrapping of the upstream side of the bellows most likely restricted the allowable movements of that bellows, requiring the downstream bellows to account for the additional movement.
Additionally, in July 2014, a reheater relief valve (V-1-132) upstream of the bellows (Y-1-26) was confirmed to be leaking past its seat. The combination of the leak-by of the relief valve with the restricted movement of the bellows created increased fatigue on the downstream bellows.
Inspection of the 2”x6” hole showed a circumferential fracture from end to end of one of the three bellow convolutions. There was also a tangential fracture at the 2”x3” hole on the same bellow convolution. A review of photos of the failure along with discussions with Subject Matter Experts (SMEs) from Exelon Corporate engineering determined the likely cause of the failure to be from fatigue cracking.
Cause of Event
Upon completion of an Equipment Apparent Cause Analysis (EACE) it was determined that the apparent cause of the bellows failure was fatigue failure due to additional loading of the downstream bellows caused by the repeated leak repairs and a leaking relief valve causing induced vibration which accelerated the bellows fatigue process leading to the failure of the unwrapped downstream portion of the bellows in Y-1-26.
Corrective Actions
The expansion joint (Y-1-26) and the relief valve (V-1-132) were replaced.
Previous Occurrences
Licensee Event Report, LER-2013-002, was submitted in the last two years.
Component Data Component IEEE 805 System ID IEEE 803A Component Expansion Joint SM EXJ