05000482/LER-2013-001
| Wolf Creek Generating Station | |
| Event date: | 01-11-2013 |
|---|---|
| Report date: | 03-11-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4822013001R00 - NRC Website | |
There were no structures, components or systems that were inoperable and contributed to the initiation or limited mitigation of the event.
DESCRIPTION OF THE EVENT
On January 8, 2013, at 0500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> Central Standard Time (CST), Required Action B.4.1 of Technical Specification (TS) 3.8.1 was entered when the 'B' diesel generator (DG) [EllS: EK] was declared inoperable for performing voluntary preplanned maintenance activities. One of the preplanned maintenance activities was the replacement of the o-ring between the cylinder head and valve housing on the number 7 cylinder. This maintenance activity was being performed due to an oil leak on the number 7 cylinder. During the removal of the rocker arm assembly, maintenance personnel contacted one of the eight cylinder head studs and identified that the stud was loose. At approximately 1213 hours0.014 days <br />0.337 hours <br />0.00201 weeks <br />4.615465e-4 months <br />, it was identified that the head stud was broken when it was removed by hand.
At 2342 hours0.0271 days <br />0.651 hours <br />0.00387 weeks <br />8.91131e-4 months <br /> on January 8, 2013, Surveillance Requirement (SR) 3.8.1.2 was performed satisfactorily on the 'A' DG in accordance with Required Action B.3.2 of TS 3.8.1. SR 3.8.1.2 verifies the DG starts from standby conditions and achieves steady state voltage and frequency. For the performance of SR 3.8.1.2, procedure STS KJ-005A, "Manual/Auto Start, Sync & Loading of EDG NE01," requires documenting any deficiencies, such as intake, exhaust or oil leaks. No deficiencies were identified.
The necessary replacement parts were available to complete repairs on the 'B' DG, however, with input from the DG vendor, it was determined that the replacement and testing of the 'B' DG to establish operability would result in exceeding the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Completion Time of TS 3.8.1, Required Action B.4.1.
The repair activities include draining the crankcase oil, removing the cylinder head, removing eight studs, non-destructive examination, replacing the broken stud and installing two new studs adjacent to the broken stud, reinstalling five of the removed studs, assembly of the cylinder head, refill the DG crankcase with oil and performance of post maintenance testing.
On January 10, 2013, enforcement discretion was sought to permit noncompliance with TS 3.8.1, i.e., to permit additional time to complete repairs and restoration of the 'B' DG before a plant shutdown was required. An additional 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> was requested to restore the 'B' DG to OPERABLE status such that the Completion Time of Required Action B.4.1 would expire at 0500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> CST on January 15, 2013.
The NRC granted approval of the requested enforcement discretion on January 10, 2013.
A ping (ring) test was performed on the remaining seven cylinder head studs on the number 7 cylinder.
This ping test was also performed on the studs of the other 13 cylinders on the 'B' DG. The ping test confirmed that there are no additional broken studs on the 'B' DG. The test only determines if the stud is fully fractured; it does not give an indication of a fatigue crack that has not yet resulted in failure.
Following the ping test, an ultrasonic test (UT) was performed on all remaining studs on the 'B' DG.
The UT can be performed with the studs installed and tensioned. The UT of all remaining studs was completed at approximately 2007 hours0.0232 days <br />0.558 hours <br />0.00332 weeks <br />7.636635e-4 months <br /> on January 9, 2013, 111 studs in total, did not identify any additional fatigue cracks in any studs of any other cylinders of the 'B' DG.
The DGs are Colt-Pielstick/Fairbanks-Morse model 14PC2.5 engines. The broken stud on the number 7 cylinder is original to the engine and has been in service for about 30 years. The overall run time of the 'B' DG is 2321.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The DGs are subjected to a one-hour run once a month, and a 24-hour run every 18 months. The most recent run on the 'B' DG was on December 14, 2012, which consisted of two one-hour runs, whereas the last 24-hour run was on March 11, 2012.
The broken stud was sent to Exelon PowerLabs, LLC, for a hardware failure analysis. Preliminary results of the analysis indicated the cause of the failure of the broken stud from the number 7 cylinder was cyclic fatigue. Beachmarks were identified on the fracture surface, which are indicative of fatigue.
A possible contributing cause may have been fretting corrosion identified on the outer diameter of the stud directly adjacent to the fracture initiation site. Fretting would cause a stress concentration on the outside diameter surface and make the material more susceptible to fatigue.
The replacement of the broken stud and two adjacent studs and testing to restore operability was completed at 2254 hours0.0261 days <br />0.626 hours <br />0.00373 weeks <br />8.57647e-4 months <br /> CST on January 12, 2013.
BASIS FOR REPORTABILITY
Wolf Creek Nuclear Operating Corporation (WCNOC) requested and received a notice of enforcement discretion (NOED) from the NRC to not enforce compliance with the actions required in TS 3.8.1 Required Action B.4.1 for a period of 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />. The 'B' DG was inoperable for a period longer than allowed by the Completion Time of TS 3.8.1, Required Action B.4.1. As such, the event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) for any operation or condition which was prohibited by the plant's TSs.
A review of this event determined that it did not meet the criteria for reporting under 10 CFR 50.73(a)(2)(v) as an event or condition that could have prevented the fullfillment of a safety function. As stated in NUREG-1022, Revision 2, Section 3.2.7, "The level of judgment for reporting an event or condition under this criterion is a reasonable expectation of preventing fulfillment of a safety function.
The original manufacturer of the equipment was contacted regarding this condition. The manufacturer provided written communication identifying the condition would not be immediately detrimental to the function of the engine and in their opinion, the engine would operate for a period of seven days at rated continuous capacity. The manufacturer completed a formal evaluation confirming that a single broken cylinder head stud would not result in overloading the remaining seven studs if the engine would be required to operate for a period of seven days at rated continuous capacity. The evaluation assumed that the remaining studs were undamaged, installed with proper tension, and met the design requirements. During repairs of the engine, detensioning of the seven intact cylinder head studs with the hydraulic tensioning tool required hydraulic pressures as would be expected for a properly tensioned stud. In addition, a review of the work order that installed the cylinder heads in October of 2006, also had hydraulic pressures necessary for proper tensioning. Quality control personnel performed non-destructive surface examination of the intact cylinder head studs and found no cracking.
The two intact adjacent studs to the broken stud were replaced as precautionary measure. Offsite lab analysis identified the broken cylinder head stud had yield strength above the design requirements identified in the manufacturers evaluation. Prior to the event on January 8, 2013, the broken cylinder head stud did not result in the inoperability of the 'B' DG.
The probable cause of this event is the fretting of the cylinder head stud from coming in contact with the cylinder liner during operation. The hardware failure analysis concluded fretting corrosion was identified on the outer diameter of the stud directly adjacent the fracture initiation site. Fretting would cause a stress concentration on the outside diameter surface and make the material more susceptible to fatigue. Examination of the cylinder head liners identified rubbing marks that suggest contact was being made between the cylinder head stud and the liner. The rubbing is the result of vibration of the diesel engine when the DG is in operation.
CORRECTIVE ACTIONS
Non-destructive surface examination for linear indications of the intact cylinder head studs was performed. The examination showed no relevant indications. The two adjacent studs and the broken stud were replaced. A ping test and an UT was performed on all remaining studs on the 'B' DG.
WCNOC performed a ping test and UT of the 'A' DG cylinder head studs on January 20, 2013. The testing results were acceptable.
A preventative maintenance activity is being established to perform UT of the DG cylinder head studs on a frequency consistent with 18 month 24-hour DG surveillance testing. An additional preventative maintenance activity is being established for the performance of a surface examination of the DG cylinder head studs on a 15-year interval. A detailed evaluation to determine the cause of the fretting is being performed by Fairbanks Morse.
SAFETY SIGNIFICANCE
The incremental conditional core damage probability (ICCDP) and incremental conditional large early release probability (ICLERP) was quantified for the requested additional time for restoring the 'B' DG.
The ICCDP calculated value was 4.39E-7 and the ICLERP calculated value was 7.89E-9. The results of the quantification are within the guidance threshold in Regulatory Issue Summary 2005-01, "Changes to Notice of Enforcement Discretion (NOED) Process and Staff Guidance.
WCNOC took the necessary actions per procedure SYS SY-120, "Sharpe Diesel Operation and Alignment to Site," to ensure that the Sharpe Station would be available during the period of the additional 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />. The Sharpe Station is capable of providing emergency back-up power for the Wolf Creek Generating Station (WCGS) in the event of a complete loss of all onsite emergency AC power, and grid perturbations or loss of a normal offsite power source to WCGS.
OPERATING EXPERIENCE/PREVIOUS SIMILAR OCCURRENCES
In Refueling Outage 15, Fall 2006, during a maintenance overhaul of the 'B' DG, the detensioning of a cylinder stud on the number 4 cylinder resulted in a broken cylinder head stud. It was determined that a crack existed on the stud prior to the detensioning. A failure analysis of this broken stud was not performed, as there was clear indication of a nick that caused a stress riser.