05000219/LER-2014-003, Regarding Technical Specification Prohibited Condition Caused by Emergency Diesel Generator Inoperable for Greater than Allowed Outage Time
| ML14325A598 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 11/11/2014 |
| From: | Dostal J Exelon Generation Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RA-14-095 LER 14-003-00 | |
| Download: ML14325A598 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2192014003R00 - NRC Website | |
text
A Exelon Generation 10 CFR 50.73 RA-14-095 November 11, 2014 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 - 0001 Oyster Creek Nuclear Generating Station Renewed Facility Operating License No. DPR-16 NRC Docket No. 50-219
Subject:
Licensee Event Report (LER) 2014-003-00, Technical Specification Prohibited Condition Caused by Emergency Diesel Generator Inoperable for Greater than Allowed Outage Time Enclosed is LER 2014-003-00, Technical Specification Prohibited Condition Caused by Emergency Diesel Generator Inoperable for Greater than Allowed Outage Time. This report is submitted in accordance with 10 CFR 50.73(a)(2)(i)(B), any operation or condition prohibited by the plant's Technical Specifications.
This event did not affect the health and safety of the public or plant personnel. This event did not result in a safety system functional failure. There are no regulatory commitments made in this LER submittal.
Should you have any questions concerning this letter, please contact Michael McKenna, Regulatory Assurance Manager, at (609) 971-4389.
Respectfully, Jeffrey P.
al Plant Manager Oyster Creek Nuclear Generating Station Enclosure: NRC Form 366, LER 2014-003-00 cc:
Administrator, NRC Region I NRC Senior Resident Inspector - Oyster Creek Nuclear Generating Station NRC Project Manager - Oyster Creek Nuclear Generating Station LJW9)
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 (01-2014)
Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by LICENSEE EVENT REP RT (LER) internet e-mail to lntocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and (See Page 2 for required number of Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC for each block) 20503. If a means used to impose an information collection does not display a currently valid OMB digits/characters fcontrol number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. PAGE Oyster Creek, Unit 1 05000219 1 OF
- 4. TITLE Technical Specification Prohibited Condition Caused by Emergency Diesel Generator Inoperable for Greater than Allowed Outage Time
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTH DAY YYEAR EREOUENTUAMB EV MONTH DAY YEAR F
N/A N/
NUMERNO.
//
I i
i iFACILITY NAME DOCKET NUMBER 0 1 2 01 4 2014 003
- - 00 1 I No/,iA N/A
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)
El 20.2201(b)
El 20.2203(a)(3)(i)
El 50.73(a)(2)(i)(C)
F] 50.73(a)(2)(vii)
Nl 20.2201(d)
El 20.2203(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4)
El 50.73(a)(2)(ii)(B)
El 50.73(a)(2)(viii)(B) 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
El 50.73(a)(2)(iv)(A)
El 50.73(a)(2)(x)
El 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El 73.71(a)(4)
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
El 50.73(a)(2)(v)(B)
El 73.71 (a)(5) 100 El 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C)
El OTHER El 20.2203(a)(2)(vi)
[
50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specity in Abstract below or in
Description of Event
On July 28, 2014, Emergency Diesel Generator No. 2 (EDG-2) was being operated for its bi-weekly one-hour load test run, when alarms "EDG 2 ENGINE TEMP HI" and "EDG 2 DISABLED" were received. Operations manually shut down EDG 2 due to an apparent cooling problem with the diesel engine.
During initial troubleshooting, the fan duct was opened to access the upper fan shaft and it was found that the cooling fan shaft had failed. Without the fan in service, radiator heat transfer performance was degraded, leading to high jacket water (coolant) temperatures and associated alarms.
Equipment Description Oyster Creek Nuclear Generating Station is equipped with two identical EDG units. The function of the EDGs is to provide AC power to the Class 1 E busses upon a loss of the off-site power. The EDG must be able to provide this power rapidly, within 10 seconds, upon demand. This condition is referred to as a fast start signal.
If started with a fast start signal, a high jacket water temperature condition will not trip the EDG.
The EDG units are General Motors Corporation, Electromotive Division (EMD) Model EMD 20-645E4, 20-cylinder, 2-cycle, turbo-intercooled diesel engines, which drive their respective EMD A20C AC generators.
The EDGs are installed in enclosures inside the EDG vaults. Engine auxiliaries retain a locomotive-type layout with the radiators in duct compartments over the engines. Cooling air to each engine is drawn into this duct by a large fan at the south end of the enclosure. The fan is supported and rotated by a belt-driven shaft that is in turn rotated by a power-takeoff shaft connected to the engine. The failure location was at a groove in which a bearing retainer is mounted. The failure location could not be visually inspected without removing the bearing retainer and as such, would not be identified on operator rounds or normal maintenance.
Analysis of Event
Following the event, a complex troubleshooting team was formed to identify and investigate possible failure modes. Bearing issues, bent fan shaft, fan imbalances, metallurgical defect, and incorrect belt tension were considered. Some of the follow-up actions to support or refute possible failure modes required laboratory failure analysis. Consequently, the shaft section and bearing parts were quarantined and sent to Exelon PowerLabs (PL) for examination.
PowerLabs Report Summary Laboratory evaluations indicated the shaft failure was caused by rotational bending fatigue. Investigations revealed that based on the smooth, flat, and planar fracture surface features and relatively small final overload area (approximately 10-20% of the total fracture surface), propagation occurred by a high cycle-low stress fatigue mechanism. The cracking initiated at the shaft groove diameter transition, which would act as a high stress concentrator. Multiple ratchet marks (which are indicative of multiple crack planes) were observed around the periphery and imply that propagation was due to rotating-bending fatigue. No material defects were observed on the shaft outer surface that would have contributed to the failure initiation.
PL also noted that the bearing contained minor damage that was considered collateral damage from the failed shaft. There was no misalignment of the roller paths on the inner or outer races. In addition, the pillow block base and bolt holes were not worn, which suggested the base was secured during operation.
Findings presented in the PL report lead to a review of fan belt tension, which was initially identified as a potential failure mode. Per Maintenance procedure MA-OC-86103-100, "Diesel Generator Fan Belt Replacement," belt tension is set at 60Hz (-5800 lbs hub load equivalent). This setting was discussed with Engine Systems Incorporated (ESI) and found to be 37% higher for the fan horsepower requirement. Technical Evaluation 01686101-06 concluded 47.4 +/-2 Hz (-3600 lbs hub load equivalent) is an appropriate setting Structural Integrity Associates (SIA) was consulted to perform a stress analysis to determine potential causes that could further explain why the fan shaft failed. Additionally, they were asked to analyze fatigue crack initiation and growth, and to determine at what point in the EDG-2 operating cycle the fan shaft had remaining life of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> (EDG mission time).
Structural Integrity Associates Report Summary SIA investigations revealed that crack growth occurred during the last test run (less than one hour).
Beachmarks, features that typically define stops and starts of fatigue crack growth, combined with simulated crack growth data show that the shaft failure would have happened during the last load test. The remaining shaft life would have met the 24-hour mission time approximately four test cycles or approximately 43 days prior to shaft failure.
SIA investigations indicated that a hub load of 5800 lbs. would produce stresses in the reduced (shaft groove transition) section of the shaft of 19.15 ksi, which is at or just below the endurance limit of the shaft material.
Endurance limit is defined as the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. A slight increase in stress for whatever reason would cause the predicted fatigue life to move from infinite to approximately 250 hours0.00289 days <br />0.0694 hours <br />4.133598e-4 weeks <br />9.5125e-5 months <br /> based on a change for 19.15 ksi to 20.5 ksi, (about 165 tests). A smaller stress increment above the 19.15 ksi, or smaller, non-continuous time above the 19.15 ksi (e.g., run starts at full belt tension plus stress increment; over time belt loosens slightly to drop stress below 19.15 ksi) would increase the number of cycles and tests to crack initiation.
SIA concluded that at the loads the shaft might experience, fatigue life (both initiation and growth) would be very sensitive to small changes in the load or Stress Concentration Factor (SCF), a dimensionless number used to quantify how concentrated the stress is in a material. The stress amplitude at some point in time must have been higher than 19.15 ksi or that SCF is higher than three for the crack to initiate. SCF is affected by the dimensions of shaft transitions, such as the bearing groove; the smaller the radii or fillet, the higher the SCF.
SIA determined the potential causes of rotational bending fatigue as: 1) Hub loading exceeded the material properties, or 2) defect that produced a stress riser at the groove location (more notched or angular versus a U-shape with radius bends). Either condition could result in stress conditions where the SCF exceeds its design limit of less than or equal to three.
Cause of Event
When reviewing the timeline for potential causes in support of SIA's conclusions, two instances were found worth considering, a fan shaft failure in 1993 and added belt tension in 2005. First, the shaft failure in 1993 shows how susceptible the bearing grooves locations are to imperfection. Questions regarding design details and manufacturing practices were discussed with ESI. Design information, such as dimensions, is limited to vendor manual references, and specifics of bearing retaining groove, as stated by ESI, was left at the discretion of the machinist.
Second, the then recommended tension frequency of 60 Hz is important because it increases the likelihood of the shaft exceeding the endurance limit due to reducing stress margin. An important result from the SIA investigations is that 60 Hz of tensioning frequency, although very high, was most likely not enough to single-
handedly cause the failure. It was noted that 60 Hz corresponds to approximately 19.15 ksi of stress, which is at or just below the endurance limit. If the shaft was consistent in diameter and stress relief contour profile throughout, it could effectively operate over an infinite number of cycles. Therefore, the plausible failure mode is attributed to manufacturing deficiency or imperfection such as a scratch or nick that increased the stress above the endurance limit. A manufacturing deficiency is the more likely of the two causes based on that fact the PL investigation found no instances of imperfections. Furthermore, the EDG sets are adapted from transportation engines. The original Equipment Manufacturer Design (EMD) was qualified for safety-related service and subsequent parts procured through companies such as ESI. The shafts in service as well as spares were provided to the plant in the late 1960s. Discussions with ESI indicated limited information (dimensions and material) was on file for shaft part number 8441753. In addition to stress relief contours, other factors such as surface finish, heat treatment, etc., can affect material fatigue resistance.
The apparent cause of the failure was found to be higher than average stress concentration factor due to manufacturing deficiency at the grooved location.
The high cycle fatigue mechanism as evaluated by PL and SIA concluded that either excessive hub loading (belt tension) or a notch type defect were the most likely failure mechanisms. It is important to note that the SIA report data approximates material properties of the shaft. Design and fabrication variables such as material hardening, surface finish, EMD shaft design process, machinist proficiency, etc. are variables whose combined impact cannot be assessed.
ESI indicated that limited detail is available on the shaft design, including control of parameters such as stress relief profiles, and finish. Further, control of the groove profile was left to the skill of the fabricating machinist. Minimal design data also challenges the ability to assess shaft health. It is likely that the failed shaft had lower stress margin due to unfavorable variances in fabrication. This condition would have lowered the margin between operational stresses and design stresses leading to its transition from infinite endurance to a finite life from installation in 1993 to end-of-life in 2014.
Therefore, a deficiency in the groove profile fabrication is the most likely cause for the shaft failure.
Contributing to the failure was that belt tension, as outlined in station procedure, did not provide adequate margin necessary to address stress risers at the notch.
In 2005, Maintenance procedure MA-OC-86103-100 was issued to specify the 60 Hz belt tension setting, but no technical evaluation was performed or vendor document referenced to review this setting. The 60 Hz belt tension setting is excessive; which resulted in:
A. Reduced margin to the shaft stress design limits.
B. A possibility that the belt was tensioned to a point where hub loading combined with stress risers originating from control of the groove configuration exceeded the shaft fatigue endurance limit.
The following immediate actions were taken:
Replaced EDG-2 fan shaft.
Performed Ultrasonic Testing of the EDG-1 fan shaft.
Obtained failure Analyses for failed fan shaft (PL and SIA).
Performed technical evaluation to specify correct fan belt tension.
Performed a "Deep Dive" check-in assessment of station EDG maintnenance and operating strategies.
Corrective Actions
In order to address the Apparent Cause the following actions were (or are being) taken:
Repaired EDG-2 failed fan shaft by completing a work order to replace the shaft.
" The EDG-1 fan shaft will be replaced by May 15, 2016.
In order to address the Contributing Cause the following actions were (or are being) taken:
Technical Evaluation was performed to determine the correct belt tension for both EDG-1 and EDG-2 Re-tensioned EDG-1 and 2 fan belts based on Technical Evaluation.
Revise station procedures to incorporate correct fan belt tension specified in Technical Evaluation.
Previous Occurrences
DR 93-387 EDG-2 Fan Shaft Failure - In August 1993, the EDG-2 fan shaft failed during routine testing. The cause of the failure was a combination of two factors: 1) the existence of a weld overlay in the vicinity of the bearing sleeve attachment that created an extremely hard subsurface layer, and 2) a machined groove immediately adjacent to the bearing sleeve attachment that extended to a depth corresponding to this extremely hard zone. The result was the initiation of a crack that propagated by torsional fatigue until failure.
Component Data Component IEEE 805 System ID IEEE 803A Function Emergency Diesel Generator EK DG