05000461/LER-2011-002
| Clinton Power Station, Unit 1 | |
| Event date: | 05-23-2011 |
|---|---|
| Report date: | 07-22-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4612011002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric — Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Main Control Room HVAC Return Fan B High Vibrations
A. CONDITION PRIOR TO EVENT
Unit: 1 Event Date: 5/23/11 Event Time: 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> CDT Mode: 1 Mode Name: Power Operation Reactor Power: 97.0 percent
B. DESCRIPTION OF EVENT
On 5/23/11, at approximately 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />, Operators in the Main Control Room (MCR) noticed an audible noise level originating from ventilation equipment outside the MCR envelope. The initial investigation revealed the elevated noise was emanating from the Main Control Room Ventilation (VC) [VI] B Return Fan [FAN] (OVCO4CB). This fan is a vane axial design manufactured by Buffalo Forge (B517). At 1430 hours0.0166 days <br />0.397 hours <br />0.00236 weeks <br />5.44115e-4 months <br />, initial vibration readings on the fan assembly were obtained and compared to data taken in February 2011. The latest data indicated that the vibration levels were higher than the February readings, specifically; Axial from 0.11 to 0.212 inches per second (in/sec), Vertical from 0.19 to 0.510 in/sec, and Horizontal from 0.21 to 0.557 in/sec. These readings placed the fan vibrations in the alert range (greater than 0.325 in/sec), but not into the shutdown (inoperable) range (0.70 in/sec).
Additional vibration readings were taken on OVCO4CB over approximately an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> period after the step change to determine if the vibration levels were continuing to increase. During that time, vibration levels remained constant. All other MCR ventilation parameters for the VC B train were normal and unchanged.
After obtaining the vibration data during the run, the VC B train was placed in standby in preparation for troubleshooting activities. An operability evaluation was prepared, which supported continued operability of VC B train with the increased vibration readings.
On 6/7/11, during a planned VC B system window to support additional inspections for vibration cause, maintenance personnel performed an inspection of the fan under Work Order 1440503 and identified a crack from the outside of the fan hub to one mounting hole. The crack (approximately four inches long) penetrated the entire thickness of the hub. The crack length and extent were confirmed after fan removal. After this discovery, contingency plans to replace OVCO4CB fan were completed. Following replacement, OVCO4CB was restored to an operable status on 6/10/11.
The fan hub was sent offsite for a formal failure analysis investigation which concluded that the crack was due to end of life fatigue, caused by low stress, high cycle loading. The failure analysis concluded that the fan could have operated for "many hours" before additional fatigue cracks initiated. However, the overall conservative conclusion is that the crack on the hub assembly could not support the ability of the fan to perform its specified safety function for the designed mission time of 30 days, and thus the fan was inoperable.
Based on this, the VC B fan would have been inoperable at point of discovery on 5/23/11 and Technical Specification 3.7.3 and 3.7.4 should have been entered. Technical Specification 3.7.3 Required Action (RA) A.1 would have placed the unit in a 7 day LCO ending on 5/30/11 at which point RA B.1 should have been entered placing the unit on a 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> LCO to be in Mode 3. Based on when the VC B fan was repaired and restored to an operable status, on 6/10/11, this constituted a condition prohibited by technical specifications.
During the time from 5/23/11 to 6/10/11 the VC A system was always operable. Therefore at all times during this event, the MCR was supported by an operable train of VC, and thus no loss of safety function occurred.
C. CAUSE OF EVENT
An equipment apparent cause analysis was conducted under Issue Report (IR) 1225739. The equipment apparent cause determined that the hub crack was the result of end-of-life fatigue, caused by low stress, high cycle loading.
OVCO4CB had approximately 115,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of service before the hub crack propagated enough to cause a step change in vibration levels, which stabilized as the crack reached the edge of the hub. The as-found data described a crack from the outside of the fan hub to one fan mounting hole (approximately 4 inches long). The crack penetrated the entire thickness of the hub. The crack length and extent were confirmed after fan removal.
D. SAFETY CONSEQUENCES
This event is reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition which was prohibited by the plant's Technical Specifications due to exceeding Limiting Condition of Operation (LCO) 3.7.3 of Technical Specification (TS) 3.7.3, Control Room Ventilation System, Required Action A.1, to restore an inoperable Control Room Ventilation subsystem to an operable status within 7 days.
As stated previously, the VC B fan hub cracking was identified and confirmed on 6/7/11 and the VC B fan was replaced and restored to an operable status on 6/10/11. Existing analysis is not sufficient to provide the high degree of confidence necessary to support past operability from the time of the increase in vibrations on 5/23/11, until the fan was replaced, or approximately 18 days.
There were no actual safety consequences impacting plant or public safety as a result of this event. The opposite division train was operable during the time of inoperability; therefore there was no loss of safety function.
E. CORRECTIVE ACTIONS
The VC B return air fan was replaced.
The VC A return air fan will be replaced in 2012 based on a lower number of operating hours compared to VC B.
The applicable Performance Centered Maintenance (PCM) templates were reviewed for similar high duty cycle fans (extent of condition) and the replacement strategy was changed from performance monitoring to time directed replacement.
F. PREVIOUS OCCURRENCES
A review of CPS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with ventilation fan failures; however, one similar previous event was identified.
On 10/26/06, the MCR Supply Fan for VC B, OVCO3CB, failed suddenly and broke apart. A root cause investigation noted that the fan failure was due to cracking of the material in the hub. The failure analysis determined that the cracks propagated by fatigue until a hub section failed in a relatively brittle manner due to mechanical overloading.
The difference between this event and the OVCO4CB event on 5/23/11, is the physical difference between the hub diameters, 54 inches for the supply fan compared to 48 inches for the return fan. The supply fan runs closer to the stall region on the fan performance curve and the failure was caused by unbalanced loading on the fan (caused by the design of the system). The supply fan was determined to be too large for the application such that normal system transients put the fan in the stall region. There were multiple cracks found in the supply fan hub, whereas the VC B return fan had one crack that had no further growth possible and no indications of additional fatigue cracks. Finally, the return fan air flow has a straight path to the suction and the VC B supply fan operates above the stall region.