05000317/LER-2001-001
| Docket Number | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3172001001R00 - NRC Website | |
I. � DESCRIPTION OF EVENT On May 16, 2001, at 1338 hours0.0155 days <br />0.372 hours <br />0.00221 weeks <br />5.09109e-4 months <br />, Calvert Cliffs Unit 1 experienced a failure of Number 11 auxiliary feedwater (AFW) pump turbine outboard bearing during a surveillance test. Subsequent investigation into the cause of the bearing failure concluded that the AFW pump turbine was not operable since the last overhaul of the pump on March 25, 2000. At the time of discovery, Calvert Cliffs Unit 1 was in Mode 1 with reactor power at 100 percent rated thermal power.
On May 16, 2001, the Unit 1 Auxiliary Feedwater System Quarterly Surveillance Test (STP 0-5A-1) was being performed on 11 AFW pump. This test is designed to demonstrate operability of portions of the AFW system by verifying that the pump develops the required total dynamic head in recirculation flow, each testable remote-operated valve is operated through one complete cycle, and the pump and applicable valves meet the acceptance criteria of American Society of Mechanical Engineers Section XI. The procedure also requires that the pump be run for a minimum of 90 minutes to ensure the bearings are operating properly.
After 88 minutes of operation with bearing temperatures at normal (about 190 degrees Fahrenheit), the 11 AFW turbine outboard journal bearing temperature started to increase rapidly. A Control Room operator manually secured the pump, as directed by STP 0-5A-1, as the outboard bearing temperature reached 210 degrees Fahrenheit.
Number 11 AFW pump was declared inoperable and an issue report (IR3-041-139) was prepared to document the problem in the corrective action system. The issue report recommended that the bearing be inspected and repaired. A priority 2 maintenance order was initiated for the work (M01200102535).
On May 18, 2001, 11 AFW turbine outboard bearing was disassembled and inspected. Based on the observations of the mechanics and AFW system engineer, it was suspected that the cause of the bearing failure was the intrusion of Permatex sealant into the bearing. Permatex 2 sealant is used to seal the bearing housing cover onto the bearing housing and prevent the lubricating oil inside the housing from leaking out. It was noticed that Permatex 2 sealant was extruded from the mating surfaces of the housing/housing cover and some was exposed along the mating surface edges inside the bearing housing. The mechanics and system engineer concluded that some of the Permatex sealant probably had become dislodged from the mating surface edges inside the housing and fallen into the lubricating oil in the bearing housing. Then the Permatex sealant could have been transported into the bearing by the lubricating oil.
Since all four steam-driven AFW pumps (11, 12, 21, and 22) at Calvert Cliffs are maintained in a similar fashion, the problem was conservatively assumed to be generic to the other three pumps. Two additional issue reports were initiated to document this generic operability concern for the other steam- driven AFW pumps (IR3-084-400 and IR3-084-380).
At 1400 on May 18, 2001, 12 AFW pump was declared inoperable because the maintenance procedure used to apply Permatex on this pump was performed at approximately the same time as the 11 AFW pump (March 2000). Thus, it was thought that 12 AFW pump turbine could be subject to the same failure mechanism as 11 AFW pump turbine had experienced.
An initial operability evaluation was completed on May 18, 2001 at 1900 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.2295e-4 months <br /> for 21 and 22 AFW pumps. The operability evaluation concluded that the 21 and 22 AFW pump turbines were operable based on the fact that their bearings had been recently inspected following overspeed trip testing during the spring 2001 refueling outage and the Permatex sealant used was freshly set, pliable and not likely to dislodge if extruded inside the housing.
A priority 1 maintenance order was initiated on May 18, 2001 to inspect the 12, 21, and 22 AFW turbine bearings for similar problems (MO2200101994 and MO1200102563). These turbine bearings were inspected on May 19, 2001. The inspection found that none of the other AFW turbines had excessive Permatex sealant at the bearing housing to bearing cover mating surface.
Number 11 AFW turbine outboard bearing was replaced and the pump was returned to service on May 18, 2001, at 2350 hours0.0272 days <br />0.653 hours <br />0.00389 weeks <br />8.94175e-4 months <br />. Number 12 AFW turbine was inspected for similar sealant intrusion issues. None were found. The 12 AFW pump was returned to service at 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br /> on May 19, 2001.
II. � CAUSE OF EVENT A multi-disciplinary root cause analysis team was assigned to investigate this event, determine the causes, and develop a comprehensive set of corrective actions to prevent recurrence of similar events. During the investigation, the team utilized subject matter experts and an independent outside consulting firm to provide information and independent opinions concerning the cause of the 11 AFW turbine outboard bearing failure.
The immediate cause of the bearing failure was determined to be intrusion of Permatex sealant into the bearing causing disruption of oil to the bearing.
Inspection of the bearing found a black sticky residue on its inside surface and hardened black particles imbedded in its inside surface. Samples and observation upon disassembly showed the black substance to be similar in smell and texture to Permatex sealant. Chemical analysis also indicated that the physical and chemical characteristics of the material in the bearing were consistent with the characteristics of Permatex 2 sealant.
The over-application of Permatex 2 at the mating surface of the bearing housing/bearing housing cover caused it to extrude to the inside edges of the turbine bearing housing next to the bearing oil slinger rings. As the sealant ages it becomes more brittle and more likely to be dislodged and fall into the bearing housing, thus becoming a potential source of foreign material in the bearing. It was verified through testing that the slinger rings can pick up fine particles of Permatex sealant in the bearing lube oil and transfer them to the inside of the bearing. These particles are large enough to disrupt the thin oil film (thickness of about 1 mil) if they enter the bearing.
The root cause of the bearing failure was determined to be inadequate direction by the AFW turbine vendor technical manual as to how thick bearing cover sealant may be applied. The technical manual specifies that the sealant can be applied at up to .015 inches thick. This thickness proved to be excessive. The excessive sealant extrudes into the bearing oil housing where it can be dislodged and become a contaminant.
The reason that the other bearing housings did not have excessive sealant present was discovered during interviews with the maintenance mechanics who performed the work. Several years ago, one mechanic was observed reassembling a bearing by a vendor representative. The vendor representative instructed the mechanic that he did not need to use the maximum amount of sealant allowed in the vendor technical manual to obtain a good seal. Since that time, the mechanic had worked 12, 21, and 22 AFW turbine bearings utilizing less sealant. He had not yet had the opportunity to work the 11 AFW turbine bearings.
III. ANALYSIS OF EVENT
The AFW system is designed to provide feedwater to the steam generators for the removal of sensible and decay heat, and to cool the reactor coolant system to 300 degrees Fahrenheit in case the condensate or main feed system are inoperable. Three AFW pumps are installed for each Unit, one train utilizes a single motor-driven pump and another train utilizes either of two steam-driven pumps. The steam-driven pumps may be used for normal system cooldown, but the motor-driven pump is designated for emergency use only. Upon automatic initiation of AFW, one motor-driven and one turbine-driven pump automatically start. However, only one pump is required to operate to shutdown the plant.
In order to evaluate the safety significance of this event, the following assumptions were made:
A. Since the other turbine-driven AFW pumps had significantly less sealant applied to their bearing housings, they were not considered to be affected by the sealant intrusion problem that caused 11 AFW pump to become inoperable.
B. The Permatex 2 sealant was applied to 11 AFW outboard turbine bearing housing on March 25, 2000. The total fault exposure of 11 AFW pump was assumed to be 387 days.
C. The 11 AFW pump failure rate due to sealant intrusion was estimated to be one failure per 10.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of pump operation since the 11 AFW pump was run for a total of 10.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> during the 387 day exposure period and experienced one failure.
The safety significance for the 11 AFW pump inoperability was evaluated using the results of the preliminary Calvert Cliffs Probabilistic Risk Assessment.
The overall core damage frequency (CDF) of the CCPRA is 1.1E-4. This model includes both internal and external events. A base calculation was performed for the 11 AFW pump inoperability assuming the pump was failed for 387 days.
This resulted in a CDF increase of 4E-5. Changing the failure likelihood to one per 10.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, and considering the potential for other AFW pump recovery actions not currently assumed in the CCPRA reduces the CDF increase for this event to approximately 2E-5.
This event is considered reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) because it resulted in a condition prohibited by the plants Technical Specifications.
IV. � CORRECTIVE ACTIONS
Immediate Corrective Actions:
All of the AFW turbine bearings housings were inspected and reassembled using appropriate amounts of Permatex 2 sealant.
Corrective Actions Planned to Prevent Recurrence:
A. The vendor technical manual for AFW turbines will be revised to incorporate more appropriate recommendations for the application of Permatex 2 to the bearing cover sealing surfaces.
B. The AFW pump turbine overhaul procedures will be revised to incorporate the lessons learned from this event.
C. A review of other applicable procedures that incorporate the use of sealants will be conducted. Appropriate changes to these procedures will be made.
D. This issue has been discussed with the mechanics qualified to perform AFW turbine work. A training session is being developed for appropriate craft personnel concerning this event, its causes, and lessons learned.
E. During the next bearing inspection for each AFW turbine bearing, mechanics will verify that the amount of sealant being used does not result in sealant extruding over the bearing casing internal wall. This will ensure that the corrective actions above have been effective.
V. � ADDITIONAL INFORMATION A. � Previous Similar Events:
As part of the causal analysis for this event, all of the AFW turbine bearing failures (for inboard and outboard bearings on 11, 12, 21, and 22 AFW pumps) since 1991 were identified. The causes of each failure and other pertinent data were also tabulated. There were 10 failures identified. None of the failures identified misuse of Permatex sealant as a cause of bearing failure. Although FME in the bearing or bearing lube oil was often considered to be a potential cause of past AFW turbine bearing failures, only one case, in 1994, found that FME was the probable cause. A collective significance analysis of the past failures is being performed to determine if there are any common causes or issues that can be addressed that will improve overall performance of AFW turbine bearings.
B. Component Identification:
IEEE 803 IEEE 805 Component EIIS Function System ID AFW Pump P BA AFW Pump Turbine TRB BA