05000413/LER-2014-001
| Catawba Nuclear Station, Unit 1 | |
| Event date: | 03-07-2014 |
|---|---|
| Report date: | 05-06-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4132014001R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
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BACKGROUND
This event is being reported under the following criterion:
10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's Technical Specifications (TS).
Catawba Nuclear Station, Unit 1 is a Westinghouse four-loop Pressurized Water Reactor (PWR) [EIIS:
RCT].
The onsite standby power source for each 4160 volt Engineered Safety Features (ESF) bus [EIIS: BU] at Catawba is a dedicated DG [EllS: EK]. For each unit, DGs A and B are dedicated to ESF buses ETA and ETB, respectively. Each DG starts automatically on a Safety Injection (SI) signal (i.e., low pressurizer pressure or high containment pressure) or on an ESF bus degraded voltage or undervoltage signal. After the DG has started, it will automatically tie to its respective bus after offsite power is lost as a consequence of ESF bus undervoltage or degraded voltage, independent of or coincident with an SI signal. The DGs will also start and operate in the standby mode without tying to the ESF bus on an SI signal alone. Following the loss of offsite power, a sequencer [EIIS: EK] strips loads from the ESF bus.
When the DG is tied to the ESF bus, loads are then sequentially connected to its respective ESF bus by the automatic load sequencer. The sequencing logic controls the permissive and starting signals to motor breakers to prevent overloading the DG by automatic load application.
In the event of a loss of offsite power, the ESF electrical loads are automatically connected to the DGs in sufficient time to provide for safe reactor shutdown and to mitigate the consequences of a Design Basis Accident (DBA) such as a Loss of Coolant Accident (LOCA).
Certain required unit loads are returned to service in a predetermined sequence in order to prevent overloading the DG in the process. Approximately one minute after the initiating signal is received, all loads needed to recover the unit or to maintain it in a safe condition are returned to service.
TS 3.8.1 governs the DGs. Limiting Condition for Operation (LCO) 3.8.1 requires two operable DGs for each unit that is in Modes 1, 2, 3, and 4. With one DG inoperable, the inoperable DG must be restored to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per Required Action B.4. If this is not accomplished, the unit must be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> per Required Actions G.1 and G.2.
The Nuclear Service Water System (NSWS) [EIIS: BI] provides a heat sink for the removal of process and operating heat from safety related components during a DBA or transient. During normal operation, and a normal shutdown, the NSWS also provides this function for various safety related and non-safety related components.
The NSWS consists of two independent loops (A and B) of essential equipment. Each loop contains two NSWS pumps [EIIS: P], each of which is supplied from a separate DG. Each set of two pumps supplies two trains (1A and 2A, or 1B and 2B) of essential equipment through common discharge piping [EIIS:
None]. While the pumps are unit designated (i.e., 1A, 1B, 2A, 2B), all train-related pumps receive automatic start signals from a corresponding train-related SI or blackout signal from either unit.
Therefore, a pump designated to one unit will supply post-accident cooling to equipment in that loop on both units. For example, the 1A NSWS pump, whose emergency power is supplied by DG 1A, will supply post-accident cooling to NSWS trains 1A and 2A.
The NSWS system is shared between the two units. The shared portions of the system must be operable for each unit when that unit is in the mode of applicability. Additionally, both normal and emergency power for shared components must also be operable. If a shared NSWS component becomes inoperable, or normal or emergency power to shared components becomes inoperable, then the required actions of the NSWS LCO must be entered independently for each unit that is in the mode of applicability of the LCO. If both units are in the mode of applicability with the NSWS operating in the normal dual supply and discharge header alignment, one unit may exit the LCO's required actions provided that unit's NSWS pump is operable and one unit's flow path to the associated train non- essential header, Auxiliary Feedwater (AFW) pumps, and Containment Spray System heat exchanger [EIIS: HX] is isolated (or equivalent flow restrictions). In this case, sufficient flow is available, however, this configuration results in inoperabilities within other required systems on one unit and the associated required actions must be entered.
One NSWS loop containing two operable NSWS pumps has sufficient capacity to supply post-LOCA loads on one unit and shutdown and cooldown loads on the other unit. Thus, the operability of two NSWS loops assures that no single failure will keep the system from performing the required safety function. Additionally, one NSWS loop containing one operable NSWS pump has sufficient capacity to maintain one unit indefinitely in Mode 5 (commencing 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> following a trip from full power) while supplying the post-LOCA loads of the other unit. Thus, after a unit has been placed in Mode 5, only one NSWS pump and its associated DG are required to be operable on each loop, in order for the system to be capable of performing its required safety function, including single failure considerations.
TS 3.7.8 governs the NSWS. LCO 3.7.8 requires two operable NSWS trains for each unit that is in Modes 1, 2, 3, and 4. With one NSWS train inoperable, the inoperable NSWS train must be restored to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per Required Action A.1. If this is not accomplished, the unit must be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> per Required Actions D.1 and D.2.
The NSWS also supports the AFW [EIIS: BA] and Containment Spray [EIIS: BE] Systems since it serves as the assured water source for AFW and cooling water for the Containment Spray System. TS 3.7.5 governs the AFW System. LCO 3.7.5 requires three AFW trains to be operable in Modes 1, 2, and 3, and one motor-driven AFW train to be operable in Mode 4 when the steam generators [EIIS: SG] are relied upon for heat removal. With one AFW train inoperable in Mode 1, 2, or 3 for reasons other than an inoperable steam supply to the turbine-driven AFW pump, the inoperable AFW train must be restored to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per Required Action B.1. If this is not accomplished, the unit must be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> per Required Actions C.1 and C.2. TS 3.6.6 governs the Containment Spray System. LCO 3.6.6 requires two containment spray trains to be operable in Modes 1, 2, 3, and 4. With one containment spray train inoperable, the inoperable containment spray train must be restored to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per Required Action A.1. If this is not accomplished, the unit must be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 within 84 hours9.722222e-4 days <br />0.0233 hours <br />1.388889e-4 weeks <br />3.1962e-5 months <br /> per Required Actions B.1 and B.2.
Throughout the duration of this event, Unit 1 operated in Mode 1 at 100% power.
EVENT DESCRIPTION
Date/Time Event (Some event times are approximate.) 11/2006 Unit 1 was in its end-of-cycle 16 refueling outage, during which maintenance work was performed on DGs 1A and 1B. On 11/24/2006 at 2216 hours0.0256 days <br />0.616 hours <br />0.00366 weeks <br />8.43188e-4 months <br />, DG 1A tripped on high vibration during a performance run. The bearing for connecting rod number 4 experienced a catastrophic failure. The bearing for connecting rod number 6 was also found to have rotated from its normal horizontal position. These two bearings were subsequently replaced. In addition, the bearing for connecting rod number 8 was also replaced since it was from the same lot number as the other two bearings. Position measurements were taken on all of the remaining bearings on DG 1A. In response to this event, an 18-month preventive maintenance inspection was established to check bearing position for both Unit 1 and both Unit 2 DGs.
5/2008 The bearings for connecting rod number 3 on DG 1A and connecting rod number 2 on DG 1 B were replaced during the Unit 1 end-of-cycle 17 refueling outage.
2009 - 2012 The 18-month preventive maintenance bearing position inspections were performed with no issues noted.
3/4/2014/0329 DG 1A was declared inoperable for pre-outage related preventive maintenance activities. These activities included the 18-month preventive maintenance bearing position inspection.
—0930 Maintenance discovered that the bearing for connecting rod number 7 had rotated approximately 25 degrees from its normal horizontal position.
1030 Maintenance and Engineering met to discuss the situation and determine its significance.
1100 A formal response team was initiated to respond to this issue.
1800-2300 Operations drained the jacket water cooling system and tagged the DG in preparation for Maintenance activities to replace the bearing.
2300 Maintenance activities to replace the bearing began. A brief summary of the major maintenance activities is as follows:
- Remove valve covers, rocker boxes, and cylinder heads.
- Remove affected piston and link rod and secure piston and master rod.
- Remove connecting rod bearing from crankshaft journal.
- Inspect crankshaft journal and install new connecting rod bearing.
- Reconnect piston and master rod and install piston and link rod.
- Install connecting rod bolting and torque and lockwire.
- Install cylinder heads, rocker boxes, and valve covers.
3/5/2014 Maintenance activities continued, management discussed the possibility of pursuing a NOED request, and the NOED request was drafted.
3/6/2014/0600 The Catawba Plant Operations Review Committee (PORC) met to review the draft NOED request. The request was approved by the PORC.
1000 A telephone conference call was held between Duke Energy and the NRC during which the NOED request was made. The request was for 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br /> beyond the TS 3.8.1 Required Action B.4 Completion Time limit of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
1638 NSWS realignments were completed, allowing Unit 2 to exit TS 3.7.8 Condition A.
2000 A second telephone conference call was held between Duke Energy and the NRC during which the NOED request was granted.
3/7-9/2014 Maintenance activities continued. Following the completion of the Maintenance activities, Operations cleared tags, refilled the jacket water cooling system, heated up the system, and topped off lubricating oil level. Operations subsequently ran the engine for its functional break-in run and required testing.
3/9/2014/0340 DG 1A was declared operable.
3/10/2014/2000 Catawba submitted the written NOED paperwork to the NRC via electronic mail.
3/12/2014/---- The NRC issued the written NOED approval (NOED No. 14-2-001).
CAUSAL FACTORS
The most probable cause of the rotation of the bearing for connecting rod number 7 on DG 1A is inadequate procedural control of lubricating oil temperature prior to starting an engine following extended periods of maintenance.
The keep warm system for the Catawba DGs only heats approximately one-third of the lubricating oil volume (between the sump tank and the crankcase) to a minimum of 120 degrees F prior to initially starting a DG following maintenance. This results in a large volume of much cooler oil being injected into the bearings as the engine-driven lubricating oil pump comes up to speed. The differential temperature causes contraction of the aluminum bearing shells, which contract faster than the forged steel connecting rod.
This contraction, combined with the additional torque applied on the bearing inside surface by the cooler, more viscous oil, is postulated to cause the bearing to temporarily lose adequate crush, resulting in enough force to shear the locating dowel pin and cause some rotation. Catawba presently utilizes bearings from three different manufacturers. The bearings from one particular manufacturer appear to be more susceptible to this phenomenon.
Independent calculations showed that at approximately 73 degrees F, the force applied by the crankpin through the oil film to the bearing is equal to the combination of the crush force holding the bearing in place inside the connecting rod housing and the force required to shear the alignment dowel. Data recorded recently indicates that during normal monthly periodic test starts, the minimum oil temperature entering the engine is approximately 120 degrees F, well above the calculated value of 73 degrees F.
Data recorded from an engine start after the keep warm system had been shut down for an extended period of time showed a minimum oil temperature of 86 degrees F entering the engine. This temperature is still above the 73 degrees F calculated threshold, but there is significantly less margin following major maintenance.
Prior to the 2006 bearing failure event, the lubricating oil had been removed from DG 1A and stored outside for several days. The outside air temperature averaged 50 degrees F and DG 1A room temperature averaged 74 degrees F. It has been postulated that the oil temperature was significantly cooler (30-40 degrees F) upon engine start than what is normally seen. The bearing for connecting rod number 4 failed and the bearing for connecting rod number 6 was found to have rotated from its normal horizontal position. Conversely, DG 1B room temperature averaged 88 degrees F during its maintenance period and the lubricating oil was not removed. In 2006, no bearings were found to have rotated on DG 1B.
During the Unit 1 end-of-cycle 20 refueling outage, outside air temperature averaged 49 degrees F. DG 1A room temperature averaged 73 degrees F and DG 1B room temperature averaged 88 degrees F.
The lubricating oil for DG 1A was stored outside for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> prior to being put back in the engine, while the lubricating oil for DG 1B was not removed. During the pre-outage maintenance window in March 2014, the bearing for connecting rod number 7 on DG 1A was found to have rotated approximately 25 degrees from its normal horizontal position. It is believed that the rotation occurred during the post- maintenance runs during the Unit 1 end-of-cycle 20 refueling outage, since bearing position had been inspected in late 2012.
Bearing rotation has never been observed on the Unit 2 DGs. A review of past outage schedules showed that Unit 1 outages occur during times that are typically cooler than Unit 2 outages. This correlates with cooler initial lubricating oil temperatures in the Unit 1 DGs prior to initial starts. A review of outside air temperatures and room temperatures for the Unit 2 DGs showed that the lowest outside air temperature was approximately 72 degrees F and the room temperature was consistently above 73 degrees F. At no time does it appear that lubricating oil temperatures on the Unit 2 DGs could have approached the calculated temperature threshold of 73 degrees F.
Based upon this information, it has been concluded that plant procedures for the DGs did not ensure that the lubricating oil was sufficiently warmed prior to starting the engine after extended periods of maintenance when the keep warm system was not in service. The operability requirement for the DGs is only for the keep warm temperature to be greater than or equal to 120 degrees F.
CORRECTIVE ACTIONS
Immediate:
1. Upon discovery of the misaligned connecting rod bearing, management decided to proactively replace the bearing and made preparations to implement 24-hour coverage for the evolution.
Subsequent:
1. Work began to replace the bearing and a NOED was requested and approved to allow time to complete the replacement.
2. A root cause evaluation of the misaligned bearing was performed.
3, Interim guidance was issued to require a minimum jacket water and lubricating oil temperature of 140 degrees F at the DG engine inlets and outlets prior to any planned engine start.
Planned:
1. Connecting rod bearing inspection preventive maintenance work orders will be revised to inspect for bearing rotation prior to the completion of refueling outages.
2. Plant procedures will be revised to preheat lubricating oil prior to putting it in the engine and to require a minimum jacket water and lubricating oil temperature of 140 degrees F at the DG engine inlets and outlets prior to any planned engine start.
3. A means will be provided to heat the non-keep warm sections of the lubricating oil system prior to post-maintenance starts of the DGs.
4. The remaining bearings that appear to be more susceptible to rotation will be replaced with bearings from a different manufacturer.
SAFETY ANALYSIS
There was no safety significance to this event. Although DG 1A connecting rod bearing number 7 was found to have rotated from its normal horizontal position by approximately 25 degrees, Engineering and vendor personnel determined that the bearing would have been able to perform its specified safety function in the as-found condition. Nevertheless, the bearing was proactively replaced. In addition, DG 1A had undergone numerous successful monthly performance tests since the last bearing inspection was conducted 18 months earlier. During the period that DG 1A was inoperable while bearing number 7 was being replaced, DG 1B was operable. Following the restoration of DG 1A to operable status, an identical bearing inspection was performed on DG 1B. Bearing number 1 on DG 1B was found to have rotated 5.5 degrees from its normal horizontal position and this bearing was also subsequently proactively replaced. It was determined that DG 1B bearing number 1 would also have been able to perform its specified safety function in the as-found condition. Bearing position was inspected on DGs 2A and 2B and no rotation was observed.
Catawba verbally requested a NOED on March 6, 2014 to allow Unit 1 to remain in Mode 1 for 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br /> beyond the TS 3.8.1 Required Action B.4 Completion Time limit of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> while bearing number 7 on DG 1A was proactively replaced. The NOED request included the results of a risk analysis which demonstrated that the risk associated with allowing Unit 1 to remain in Mode 1 for an additional 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br /> while the bearing replacement was completed remained acceptable. The NRC subsequently verbally approved the NOED request on March 6, 2014. The written NOED documentation was submitted to the NRC on March 10, 2014 (ADAMS Accession Number ML14072A006). The NRC subsequently issued the written NOED approval (NOED No. 14-2-001) on March 12, 2014 (ADAMS Accession Number ML14071A568). A NOED request was not needed in conjunction with the DG 1B bearing number 1 replacement.
This event had no adverse effect upon the health and safety of the public.
ADDITIONAL INFORMATION
Within the previous three years, there were LER events involving DG inoperability due to failed subcomponents. However, these events did not involve misaligned connecting rod bearings. In addition, it was subsequently determined by Engineering and vendor personnel that the as-found bearing misalignment condition would not have actually rendered DG 1A or DG 1B inoperable. The DGs were only declared inoperable for the bearing inspections and they remained inoperable while the affected bearings were proactively replaced. Therefore, corrective actions taken as a result of the previous DG related LER events could not have prevented this event from occurring. This event is therefore considered to be non-recurring.
Energy Industry Identification System (EIIS) codes are identified in the text as [EIIS: ) made to post this event to the INPO Consolidated Event System (ICES) (formerly called the Equipment Performance and Information Exchange (EPIX) program). However, it should be noted that no actual component failure occurred during this event.
This event is not considered to constitute a Safety System Functional Failure. There was no release of radioactive material, radiation overexposure, or personnel injury associated with the event described in this LER.
10 CFR Part 21 Applicability:
This event was evaluated for 10 CFR Part 21 applicability in accordance with Duke Energy administrative procedures. This event was determined to be not reportable in accordance with 10 CFR Part 21. The details of this evaluation are documented in the corrective action program entry for this event.