05000370/LER-2011-001
| Mcguire Nuclear Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 3702011001R00 - NRC Website | |
BACKGROUND
The following information is provided to assist readers in understanding the event described in this LER.
Applicable Energy Industry Identification [EIIS] system and component codes are enclosed within brackets.
McGuire unique system and component identifiers are contained within parentheses.
McGuire has four Emergency Diesel Generators [DG] (EDGs), two for each unit to supply emergency power to "A" and "B" train equipment in the event of a Loss of Coolant Accident and/or Loss of Offsite Power. During emergency conditidns, essential equipment is load shed from the Essential Busses and sequenced on to the EDGs. In this mode of operation, the EDGs operate isochronously (Isolated Bus); this mode of operation is distinct from operation during online testing. During normal online testing, the EDGs operate in parallel with the offsite power grid.
Each EDG is rated for continuous operation at 4000 kW with added capacity to operate between 4200 - 4400 kW for a period of two hours out of every twenty-four hours of operation without adversely affecting the life of the EDG. The design basis accident load level for each EDG does not exceed the 4000 kW continuous rating.
Technical Specification 3.8.1 requires two operable EDGs for each unit that is in Modes 1 through 4. With one EDG inoperable, the inoperable EDG 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 />. With two EDGs inoperable, one EDG must be restored to operable status within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. 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 />.
EVENT DESCRIPTION
Date Event 10/26/10 EDG 2B valve timing linkshaft actuator diaphragm (Bellofram seal) was replaced.
02/14/11 EDG 2B engine performance analysis indicated increased cylinder compression pressure.
Bellofram seal was suspected to be leaking.
03/05/11 During Unit 2 refueling outage, EDG 2B governor was replaced. Leaking Bellofram seal was not replaced.
03/10/11 During Unit 2 refueling outage, the replacement governor was tuned and EDG 2B was tested between 4200 - 4400 kW successfully. No fuel rack linkage adjustment was performed.
07/15/11 Leaking Bellofram seal was replaced. No governor tuning or fuel rack linkage adjustment was performed.
07/16/11 EDG 2B operability surveillance was completed successfully and an overload verification was not performed.
NRC E.URM J(0(0A (1U-2010) 08/02/11 EDG 2B operability surveillance with overload test was unsuccessful. EDG 2B could not load to > 4200 kW. Technical Specification Surveillance Requirement (TSSR) 3.8.1.14 was not satisfied.
08/04/11 EDG 2B governor actuator linkage rod to the fuel metering racks was adjusted with assistance from the vendor (Engine Systems, Inc.) 08/05/11 EDG 2B operability surveillance with overload test was completed successfully.
08/07/11 It was determined EDG 2B was incapable of achieving overload operation as required by TSSR 3.8.1.14 from 07/16/11 to 08/05/11. During this window, EDG 2A was tagged out of service from 07/19/11, 04:36 to 07/20/11, 04:01 for planned maintenance. Since TS 3.8.1 Required Actions were not complied with during this window, a Licensee Event Report is required pursuant to 10 CFR 50.73(a)(2)(i)(B).
CAUSAL FACTORS
Root Cause:
The mechanical governor actuator installation procedure did not set/validate appropriate fuel rack travel due to lack of documented guidance in the original OEM drawings.
When the new governor actuator was installed on 3/5/11, it could only operate the fuel racks to 53 millimeters (mm). The low fuel rack movement was discovered during the investigation on 8/4/11. The reason the governor actuator could not move the racks further than 53 mm was that the setup at the vendor was different than the setup of the old governor actuator even though it was still within the vendor's specification. The vendor setup ensures the governor actuator terminal output shaft can operate through the full range of 45 degrees of motion, but there is no reference to where the 45 degrees of motion occurs on the governor actuator output shaft. The vendor expects the fuel racks will be adjusted to meet the range of motion of the governor actuator; however, this was not communicated by the vendor and there is no original literature from the OEM which directs adjusting the fuel racks following governor actuator replacement. When the new governor actuator was installed, there was no adjustment made to the fuel racks to ensure they could travel to the range of 56 mm - 58 mm as normally expected to achieve overload operation (4200 kW to 4400 kW). The governor manufacturer did not indicate the new governor actuators could operate in a range of motion different than previously seen, and therefore require fuel rack adjustments.
Contributing Cause:
An inadequate retest of EDG 2B after the 7/15/11 Bellofram seal replacement did not identify that the EDG could not achieve 4200 kW. Had the fuel rack travel been appropriately set/validated prior to the Bellofram seal replacement, the EDG could have achieved 4200 kW after the seal replacement.
The governor installation procedure verifies the engine can achieve overload, which would normally be an indirect indication of proper fuel rack range of travel; however on EDG 2B, there was an off-design condition occurring at the time the governor actuator was replaced. The variable inlet valve timing system was not operating as it is designed due to a leaking Bellofram seal. This condition allowed more air charge in the cylinders, which masked the lower than expected fuel rack operation. Had this condition not existed at the time the governor actuator was replaced, testing of the governor after replacement would have identified the engine's inability to achieve overload operation. The investigation would have arrived at the proper conclusion, and an adjustment to the fuel rack would have been made to allow 56 mm to 58 mm of fuel rack at maximum governor actuator output and 4200 kW would have been obtained.
The leaking Bellofram seal caused load to be achieved with less than normal fuel delivery to the engine.
Therefore, when the seal was replaced on 7/15/11, the normal load versus fuel delivery was re-established.
However, due to the small reduction in governor actuator output shaft range of movement for the new governor actuator installed during the outage, the needed fuel rack travel was not available to achieve 4200 kW. The functional verification of the 7/15/11 work only loaded the engine to 3800 kW. Had an attempt been made to load EDG 2B to overload, the adverse rack travel would have been discovered. Not performing the overload verification was a missed opportunity to discover the overload issue prior to declaring the EDG operable and was a contributing cause to this event. This issue did not contribute to the adverse overload condition but prevented its timely discovery.
CORRECTIVE ACTIONS
Immediate:
1. Adjusted fuel rack linkage to allow range of motion to reach overload operation.
2. Retested engine to verify overload margin has been restored.
Subsequent:
1. Placed procedure MP/0/A/7400/014, "Nordberg Diesel Governor, Governor Oil Cooler and Booster Servomotor Maintenance" on Tech Hold.
2. Revised procedure MP/0/N7400/014 to ensure engine fuel racks are adjusted to achieve appropriate rack travel any time a governor is removed and replaced.
SAFETY ANALYSIS
This event has no significance with respect to the health and safety of the public. Had EDG 2B been called upon to perform its emergency function, it would have met its required emergency load of approximately 3900 kW since it could achieve a load of 4150 kW.
NRC FORM sitptA (1U-2U1U)
ADDITIONAL INFORMATION
To determine if this event is recurring, a search of the McGuire Problem Identification Process (PIP) database was conducted for a time period covering 5 years prior to the date of this event. Three PIPs were found but they were not recurring because the combination of equipment, administrative controls, personnel actions and causes for those events are not similar to those in this event. Therefore, this event is not considered recurring.