05000457/LER-2004-001
| Event date: | 09-25-2004 |
|---|---|
| Report date: | 11-24-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 4572004001R00 - NRC Website | |
A. Plant Operating Conditions Before The Event:
Event Date: September 25, 2004�Event Time: 2340 Unit: 2 MODE: 1� Reactor Power: 100 percent Unit 2 Reactor Coolant System (RCS) [AB] Temperature: 581 degrees F, Pressure: 2233 psig 3. Description of Event:
There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.
On September 25, 2004, the Unit 2 Pressurizer [AB] 2D backup heater group was being placed on line in power reduction to support throttle valve and governor valve testing. After the attempt to energize the 2D backup heater group in the Main Control Room, there was no response. Pressurizer pressure, heater group 2C demand current, and spray valve position was unchanged over a 10-minute time frame. The 2A backup heater group was then placed "ON" in manual and again Pressurizer pressure, heater group 2C demand current, and spray valve position remained unchanged, indicating that the "2A" backup heater group also would not energize. On September 25, 2004, at 2340, unplanned LCO 3.4.9, Condition B was entered due to the Pressurizer backup heater groups 2A and 2D being inoperable.
On September 26, 2004, an investigation was initiated to identify the cause of the heater failure. A visual examination revealed that the contactor armature stop bracket assemblies for the 2A and 2D heaters were misaligned and interfered with the rotation of the contactor armatures. The 2A and 2D contactor armature stop bracket assemblies were properly re-aligned and subsequently verified to be operable, and on September 26, 2004, at 1038, LCO 3.4.9 was exited.
It could not conclusively be determined when the 2A and 2D Pressurizer backup heater groups became inoperable.
Post maintenance tests were performed on March 3, 2004 and March 4, 2004, which verified the operation of the heaters via a 60-amp increase on the feed to the heaters. A review of the historical SER data revealed several instances, after March 4, 2004, that a demand signal existed for the energization of the 2A or 2D Pressurizer heater groups. However, sufficient evidence could not be produced to conclusively prove that the heater groups energized upon the demand signals. Therefore, the conservative decision was made that the 2A and 2D heaters were last operable as of March 3, 2004 (2A) and March 4, 2004 (2D). The LCO AOT for inoperable heaters is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />; therefore, the 2A and 2D Pressurizer backup heaters were determined to be inoperable for greater than the AOT for LCO 3.4.9.
C. Cause of Event
During this investigation the root cause was determined to be the lack of torque specifications for the armature mounting screws in the equipment vendor manual. Since no vendor instructions exist for assembly of the armature mounting assembly, torque specifications were not included in the March 2004 work instructions for coil replacements.
Additionally, when inspected, during troubleshooting, the 2D heater armature mounting screws had lock-washers, whereas the 2A heater armature mounting screws did not. The application of lock washers increases the rigidity of the armature in relation to the operating shaft and reduces axial movement. The absence of the lock washers on the 2A occurred, once again, because of the lack of instructions for this maintenance activity. There is no detail on either local or vendor drawings that provide the details for installation of lock washers.
The corrective action to prevent recurrence is to develop a maintenance procedure for this type contactor assembly.
This procedure will firovide the appropriate torque requirements for the armature mounting screws, and instructions for washer installation and armature centering.
D. Safety Consequences:
In Modes 1, 2, and 3, the LCO requirement for a steam bubble is reflected implicitly in the accident analyses.
Safety analyses performed for lower Modes are not limiting. All analyses performed from a critical reactor condition assume the existence of a steam bubble and saturated conditions in the pressurizer. In making this assumption, the analyses neglect the small fraction of non-condensible gases normally present.
Safety analyses presented in the Updated Final Safety Analysis Report does not take credit for pressurizer heater operation; however, an implicit initial condition assumption of the safety analyses is that the RCS is operating at normal pressure.
The maximum pressurizer water level limit, which ensures that a steam bubble exists in the pressurizer, satisfies provide the capability to maintain sub-cooling in the long term during loss of offsite power, as indicated in NUREG A risk assessment classified this event as being of very low risk significance. The primary purpose of the pressurizer heaters is to maintain the assumption in the safety analysis and probabilistic risk assessment that all accidents start with a steam bubble and saturated conditions in the pressurizer. As these conditions were maintained during the event, there was no impact on either the safety analysis or probabilistic risk assessment.
This event did not result in a safety system functional failure.
E. Corrective Actions:
Develop a maintenance procedure to standardize and provide an additional level of detail for the replacement of the coils and/or inspection/maintenance of the contactor assembly. These instructions will include torque specification requirements for the armature mounting fasteners, the clearance (gap) for the armature to side-plates, and inspection for, or installation of, locking washer onto the armature mounting fasteners.
F. Previous Occurrences:
There have been no similar Licensee Event Report events at Braidwood Station.
In 1988, the Braidwood 2D back-up heaters failed to operate due to armature misalignment, and repairs completed on 2/4/1988.
G. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A