05000390/LER-2009-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | 07-17-2009 |
|---|---|
| Report date: | 09-15-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3902009002R00 - NRC Website | |
I. PLANT CONDITIONS:
The event in this LER occurred on July 17, 2009, while the unit was at 100% power. The plant was in Technical Specification (TS) 3.8.1, "AC Sources - Operating," Condition A for one offsite circuit being inoperable. The Common Station Service Transformer (CSST) D [Energy Industry Identification System (EllS) Code XFMR] had been removed from service to perform corrective maintenance. In support of this maintenance the 2B-B Shutdown Board (El IS Code ECBD) had been aligned to the alternate power supply.
II. DESCRIPTION OF EVENT:
A. Event:
On July 17, 2009, the plant was in TS 3.8.1 Condition A for one offsite circuit being inoperable. While corrective maintenance was being performed on CSST D, the 2B-B Shutdown Board was aligned to CSST C.
Upon completion of this maintenance TVA was preparing to perform a manual fast transfer of the 2B-B Shutdown Board from its alternate supply breaker (El IS Code BKR) to the normal supply breaker (El IS Code BKR). This transfer is performed by first turning the normal supply breaker control switch to CLOSE, and then the alternate supply breaker control switch to OPEN. The fast transfer function is enabled by a microswitch (EllS Code IIS) that prevents actual closure until loss of voltage is detected at the alternate supply. However, when the normal supply breaker control switch (EllS Code IS) was placed in CLOSE, the breaker immediately closed, which resulted in both breakers tripping open. This deenergized the board causing all four emergency diesel generators (EDGs, El IS Code DG) to start. The EDG for this board tied on and reenergized the board. The other EDGs did not tie on to their respective boards, as they never lost power.
Investigation found that the alternate supply breaker actuation arm that engages the fast transfer microswitch associated with control circuitry for the normal supply breaker was misaligned. Thus, the fast transfer microswitch was not properly positioned to prevent the normal supply breaker from closing while the alternate supply breaker was closed. The breaker involved was a General Electric (GE) Magne-Blast, component model number AM-7.2-500-6HB.
This event is addressed in TVA's Corrective Action Program as Problem Evaluation Report (PER) 176604.
B. Inoperable Structures, Components, or Systems that Contributed to the Event No structures, components, or systems were inoperable that contributed to the event besides the aforementioned alignment issues between the breaker actuation arm and the microswitch.
C.�Dates and Approximate Times of Major Occurrences - all times are in Eastern Daylight Time Date�Time� Event � July 17, 2009 1742�Operations personnel aligned the alternate to normal transfer switch to the manual transfer position for the 2B-B Shutdown Board.
While holding the hand switch (EllS Code HS) for the normal supply breaker in the closed position, the normal supply breaker closed. This caused the normal and alternate supply breakers to be in the closed position. Both breakers tripped open, and all four EDGs started.
EDG 2B-B tied on and reenergized the 2B-B Shutdown Board.
July 18, 2009�1005�Supply for the 2B-B Shutdown Board was returned to the normal supply breaker and EDG 2B-B was removed from service.
D. Other Systems or Secondary Functions Affected
No other systems were affected by this event.
E. Method of Discovery
The diesel generator actuations were self-revealing by multiple alarms (EllS Code ALM) and indications in the main control room.
F. Operator Actions
TVA operations staff (licensed personnel) followed appropriate procedures for the manual fast transfer.
Once the EDGs started, the Operations crew entered Abnormal Operating Instruction (A01) 43.04, "Loss of Unit 2 Train B Shutdown Boards." Since only the 2B-B Shutdown Board was affected, a limited number of automatic equipment actions were initiated. The 2B component cooling water (El IS Code CC) pump (EllS Code P) started automatically after the required time delay and the selected Emergency Raw Cooling Water pump restarted after its required time delay. Numerous non-essential loads that automatically strip on a blackout also functioned as required and were reset by the Operations crew. The crew used the System Operating Instructions (SOls) for the EDGs to shutdown all diesel generators from the auto start and return all equipment back to normal after the load stripping on the 2B-B Shutdown Board. All unloaded EDGs were stopped by 1852. The 2B-B Diesel Generator was removed from service at 1005 July 18, 2009.
G. Safety System Responses
As a result of this event, all four EDGs started and the 2B-B EDG assumed the load of the 2B-B Shutdown Board.
III. CAUSE OF EVENT
The cause of this event was installation of the alternate supply breaker with a misaligned breaker actuation arm. The misaligned arm prevented the proper positioning of the fast transfer microswitch, thereby allowing the normal breaker to close prior to the alternate breaker opening. The closure of the normal breaker with the alternate breaker closed resulted in both breakers tripping open, thus deenergizing the board.
The cause for the installation with misalignment of the actuation arm was a lack of vendor manual criteria and thus site maintenance procedures to perform adjustment of the actuation arm in relation to the breaker's microswitch.
IV. ANALYSIS OF THE EVENT
The 2B-B 6.9 kV Shutdown Board was de-energized during the process of returning the board to its normal supply after maintenance on CSST D. When the 2B-B Shutdown Board relays detected a loss of voltage, all four diesel generators started as designed. The 2B-B Diesel Generator re-energized the 2B-B Shutdown Board. The other diesel generators remained running unloaded since their respective shutdown boards were energized from their normal supply.
V. ASSESSMENT OF SAFETY CONSEQUENCES
During this event, the EDG automatically started and all the required loads fed from the 2B-B Shutdown Board were assumed by the 2B-B EDG. The operators entered the appropriate AOls and SOls.
Performance of these procedures is an anticipated action given the loss of the 2B-B Shutdown Board.
Operators are trained to respond to these conditions. There were no human performance issues associated with the operator recovery efforts.
This condition did not adversely affect the safe operation of the plant or health and safety of the public. This condition has no impact on the ability of the board to fast transfer on a loss of normal power. The manual transfer from alternate to normal is not a safety related function. It did not affect or result in any other Engineered Safety Feature actuations. The EDGs operated within their design basis requirements, and therefore there was no impact to the safety analysis or consequences due to this event. This event was not considered to be risk significant since this actuation did not impact the ability of any safety systems to perform their design basis function during this event.
VI. CORRECTIVE ACTIONS- The corrective actions are being managed within TVA's Corrective Action Program (PER 176604). An overview of the corrective action plan is provided below.
A.�Immediate Corrective Actions 1. The alternate supply breaker was replaced with a spare breaker that successfully passed a functionality test of the fast transfer microswitch.
2. Functionality of the fast transfer microswitches was verified on the altemate supply breakers for the other 6.9 kV Shutdown Boards. The alternate supply breaker for the 2A-A Shutdown Board also had a misaligned actuation arm, and the breaker was replaced with a verified spare.
VI.�CORRECTIVE ACTIONS (continued) B.�Corrective Actions to Prevent Recurrence 1.�The vendor provided additional guidance for aligning the actuation arm with respect to the microswitch. This guidance will be incorporated into site maintenance procedures.
VII.�ADDITIONAL INFORMATION
A. Failed Components
The failed component was a misalignment of the actuation arm and the microswitch on the 2B-B Shutdown Board alternate supply breaker, which caused the breaker's microswitch to stay in the closed position when the breaker was closed. As part of the extent of condition review, a similar misalignment was discovered on the 2A-A Shutdown Board alternate supply breaker.
B. Previous LERs on Similar Events There have been other instances at Watts Bar of EDG actuations, but none have been due to this misalignment of the supply breaker's actuation arm and microswitch. Additionally, an industry operating experience search yielded no events related to this type of fast transfer microswitch failure.
C. Additional Information
None.
D. Safety System Functional Failure This event did not involve a safety system functional failure as defined in NEI 99-02, Revision 5.
E. Loss of Normal Heat Removal Consideration There was no loss of normal heat removal due to this condition.
VIII. COMMITMENTS
None.