05000272/LER-2010-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2722010002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse — Pressurized Water Reactor (PWR/4) Main Power Transformer {EA/XFMR} * Energy Industry Identification System {EIIS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date: July 7, 2010 Discovery Date: July 7, 2010
CONDITIONS PRIOR TO OCCURRENCE
Salem Unit 1 was in Operational Mode 1.
No structures, systems or components were inoperable at the time of the discovery that contributed to the event.
DESCRIPTION OF OCCURRENCE
On July 7, 2010, at approximately 1116, with Unit 1 at 100% power and steady state conditions, several fire protection system control room overhead alarms annunciated in the control room. At 1118, shortly after the receipt of these alarms, Unit 1 automatically tripped due to a turbine trip above 50% reactor power. The turbine trip was a result of an actuation of the regular and backup phase B-C differential relays in the main generator protection scheme caused by a fault on the phase "B" of the main power transformer (MPT) {EA/XFMR}. Control room personnel entered the emergency operating procedures to stabilize the plant following the unit trip. At 1156 control room personnel exited the emergency operating procedures and entered the integrated operating procedures, with Unit 1 stable in Mode 3 at normal operating temperature and pressure. All safety related equipment responded as designed.
Unit 1 returned to service (generator output breaker closed) on July 25, 2010, following replacement of the phase "B" main power transformer bushing.
�NRC FORM 366 (9-2007) PRINTED ON RECYCLED PAPER DESCRIPTION OF OCCURRENCE (cont'd) This report is being made in accordance with 10CFR50.73(a)(2)(iv)(A),"any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B)....
CAUSE OF OCCURRENCE
Salem Unit 1 automatically tripped due to a turbine trip above 50% reactor power. The turbine trip was a result of an actuation of the regular and backup phase B-C differential relays in the main generator protection scheme caused by a fault on the "B" phase of the main power transformer.
An ongoing root cause investigation has determined that failure of the main power transformer "B" phase bushing was the result of an arc flash across the bushing following an inadvertent actuation of the transformer fire protection deluge system. The heat from the transformer, unusually high ambient temperatures, direct sunlight and restricted ventilation due to three sides of the main transformer "B" phase being surrounded by concrete walls caused one of the air-pilot sprinkler heads to fuse, with the resultant discharge. The air-pilot sprinkler heads set point was determined to be 165 degrees F per design which was validated by testing 12 other similar air-pilot sprinkler heads.
Although the design of the transformer deluge system is such that upon discharge it does not provide direct water spray onto the transformer bushing, the deluge mist rose above the main power transformer "B" phase bushing, driven by the transformer operating fans, heat rising from the transformer and the close proximity of the block wall enclosure, resulting in the observed arc flash and bushing failure.
PREVIOUS OCCURRENCES
A review of LERs at Salem Station dating back to 2007 identified one reactor trip due to a failure of a major electrical component. LER 272/2007-003 "Salem Unit 1 Automatic Reactor Trip Due to The Failure of 12 Station Power Transformer Load Tap Changer" was issued on February 25, 2008. The cause of the 2007 event was an inadequate scope of maintenance procedures performed on load tap changer internal components and insufficient performance monitoring of degrading load tap changer conditions. The corrective actions taken were specific to the 2007 event and would not have prevented this event.
SAFETY CONSEQUENCES AND IMPLICATIONS
There was no actual safety consequence associated with this event. Operators appropriately responded to the failure of the main power transformer bushing and subsequent automatic reactor trip.
Plant response to the reactor trip was as expected and as designed. All safety systems operated as required.
A review of this event determined that a Safety System Functional Failure (SSFF) as defined in NEI 99- 02, Regulatory Assessment Performance Indicator Guidelines, did not occur. This event did not result in a condition that alone could have prevented the fulfillment of a safety function of a system needed to remove residual heat.
CORRECTIVE ACTIONS
- 1. All Unit 1 main power transformer deluge system air-pilot sprinkler heads were replaced via a design change with air-pilot sprinkler heads set at 286 degrees F.
2. The fire protection deluge system has been isolated requiring manual actuation if necessary.
The Unit 2 main power transformer deluge system air-pilot sprinkler heads will be replaced via design change during the next scheduled refueling outage.
3. Additional corrective actions may be taken as appropriate at the conclusion of the root cause investigation.
COMMITMENTS
No commitments are made in this LER.
FORM 366A(9-2007)