05000272/LER-2010-005
| 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 |
| 2722010005R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse — Pressurized Water Reactor (PWR/4) Electric Voltage Regulator {-/RG} * Energy Industry Identification System {El IS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date: October 15, 2010 Discovery Date: October 15, 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 October 15, 2010, at approximately 2312, with Unit 1 at 100% power and steady state conditions, the Electrical System Operator (Load Dispatcher) requested control room personnel to lower MVARS from 280 MVAR out to 230 MVAR out. When the Voltage Regulator {-/RG} Digital Control Auto Adjuster "Lower auto setpoint" console pushbutton was depressed VARS rapidly went negative to approximately -100 MVARS and immediately reversed to +270 MVARS and stabilized. A number of alarms were received, and in accordance with alarm response procedure, an operator was dispatched to the voltage regulator panel to investigate these alarms. At approximately 2321, the Unit 1 reactor tripped due to a turbine trip. The cause of the turbine trip was the actuation of the loss of field relay.
All safety related equipment responded as designed, including the Auxiliary Feedwater Pumps which started due to steam generator low level.
Unit 1 was returned to service (generator output breaker closed) on October 18, 2010, with the voltage regulator in manual control.
�NRC FORM 366A (10-2010) PRINTED ON RECYCLED PAPER DESCRIPTION OF OCCURRENCE (cont'd) This report is being made in accordance with 100 FR50.73(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
The cause of the reactor trip was the result of an automatic turbine trip. The cause of the turbine trip was the actuation of the loss of field relay, which provides the regular (primary) protection for loss of excitation for the main generator.
The exact cause for the loss of excitation has not been determined yet. A root cause evaluation is currently in progress. The most probable cause for the actuation of the loss of field relay is a defective primary Automatic Digital Regulator computer (WDR 2000).
Immediate investigation performed at the time of the event found a broken latch pin on the Phase C Main Generator Regulator Potential Transformer (PT) drawer. This finding originally led to the conclusion that the missing latch pin could have caused intermittent contact of the primary and/or secondary PT stabs causing erroneous input signals to the Main Generator Voltage Regulator (VR) automatic controls. These PT's provide voltage sensing and feedback for generator output (voltage) control when the VR is in automatic (digital) mode. Further review of alarms and relay actuations did not support the original conclusion. The broken latch pin on the PT drawer was repaired.
Troubleshooting performed during the restart of Unit 1, with the voltage regulator in manual control, found that the Maximum Excitation Limiter (MEL) WDR 2000 was actuating at no load field current levels. This early MEL actuation, with the voltage regulator in automatic control, would drive full load field excitation levels down to no load values, which could actuate the regular (primary) protection for loss of generator field. Further review of alarms and relay actuations supported the conclusion of the WDR 2000 failure.
PREVIOUS OCCURRENCES
A review of LERs at Salem Station dating back to 2007 did not identify any similar events.
SAFETY CONSEQUENCES AND IMPLICATIONS
There was no actual safety consequence associated with this event. Operators appropriately responded to the automatic reactor trip. Plant response to the reactor trip was as expected and as designed. All safety systems operated as required.
The potential safety consequences associated with this event have been analyzed in Chapter 15 of the Salem UFSAR.
�A Loss of External Electrical Load and/or Turbine Trip is categorized as condition 2 event of moderate frequency. The results of the analyses show that the plant design is such that a total loss of external electrical load without a direct or immediate reactor trip presents no hazard to the integrity of the RCS or the Main Steam System. Pressure relieving devices incorporated in the two systems are adequate to limit the maximum pressures to within the design limits.
The integrity of the core is maintained by operation of the Reactor Protection System, i.e., the DNBR will be maintained above the limit value. Therefore, there will be no cladding damage and no release of fission products to the RCS.
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 control the release of radioactive material, or mitigate the consequences of an accident.
CORRECTIVE ACTIONS
1. The phase C main generator potential transformer spring loaded detent drawer locking mechanism was repaired, and fuses were replaced.
2. Unit 1 was returned to service with the voltage regulator in manual control. During the period the voltage regulator is in manual control, daily briefings with the operating shifts are being conducted to heighten the attention to this condition.
3. The voltage regulator will be replaced during the next scheduled refueling outage via a design change.
4. A root cause has been initiated to determine the cause of the trip.
COMMITMENTS
No commitments are made in this LER.
FORM 366A (10-2010)