05000311/LER-2010-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3112010003R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse — Pressurized Water Reactor (PWR/4) Main Generator Voltage Regulator {EL/90} Reactor Coolant Pump {AB/P} * Energy Industry Identification System {EllS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date: October 17, 2010 Discovery Date: October 17, 2010
CONDITIONS PRIOR TO OCCURRENCE
Salem Unit 2 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 17, 2010, at approximately 0512 hours0.00593 days <br />0.142 hours <br />8.465608e-4 weeks <br />1.94816e-4 months <br />, Salem Unit 2 automatically tripped due to bus undervoltage for the 21 through 24 Reactor Coolant Pumps (RCPs) {AB/P}. The non-safety related group buses which power the RCPs were being fed from the Unit 2 Auxiliary Power Transformer (APT). The APT is directly powered from the Unit 2 main generator. At the time of the trip the main generator voltage regulator {EL/90} was in manual control and was being transferred to automatic control. During the voltage regulator transfer to automatic, a loss of excitation occurred. This loss of excitation led to the undervoltage condition on the group buses. The 22 through 24 RCPs tripped following the reactor trip when their respective breakers opened on undervoltage, the 21 RCP continued to run through the event. The 21 and 22 Auxiliary Feedwater (AFW) pumps started as a result of the trip of both main feedwater pumps. The 23 AFW pump started as a result of low steam generator levels in-all-four steam-generators: Unit-2-was stabilized-in--Mode-3-with-a- reactorcoolant temperature of 547°F and a reactor coolant pressure of 2235 psig.
Unit 2 returned to service (generator output breaker closed) on October 20, 2010.
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)....
�NRC FORM 366A (10-2010) PRINTED ON RECYCLED PAPER
CAUSE OF OCCURRENCE
The cause of the automatic reactor trip was an undervoltage condition on the non-safety related group buses which power the RCPs. The undervoltage condition was the result of a loss of main generator excitation during the transfer of the voltage regulator from manual to automatic. Troubleshooting determined that loss of excitation was the result of a degraded 43A transfer relay that failed to transfer excitation control from the manual voltage regulator to the automatic voltage regulator.
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 safety consequence associated with this event. All safety systems operated as required in response to the RCP bus undervoltage and subsequent reactor trip.
The RCP bus undervoltage trip provides reactor core protection against departure from nucleate boiling (DNB) as a result of loss of voltage to more than one RCP. The undervoltage set points assure a reactor trip signal is generated before the reactor coolant low flow trip setpoint is reached. The potential safety consequences associated with this event are bounded by the Salem UFSAR Chapter 15 Condition II event for the partial loss of forced reactor coolant flow. The Chapter 15 analysis only credits the reactor trip on loss of flow which occurs later in time sequence than the RCP undervoltage trip.
Tripping the reactor on RCP undervoltage increased the margin to DNB as analyzed in the UFSAR.
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 could have prevented the fulfillment of a safety function of a system needed to shu tdown the reactor and maintain it in a safe shutdown condition, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident.
CORRECTIVE ACTIONS
1. The 43A transfer relay was replaced prior to placing the main generator back into service.
2. A technical evaluation was performed to review why the 22 through 24 RCP breakers opened and the 21 RCP did not. The degradation in bus voltage was close to the drop out values of the undervoltage relays for each bus. The 21RCP not opening was a slight difference in the dropout value of the undervoltage relay but within the tolerance range for this relay. Proper relay calibration was verified following the reactor trip.
COMMITMENTS
No commitments are made in this LER.