05000368/LER-2009-004
| Arkansas Nuclear One, Unit 2 | |
| Event date: | 09-20-2009 |
|---|---|
| Report date: | 11-19-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3682009004R00 - NRC Website | |
10. POWER LEVEL
000
12. LICENSEE CONTACT FOR THIS LER
FACILITY NAME
David B. Bice, Acting Manager, Licensing TELEPHONE NUMBER (Include Area Code) 479-858-4710
13. COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT
CAUSE SYSTEM COMPONENT MANU- FACTURER
REPORTABLE
TO EPIX CAUSE SYSTEM COMPONENT
MANU-
FACTURER
REPORTABLE
TO EPIX
X EA BKR 6080 Y
14. SUPPLEMENTAL REPORT EXPECTED
- YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 0 @ 0 NO
15. EXPECTED
SUBMISSION DATE
MONTH DAY YEAR
ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) On September 20, 2009, at approximately 0511 Central Daylight Time, during the performance of planned surveillance testing, the 2K-4A Emergency Diesel Generator automatically started due to an undervoltage signal. An Offsite Power Transfer Test was being performed to test the automatic transfer from the Startup 3 Offsite Transformer (SU3) to the Startup 2 Offsite Transformer (SU2).
When the test was initiated, a slow transfer of the Red Train 4160 Volt Electrical Bus 2A1 from SU3 to SU2 occurred instead of the expected fast transfer. The slow transfer of the 2A1 Bus resulted in a momentary loss of power, for approximately two seconds, to the Red Train 4160 Volt Safety Electrical Bus 2A3 which is powered from the 2A1 Bus. The undervoltage condition on the 2A3 Bus caused the 2K-4A Emergency Diesel Generator to auto start as designed, but did not power the 2A3 Bus, since the 2A3 Bus was successfully powered from the 2A1 Bus after the slow transfer completed. The cause of this event was attributed to a high resistance contact on a contact block located in the SU2 to 2A1 Bus Feeder Breaker. This high resistance contact interfered with the voltage supply to the 2A1 Bus Synchronizing Relay, resulting in the slow transfer of the 2A1 Bus.
The wiring located on the high resistance contact was subsequently moved to an alternate available contact on the subject contact block with appropriate resistance.
A. Plant Status On September 20, 2009, at approximately 0511 Central Daylight Time, Arkansas Nuclear One — Unit 2 (ANO-2) was in Mode 5 for a scheduled refueling outage. The Reactor Coolant System (RCS) [AB] Pressurizer level was 85 percent and the RCS temperature was 139 degrees Fahrenheit. The Red Train Shutdown Cooling (SDC) [BP] loop was in service with the Green Train SDC Pump and Containment Spray [BE] pump available in standby.
B. Event Description
During the performance of planned surveillance testing, the 2K-4A Emergency Diesel Generator (EDG) [EK, DG] automatically started. An Offsite Power Transfer Test was being performed to test automatic transfer from the Startup 3 Offsite Transformer (SU3) [EA, XFMR] to the Startup 2 Offsite Transformer (SU2) [EA, XFMR]. This transfer test is accomplished by momentarily using a handheld jumper to simulate the loss of voltage on SU3. This causes the SU3 feeder breaker 2A-113 [EA, BKR] to the 4160 Volt Bus 2A1 (2A1) [EA, BU] to open, which in turn closes a permissive contact in the SU2 feeder breaker 2A-111 [EA, BKR] to Bus 2A1. If SU2 and the 2A1 Bus are synchronized, the SU2 to 2A1 feeder breaker will then close immediately, thus completing a fast bus transfer. If they are not synchronized, the SU2 to 2A1 feeder breaker will not close immediately; instead, 2A1 will momentarily de-energize, then the 2A1 undervoltage relay will close the SU2 to 2A1 feeder breaker, completing a slow bus transfer. When the Offsite Power Transfer Test was initiated, a slow transfer of the 2A1 Bus occurred instead of the expected fast transfer. The slow transfer of 2A1 resulted in a momentary loss of power, for approximately two seconds, to the 4160 Volt Safety Electrical Bus 2A3 (2A3) [EB,BU] which is powered from 2A1. The momentary undervoltage condition on 2A3 caused the 2K-4A EDG to auto start as designed. The EDG did not power 2A3, since 2A3 was successfully powered from 2A1 after the slow transfer completed. During the momentary loss of power, 2A3 automatically shed all loads as designed. This caused the running Shutdown Cooling Pump (Low Pressure Safety Injection Pump 2P-60A) [BP, P] to secure, which resulted in a loss of Shutdown Cooling flow to the Reactor Coolant System for approximately three and one half minutes. Shutdown Cooling flow was successfully restored using the applicable procedure.
voltage inputs that feed the 2A1 Bus synchronizing check relay (125-111) [EA,25], located in the 2A-111 breaker cubicle, due to a high resistance condition on a contact (11-12 contact) [BU, CON] within the 2A-111 contact switch block (General Electric Model SB-12). This high resistance condition blocked one of the voltage inputs to the synchronizing check relay, causing the relay to falsely indicate that the SU2 Transformer and the 2A1 Bus were not synchronized.
This condition resulted in a slow transfer of the 2A1 Bus in approximately two seconds, instead of a fast transfer, which would normally be expected to occur within a few electrical cycles.
D. Corrective Actions
Immediate Actions: Wires connected at the 11-12 contacts on the 2A-111 SB-12 contact block were moved to alternate contacts with appropriate resistance. Additional long term corrective actions are being tracked in the Arkansas Nuclear One (ANO) corrective action process.
E. Safety Significance
The event did not prevent the fulfillment of any safety functions, and there were no safety system functional failures. Offsite and onsite emergency power supplies were available during the event. The Shutdown Cooling pump that stopped during the event remained available for restart following the automatic restoration of power, 2 seconds after the initial de-energization of the 2A1 Bus. Additionally, the standby Shutdown Cooling train was available during the event.
RCS temperature increased approximately five degrees Fahrenheit during the event; however, no RCS heat up or cool down limits were exceeded.
F. Basis for Reportability 10 CFR 50.73(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).
10 CFR 50.73(a)(2)(iv)(B) "The systems to which the requirements of paragraph (a)(2)(iv)(A) of this section apply are: (8) Emergency ac electrical power systems, including: emergency diesel generators (EDGs);"
G. Additional Information
A review of the last five years of reportable events did not reveal any previous similar events reported by ANO pertaining to automatic actuations of systems identified in 10 CFR 50.73(a)(2)(iv)(B) that were caused by high resistance contact failures.
Energy Industry Identification System (El IS) codes and component codes are identified in the text as [XC].