05000327/LER-2009-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No Sqn Unit 2 05000328 | |
| Event date: | 03-26-2009 |
|---|---|
| Report date: | 12-21-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3272009003R01 - NRC Website | |
1. PLANT CONDITION(S)
Units 1 was operating at 96 percent power and Unit 2 was operating at 100 percent power.
II. DESCRIPTION OF EVENT
A. Event:
On March 26, 2009 at 0452 Eastern daylight time (EDT), Sequoyah Units 1 and 2 received an automatic reactor trip on reactor coolant pump (RCP) buses undervoltage (EllS code EA). A loss of Common Station Service Transformer (CSST) C (EllS code EA) caused a loss of power to two unit boards that feed RCPs (EllS code AB) on each unit. CSST C is supplied by the same switchyard power circuit breakers (EllS code FK) as CSST D. CSST C was lost because the switchyard power circuit breakers tripped as a result of a differential relay actuation on CSST D. CSST D experienced a secondary side bus phase to phase fault. The differential relays actuated as designed to clear the fault and protect CSSTs C and D, as well as the rest of the offsite power switchyard. CSST B continued to supply offsite power to the Train B safety-related shutdown boards for both units. The emergency diesel generators (DGS) (EllS code EK) started and powered the Train A safety-related shutdown boards (EllS code EB) for both units. All three auxiliary feedwater pumps (EllS code BA) on each unit started automatically and supplied the steam generators (SGs) as designed. Decay heat was removed via forced circulation of the reactor coolant system (RCS) by two RCPs, with steam relief through the SG atmospheric relief valves (EllS code SB). Both units were stabilized in hot standby (Mode 3). CSST C was isolated from CSST D and reenergized from the switchyard power circuit breakers (PCBs). Offsite power was restored to the Train A safety-related shutdown boards for both units. After the dual unit trip, with one offsite power source inoperable, the remaining offsite power source was not demonstrated operable within one hour as specified by technical specification (TS) 3.8.1.1 Action a. The TS action was subsequently completed.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
Date Description March 26, 2009 Units 1 and 2 reactor trip caused by loss of CSSTs C and at 0452 EDT D. Following the trip, the Unit 1 and 2 RCS temperature March 26, 2009 at 0508 EDT March 26, 2009 at approximately 0540 EDT March 26, 2009 at 0649 EDT March 26, 2009 at 1034 EDT March 26, 2009 at 1430 EDT March 26, 2009 at 1432 EDT March 26, 2009 at 1436 EDT March 26, 2009 at 1555 EDT was controlled by SG atmospheric relief valves. An auxiliary building isolation (ABI) was received on each unit as a result of the loss of power to a radiation monitor. All four DGS started as a result of the loss of power. DGS 1A and 2A connected to their respective shutdown boards.
Unit 1 RCS temperature dropped to 528 degrees Fahrenheit following the trip. Emergency boration was established by operations as required by Reactor Trip. Response ES-0.1.
The loss of power caused the condenser circulation water (CCW) pumps B and C to trip on each unit. The remaining CCW pump A on both units was stopped in accordance with plant procedures. The loss of CCW flow resulted in loss of condenser vacuum on both units.
The performance of the offsite power verification surveillance instruction was completed because of loss of one offsite power source. A problem event report was initiated because the offsite power verification was not performed within an hour as required by Technical Specifications (TS) 3.8.1.1.a.
While removing the 1B DG from service, the 1B DG tripped on phase imbalance relay. Both units entered TS 3.8.1.1.c for one offsite power and one DG being inoperable. The 1B shutdown board remained energized to offsite power.
Because of the abnormal annulus vacuum system alignment following the ABI, Unit 2 containment pressure reached 0.321 pounds per square inch differential (psid).
Unit 2 entered TS 3.6.1.4.
Train B emergency gas treatment system (EGTS) was placed in service to Unit 2 annulus in order to vent containment pressure. This alignment caused Train B EGTS to be inoperable for Unit 1. Since the normal power supply was inoperable for Train A EGTS, TS 3.0.5 was entered for Unit 1.
Unit 2 exited TS 3.6.1.4. Unit 2 containment was vented and containment pressure was less than 0.3 psid.
Unit 1 exited TS 3.0.5. Offsite power was restored to the 1A and 2A shutdown boards. Two offsite power sources were verified operable.
D. Other Systems or Secondary Functions Affected:
The main control room and most of the plant was without lighting, except for emergency lighting, until normal electrical board alignment was restored. The Unit 2 hydrogen side seal oil pump was secured because of an oil leak that occurred after the reactor trip. The Unit 2 main generator was depressurized and the seal oil pump was repaired.
E. Method of Discovery:
Switchyard, reactor trip, and blackout sequence alarms were annunciated on the main control room panels.
F. Operator Actions:
The operators promptly diagnosed the plant conditions and took actions as prescribed by plant procedures to stabilize the units in the hot standby condition (Mode 3).
G. Safety System Responses:
Safety systems performed as designed for the reactor trips and loss of power from CSST C . These included auxiliary feedwater auto-start, automatic feedwater isolation, and auto-start of all four DGs. The 1A and 2A DGs connected to and supplied the 1A and 2A shutdown boards as designed and functioned properly.
Ill.0CAUSE OF THE EVENT
A. Immediate Cause:
The cause of the loss of CSST C was determined to be a fault on the secondary side (6900 volt) bus of CSST D.
B. Root Cause:
The cause analysis found that insulating bus sleeve cracking at the bottom support plate, co-incident with water intrusion, caused the bus fault. The CSST D outdoor non-segregated 3 phase bus was designed with Noryl insulating sleeves, Micarta insulating supports, and is maintained inside a bus enclosure. The plant had experienced several days of rain, and there were signs of water tracking into the bus enclosure. It appears that water was able to accumulate at the bottom insulating support plate and then soak into the bus sleeve cracks to create a phase to phase fault.
The cause of the failure to perform TS 3.8.1.1 Action a was the result of initial operator focus responding to a dual unit plant trip and not performing the TS required action within one hour.
C. Contributing Factor:
Loads from CSST C would normally auto transfer to the CSST B in a fault condition.
However, CSST A loads were already aligned to CSST B, in order to support scheduled maintenance on CSST A in an upcoming outage. This alignment prevented an auto transfer of CSST C loads to CSST B.
IV.� ANALYSIS OF THE EVENT Unit 1: Prior to the event, Unit 1 was coasting down for the Unit 1 Cycle 16 refueling outage scheduled to start on March 29, 2009. The plant was operating at 96 percent power. The minimum RCS pressure following the.trip was approximately 1978 pounds per square inch gauge (psig), which is well above the pressure that would have initiated a safety injection signal (1870 psig). The minimum RCS temperature following the trip was approximately 528 degrees Fahrenheit. Emergency boration was required and initiated by Operations.
Adequate shutdown margin was maintained. The minimum pressurizer (PZR) level following the reactor trip was about 15 percent, below the availability range for the PZR heaters of 17 percent. The PZR level was below 17 percent for 3 minutes, making the PZR heaters unavailable for that amount of time. No TS limits were exceeded and the Updated Safety Analysis Report (UFSAR) analysis of this event remained bounding. The plant responded as expected for the conditions of the trip.
Unit 2: Unit 2 was operating at approximately 100 percent power prior to the reactor trip. The minimum RCS pressure following the trip was approximately 2025 psig, well above the pressure that would have initiated a safety injection signal (1870 psig). The minimum RCS temperature following the trip was approximately 532 degrees Fahrenheit. Emergency boration was not required and adequate shutdown margin was maintained. The minimum PZR level following reactor trip was about 23 percent, well above the availability range of the PZR heaters. No TS limits were exceeded and the UFSAR analysis of this event remained bounding. The plant responded as expected for the conditions of the trip.
V.�ASSESSMENT OF SAFETY CONSEQUENCES Based on the above "Analysis of The Event," this event did not adversely affect the health and safety of plant personnel or the general public.
VI.�CORRECTIVE ACTIONS A. Immediate Corrective Actions:
Corrective actions included replacement of a section of the CSST D bus and duct and inspection of the other CSSTs and cooling tower transformers (CTT).
B. Corrective Actions to Prevent Recurrence:
The other CSST and CTT buses were inspected to identify any degraded bus and to ensure bus enclosures were sealed properly. Degraded bus insulation on CSST C, CTT A, and CTT B was repaired. The preventative maintenance procedures for bus inspections will be enhanced to provide specific bus inspection instructions.
Corrective actions to address the failure to perform TS 3.8.1.1 Action a included communicating to all licensed operators that a timely, thorough, and detailed assessment of technical specifications must always be performed following any plant event or transient. In addition, emergency operating procedures were revised to evaluate operability of offsite power sources during recovery actions.
VII. ADDITIONAL INFORMATION
A. Failed Components:
The failed component of this event was a 6900-volt CSST D outdoor non-segregated three phase bus.
B. Previous LERs on Similar Events:
A review of previous reportable events for the past 10 years did not identify any previous similar events.
C. Additional Information:
None.
D. Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v).
E. Unplanned Scram with Complications:
This condition resulted in an unplanned scram with complications. Main feedwater was not available, as designed, following the unplanned scram, because the loss of power resulted in a loss of condenser vacuum and the loss of main feedwater pumps.
VIII. COMMITMENTS
None.