05000387/LER-2012-010
| Susquehanna Steam Electric Station Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 48595 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 3872012000R01 - NRC Website | |
On December 14, 2012 at 1350, while the 'A' Control Structure (CS) HVAC was inoperable during routine maintenance, Operations declared the 'B' CS HVAC inoperable when the 'B' loop chilled water circulating pump tripped during a swap from the 'A' to the 'B' train. Consequently, both trains of CS HVAC were inoperable simultaneously, although at least one train of CS HVAC was in operation during the event. This event required and at 1459 for Unit 2. This event was reported on Event Notification#48595.
The 'A' CS HVAC system was declared inoperable per LCO 3.7.3 Condition A and LCO 3.7.4 Condition A for replacement of the fan belts. Following completion of the physical work, the 'A' CS subsystem had operated for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> during a post maintenance break-in period for the new belts and to monitor fan vibrations. At the conclusion of the 'A' break-in run, the 'B' CS chiller recirculation pump was being placed in service and it tripped when started. This resulted in the declaration that the 'B' train of CS HVAC was inoperable. The concurrent inoperability of both trains of CS HVAC resulted in entry into LCO 3.0.3 for Units 1 and 2 in accordance with the TS. The 'A' CS HVAC train remained in operation while the 'B' Control Structure HVAC was successfully placed in service.
Sequence of Events The following is a summary of the events related to this reportable condition:
- 12/10/2012 —At 0:09 entered LCO's 3.7.3 and 3.7.4 for CS 'A' train fan belt replacements
- 12/14/2012 — At 13:50 during start of CS 'B' loop chilled water circulating pump the breaker tripped
- 12/14/2012 —At 13:50 entered LCO 3.0.3 for both Units 1 and 2
- 12/14/2012 —At 14.08 CS 'B' loop circulating pump breaker reset
- 12/14/2012 — At 14:53 Unit 1 began power reduction
- 12/14/2012 — At 14:59 Unit 2 began power reduction
- 12/14/2012 — At 15:00 provided 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> ENS notification to NRC
- 12/14/2012 — At 16:25 removed clearances from CS 'A' train fans
- 12/14/2012 — At 16:30 transferred CS chillers, 'A' train placed in-service, '8' train shutdown
- 12/14/2012 — At 17:10 cleared LCO 3.0.3
- 12/14/2012 — At 18:19 Unit 1 returned to 98% power
- 12/14/2012 —At 19:43 Unit 2 returned to 98% power
- 12/15/2012 —At 13:48 cleared LCOs 3.7.3 and 3.7.4
CAUSE OF THE EVENT
The direct cause of the event was a failure of the 'B' CS HVAC system when the 'B' loop circulating pump tripped during starting. The apparent cause is electrical breakdown of the 'B' CS Chilled Water Circulating Pump motor such that the motor no longer operates within the manufacturer's nameplate rating under startup conditions. This condition introduces higher than expected starting currents, removing the normal operating margin between motor startup conditions and the breaker trip setting. This condition has led to intermittent trips of the circuit breaker supplying the 'B' CS Chilled Water Circulating Pump motor.
There were no adverse consequences to the health and safety of the public as a result of this event. The 'A' CS HVAC subsystem remained in service and functional at all times throughout the event. With the 'A' train in service, the failure of the 'B' train to start when called upon did not create any adverse consequences to public health and safety.
One train of CS HVAC functionally remained in service at all times to maintain ventilation and cooling to the Control Structure spaces. However, both trains of CS HVAC were inoperable simultaneously. The 'A' train of CS HVAC was INOPERABLE for a scheduled maintenance work window. The 'B' train of CS HVAC was rendered INOPERABLE when the breaker for the 'B' CS Chilled Water Circulating Pump tripped. In accordance with plant Technical Specifications (TS), this condition required both units to reduce power in accordance with LCO 3.0.3 action statements. Both units reduced power to approximately 80%. The maintenance work on the 'A' CS HVAC system was completed prior to requiring a further power reduction and it was restored to service.
At this point, both SSES units exited LCO 3.0.3 and returned to full power operation.
This event is being reported under 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of a safety function and 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by Technical Specifications.
Potential Consequences:
A potential consequence of this event is a dual unit shutdown. If the 'A' train of CS HVAC could not be restored promptly, further power reductions on both units would have been required, possibly resulting in complete shutdown to MODE 4 on both units.
Another potential consequence is the loss of both trains of CS HVAC due to a failure of the in-service 'A' train followed by failure of the 'B' train white it attempted to auto-start . The 'B' train failure is postulated to be caused by a 'B' loop chilled water circulating pump breaker trip
CORRECTIVE ACTIONS
An immediate corrective action was to restore the 'A' CS HVAC system to OPERABLE and suspend the power reductions on both units followed by restoration to pre-event power levels.
An interim corrective action taken was to temporarily raise the breaker setting for the 'B' CS Chilled Water Circulating Pump by replacing the breaker with a higher trip setpoint.
A long term corrective action to replace the 'B' CS Chilled Water Circulating Pump motor is planned for the 41h quarter 2013.
Additional corrective actions are scheduled for completion in 2013 which include:
- reviewing previous breaker trips of other plant components to determine if a common cause exists; and
- evaluating starting current data from other safety related motors.
PREVIOUS SIMILAR EVENTS
Following is a listing of similar Susquehanna related CS HVAC LER's:
Unit 1 - LER 2012-001, Both Control Structure Chillers Inoperable' Unit 1- LER 2012-002, 'Inoperability of V Control Structure Chiller coincident with inoperability of 'A' EDG' Unit 2 - LER 2012-001, toss of Two Control Room Floor Cooling Systems' NRC FORM 3613A (10-2010)