05000389/LER-2013-005
| St. Lucie Unit 2 | |
| Event date: | 12-19-2013 |
|---|---|
| Report date: | 02-18-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3892013005R00 - NRC Website | |
Description of the Event
On December 19, 2013, with St. Lucie Unit 2 in Mode 1 at 100% reactor power, it was identified that atmospheric conditions sustaining high winds exceeding approximately 50 miles per hour from specific directions could result in inadequate heat removal to support continuous operation of the emergency diesel generators (EDG). Should weather forecasts identify conditions that would likely result in this condition, an adverse condition monitoring plan was issued including compensatory measures to assure adequate emergency diesel generator cooling (EIIS:LB).
Exhaust air from the emergency diesel generator radiators discharges into concrete box structures on the east and west ends of the diesel generator building. The outer walls of the box structure provide a missile barrier for the radiator, form a vertical plenum to the roof line and discharge horizontally. The postulated concern is for high sustained winds to directly oppose EDG radiator discharge air flow. As a result, the EDG air-cooled radiators may have lost their cooling function during high sustained winds from plant east and west directions (i.e., east-northeast or west- southwest from true north).
Cause
The potential for sustained winds to adversely impact the Unit 2 EDG radiator fan performance was not considered in the original plant design.
Analysis of the Event
The 2A and 2B EDGs are an emergency power source for Unit 2. Each EDG has a radiator at each end for a total of four radiators. Wind generated backpressure at the EDG radiator exhaust vents on the east and west sides of the diesel generator building could have potentially decreased air flow through the EDG radiator cooling fans. This condition was adverse with respect to radiator heat transfer performance and could have caused a loss of function depending on wind direction, strength and duration.
The Unit 1 EDGs are not susceptible to their air-cooled radiators losing their cooling function during sustained high winds. On the Unit 1 EDGs, the vertical plenum extends above the roof line and discharges vertically at an angle.
Safety Significance
Had wind speeds greater than or equal to 47 mph with a wind direction from 32 to 92 degrees or from 212 to 272 degrees been experienced without the currently installed compensatory measures, the EDGs may not have been capable of performing their design function. Technical Specification 3.8.1.1 requires both EDGs operable in Modes 1 through 4. Technical Specification 3.8.1.2 requires one operable EDG in Modes 5 and 6.
The currently installed compensatory measures will provide a temporary wind barrier to mitigate wind speeds up to 74 mph. In the unlikely event that hurricane force winds (i.e., greater than 74 mph) are experienced from the wind direction 32 to 92 degrees or from 212 to 272 degrees, entering the applicable sections of the Technical Specifications (3.8.1.1 for Modes 1 through 4 or 3.8.1.2 for Modes 5 and 6) will be required. The EDGs are considered operable but degraded below full qualification.
The sustained winds that are adverse to EDG radiator heat transfer are normally associated with tropical storms and hurricanes. Developing weather patterns that could produce tropical storms or hurricanes are predictable and slow moving. This allows ample time to prepare for high wind conditions.
This licensee event report is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as a condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety. Had these sustained winds been experienced at the EDGs without the currently installed compensatory measures, this condition could have prevented the fulfillment of a safety function. Therefore, this event is also reportable pursuant to 10 CFR 50.73(a)(2)(v)(A) & (D).
This event had no effect on the health and safety of the public. During this period other sources of emergency AC power were available through the 1A and 1B EDGs.
Corrective Actions
Temporary wind barriers have been installed as compensatory measures to reduce the potential for sustained winds to adversely impact the Unit 2 EDG radiator fan performance.
The corrective action listed below has been entered into the site corrective action program. Any changes to the action will be managed under the corrective action program.
1. To correct the problem, a permanent wind deflector is being installed to direct the radiator fan discharge flow vertically to restore full design qualification of the EDGs.
Failed Component(s) None Manufacturer Engine Systems, Inc. is the Emergency Diesel Generator supplier.