ENS 42171
ENS Event | |
|---|---|
10:30 Nov 27, 2005 | |
| Title | Loss and Restoration of Back-Up Instrument Air to Feedwater Control Valves |
| Event Description | Event:
At 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br /> on 11/27/2005, while in Mode 3 following a Unit 2 refueling outage, it was discovered that the nitrogen backup supply to 2FDW-315 and 2FDW-316 was valved out. 2FDW-315 and 2FDW-316 are the emergency feedwater (EFW) control valves which function to regulate steam generator level on a loss of main feedwater event. Motive force to these valves is normally supplied by instrument air and nitrogen provides backup to instrument air in the event that instrument air becomes unavailable. At 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br /> on 11/27/2005, both EFW flow paths were declared inoperable and Technical Specification 3.7.5, Conditions B and E were entered. Condition E of TS 3.7.5 specifically deals with the inoperability of two EFW flow paths. The Required Action is to initiate actions to restore one EFW flow path to operable status, immediately. Initial Safety Significance: 2FDW-315 and 2FDW-316 are the control valves for the Unit 2 'A' and 'B' steam generators, respectively. These valves must be able to function in order to control steam generator level during an event where main feedwater is lost. The motive force for these valves is normally instrument air. However, in the unlikely event where main feedwater is lost, and instrument air is unavailable, these valves rely on a nitrogen backup to supply their motive force. The events where this nitrogen backup is credited are a station blackout (SBO) and a loss of instrument air. While nitrogen backup to 2FDW-315 and 2FDW-316 was unavailable between 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br /> and 0710 hours0.00822 days <br />0.197 hours <br />0.00117 weeks <br />2.70155e-4 months <br /> on 11/27/2005, it is reasonable to assume that the ability to remove decay heat would have been available. In the case of a loss of instrument air, 2FDW-315 and 2FDW-316 fail open, with the motor driven EFW pumps providing EFW supply to both steam generators. In this case, the Emergency Operating Procedure (EOP) provides guidance to realign EFW through the electric powered startup control valves. Should this realignment attempt fail, the EOP provides further guidance to control EFW flow by manually throttling closed on 2FDW-315 and 2FDW-316. In the event of an SBO, power to the instrument air compressors would be lost, resulting in a loss of instrument air header pressure. The motor driven EFW pumps would be unavailable, however, the turbine driven EFW pump would automatically start and provide EFW flow. As with the loss of instrument air event, 2FDW-315 and 2FDW316 would fail open. The EOP provides guidance to dispatch operators to manually throttle closed on these valves to control flow to the steam generators. Additionally, while not credited for this event, Oconee has diesel driven air compressors which auto start on low instrument air pressure and supply the instrument air header. Also, in the event of a SBO, the Standby Shutdown Facility (SSF) is manned within 10 minutes of event initiation. The SSF is a SBO coping facility with a dedicated diesel generator and the pumps and valves necessary to supply and control steam generator feed in the event of an SBO. Corrective Action(s): At 0702 hours0.00813 days <br />0.195 hours <br />0.00116 weeks <br />2.67111e-4 months <br /> on 11/27/2005, nitrogen was realigned to 2FDW-316 and TS 3.7.5, Condition E was exited. At 0710 hours0.00822 days <br />0.197 hours <br />0.00117 weeks <br />2.70155e-4 months <br /> on 11/27/2005, nitrogen was realigned to 2FDW-315 and TS 3.7.5 Condition B was exited. The licensee is investigating the cause of the valve line up problem. The licensee notified the NRC Resident Inspector.
After detailed review of this event, it was concluded that Technical Specification 3.7.5 limiting conditions for operation were met at all times during this event. Control valves 2FDW-315 and 2FDW-316 fail open on loss of air/nitrogen by the spring return design of the actuators. Therefore, the required flow path would not have been lost due to the failure of normal air supply and the isolation of nitrogen supplies to the actuators. It has been further concluded that all throttling functions of these control valves could have been performed manually by use of a local handwheel on each valve actuator. Appropriate procedure guidance exists to operate the system with these valves failed open until the operators can access the local handwheel and take local control. Therefore, the valves and the associated flow paths were operable during this event. As a result of this determination, no reportability criteria under 50.72 and 50.73 apply to this event and the associated event report is hereby retracted. The licensee notified the NRC Resident Inspector. R2DO(Payne) was notified. |
| Where | |
|---|---|
Oconee  South Carolina (NRC Region 2) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat | |
| Time - Person (Reporting Time:+0.68 h0.0283 days <br />0.00405 weeks <br />9.31464e-4 months <br />) | |
| Opened: | Noel Clarkson 11:11 Nov 27, 2005 |
| NRC Officer: | Bill Gott |
| Last Updated: | Jan 26, 2006 |
| 42171 - NRC Website
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Unit 2 | |
|---|---|
| Reactor critical | Not Critical |
| Scram | No |
| Before | Hot Standby (0 %) |
| After | Hot Standby (0 %) |