ENS 40724
ENS Event | |
|---|---|
20:55 May 4, 2004 | |
| Title | Unanalyzed Condition and Potential Loss of Containment Safety Function |
| Event Description | NOTE: Oconee Unit 2 is DEFUELED (No Mode) in a refueling outage.
Oconee has discovered a new scenario where the double ended guillotine rupture addressed by IE Bulletin 80-04 is not the worst case with respect to containment pressure following a Main Steam Line Break (MSLB). Therefore this scenario is being reporting as an Unanalyzed Condition and a potential loss of the containment safety function. This new scenario involves a range of MSLB break sizes smaller than the double ended guillotine rupture addressed by IE Bulletin 80-04, with the break located inside containment, and with a concurrent Loss Of Offsite Power (LOOP). This event scenario is still being evaluated but it appears to result in containment pressures in excess of the 59 psig limit discussed in the UFSAR. Preliminary results indicate that pressure would not exceed the containment 'ultimate capacity' of 144 psig. Analysis has shown that the Steam Generator tube stresses remain within acceptable limits for this event scenario so that no additional RCS leakage would occur. As a result, the offsite dose for this scenario is bounded by previous scenarios for a MSLB outside containment. Background The AFIS (Automatic Feedwater Isolation System) Circuitry required by Tech Specs 3.3.11, 3.3.12, and 3.3.13 is intended to isolate Feedwater from the steam generators in the event of a Main Steam Line Break (MSLB) by closing the Feedwater Control Valves and preventing operation of the Emergency Feedwater Pumps. Tech Spec 3.7.3 requires the Feedwater Control Valves to be operable and provides an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Allowed Outage Time (AOT) if they are not. If the AOT is exceeded, the affected unit must be in Mode 3 within an additional 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The Feedwater Control Valves are air operated and fail as-is on loss of Instrument Air. Low MS pressure causes AFIS to signal the valves to close. Emergency Feedwater Pumps are interlocked based on rate of MS pressure drop. Scenario: ONS identified a new scenario involving a MSLB smaller than the double ended guillotine rupture addressed by IE Bulletin 80-04, located inside containment, and occurring concurrent with a LOOP. In this scenario, the LOOP results in the loss of power to the permanently installed plant instrument air compressors, so that instrument air pressure would begin to decay. The smaller MSLB break size slows the rate of pressure loss from the affected MS line. Break sizes exist such that by the time the MS system pressure decays to the AFIS actuation setpoint, instrument air pressure might become inadequate to close the Feedwater Control Valves. Also, the low rate of change of MS pressure might not satisfy the AFIS rate based setpoint for actuating the EFW pump interlock. If the control valves remain open or if the EFW pumps are allowed to operate, additional inventory would reach the steam generators and flash to steam. Since the scenario requires the break to be located inside containment, this would result in increased containment pressures. Initial Safety Significance: The Main Steam Line Break inside containment is a low probability event. Also, although the MSLB scenario includes the assumption of some primary to secondary leakage, the analysis shows that Steam Generator tube stresses remain within acceptable limits for these break sizes, so that no additional RCS leakage would occur. As a result, the offsite dose for this scenario is still bounded by the scenario for a break outside containment. Therefore this event is not significant with respect to the health and safety of the public. Corrective Action(s): A diesel compressor connected to the Instrument Air header has been placed in operation to assure a source of instrument air in the event of a LOOP. Additional actions for permanent resolution are being evaluated. The licensee will notify the NRC Resident Inspector. |
| Where | |
|---|---|
Oconee  South Carolina (NRC Region 2) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| LER: | 05000269/LER-2004-002 |
| Time - Person (Reporting Time:+-1.78 h-0.0742 days <br />-0.0106 weeks <br />-0.00244 months <br />) | |
| Opened: | Randy Todd 19:08 May 4, 2004 |
| NRC Officer: | Arlon Costa |
| Last Updated: | May 4, 2004 |
| 40724 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |
Unit 3 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |