05000389/LER-2014-002
| St. Lucie Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3892014002R01 - NRC Website | |
Description of the Event
On November 12, 2014 Unit 2 was manually tripped due to a lowering 2B steam generator level caused by the spurious (slow) closure of 2B steam generator main feedwater isolation valve (MFIV)[EIIS:JBI, HCV-09-2B. All control element assemblies (CEAs) fully inserted into the core. All safety systems responded as expected with the auxiliary feedwater actuation system channel 2(AFAS 2) actuating on low 2B steam generator level. Decay heat removal was from main feedwater to the 2A steam generator and manual control of auxiliary feedwater to the 2B steam generator.
Cause of the Event
A root cause evaluation (RCE) determined the MFIV HCV-09-2B spuriously stroked closed in the slow mode as a result of a preventive maintenance (PM) strategy that was insufficient to detect and mitigate the single point vulnerability induced by the environmental conditions affecting the relay box. Contributing causes included 1) the relay box for HCV-09-2B was insufficient in controlling humidity in the environment experienced by the relays in the steam trestle area that would ensure non-safety function reliability and 2) a previous RCE performed for HCV-09- 2A spurious closure, identified an issue with top entry conduits into electrical boxes.
Analysis of Safety Significance The safety function of HCV-09-2B is to close in response to a main steam isolation signal, or an auxiliary feedwater actuation signal (AFAS). Spurious closure of HCV-09-2B isolated feedwater to the 2B steam generator, resulting in lowering level. When lowering steam generator level was approaching the 50 percent narrow range operating limit, a manual trip was initiated in accordance with site procedure. The trip was uncomplicated and the Unit 2 risk remained Green. The AFAS actuated on low steam generator narrow range level as expected for the plant conditions during the event. There were no unexpected automatic safety system actuations as a result of the trip.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) for the reactor protection system (RPS) and AFAS actuation and 10 CFR 50.73(a)(2)(i)(A) for a plant shutdown required by technical specifications. All plant systems responded as designed and there was minimal safety significance associated with this event.
St. Lucie Unit 2 05000389
Corrective Actions
The corrective actions listed below are entered into the site corrective action program. Any changes to the actions will be managed under the corrective action program.
Completed Actions
1. Replaced the degraded relays 3Y/672 and HCV-09-2B for HCV-09-2B.
2. Installed conduit seals on conduit leading to the HCV-09-2B relay box.
3. Sealed remaining conduit to MFIV relay boxes to the HCV-09-1A relay box.
4. Replaced degraded relays in the HCV-09-2A relay box.
Corrective Actions
1. Modify Unit 2 MFIVs to eliminate location of relays in adverse environment.
2. Replace HFA and HGA relays in the Unit 2 MFIV relay boxes in conjunction with 0-PME-100.10 18 month PM (SL2-22).
SIMILAR EVENTS
Internal OE A review of similar events identified two previous related events:
- St. Lucie LER 2013-004-00, "Manual Trip Following Spurious Closure of Main Feedwater Isolation Valve (MFIV) and Lowering of Steam Generator Levels," involved an MFIV closing as a result of corrosion of two relays (3Y/671 or 20X/671) located inside the relay box caused by internal water intrusion in the conduits. Corrective actions included replacing the degraded relays and installing internal conduit seals. The RCE performed for HCV-09-2A spurious closure identified an issue with top entry conduits to electrical boxes but did not create a corrective action to seal the similar HCV-09-2B relay box.
For moisture intrusion, St. Lucie has revised a PM activity which addresses electrical box degradation on a pre-determined population.
Procedure, 0-PME-100.10, "Periodic Inspection of Electrical Boxes and Cabinets,".
There were no additional actions needed to address the concerns from this information notice.
Failed Component(s) Model No. 12HFA151A2H - HFA Relay Manufacturer: General Electric