05000306/LER-2015-003
| Docket Number | |
| Event date: | 6-7-2015 |
|---|---|
| Report date: | 8-5-2015 |
| 3062015003R00 - NRC Website | |
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FACILITY NAME 3. PAGE 2. DOCKET 6. LER NUMBER 2015 - 003 - 0 On June 7, 2015, Unit 2 Turbine Bearing Oil Pressure was normal at approximately 19 psig. At approximately 05:00, the Unit 2 Turbine Bearing Oil Pressure started to decrease. The Unit 2 Turbine Bearing Oil Pressure decreased at an increasing rate until at 07:35 oil pressure sharply decreased from approximately 17.7 psi to a level that required a turbine trip (trip setpoint is 6 psi). Unit 2 Turbine Bearing Oil Pressure quickly recovered from a low point as indicated on Emergency Response Computer System (ERCS) of 4.3 psi at 07:36:19 to above 16 psi at 07:36:21. Turbine Bearing Oil Pressure was able to be maintained by the Turning Gear Oil Pump while the turbine was on turning gear at a nominal pressure of 21 psi. The Unit 2 Turbine was removed from the turning gear at 00:30 on June 8, 2015. The oil system was shut down a short time thereafter. Oil was drained from the Unit 2 Turbine Oil Reservoir on June 8, 2015 at 06:00. As a result of the investigation, the site identified a circumferential crack on a welded joint upstream of check valve 2T0-303 on June 8, 2015 at 18:23. (Corrective Action Program (CAP) Action Request (AR) 1482236). The weld was repaired by Work Order 524416-06 on June 10, 2015.
The Auxiliary Feedwater Systeml actuated to start the auxiliary feedwater pumps as designed on low narrow range steam generator level and provided makeup flow to the steam generators. The auxiliary feedwater actuation was reportable per 10 CFR 50.72(b)(3)(iv)(A). Steam generator levels returned to normal. Auxiliary Feedwater was secured.
EVENT ANALYSIS
Troubleshooting performed in support of the event identified a cracked and broken weld on a reducing flange for the line that supplies high pressure oil to the Seal Oil skid and Auto Stop Oil (Direct Cause). This was the component that failed and resulted in the loss of pressure to the turbine oil bearings.
A Failure Modes and Effects Analysis (FMEA) was performed using available information. The component that failed was not available for inspection in support of the FMEA development as it was required to be reused to restore the system. New spare parts were not available.
Under WO 524416, the site ground the failed welded fitting, repaired damaged material, and reused the reducing flange. The site discovered that the weld that had broken lacked weld penetration and had significant voids in the weld. There also appears to be a brittle failure of some of the base metal of the flange. However, the majority of the failure occurred in the weld region.
The turbine oil reservoir and associated components and piping were supplied as a skid from Westinghouse with no unique equipment identifiers for the piping or valves inside the reservoir. Based on the condition of the weld on the reducing flange, it is most likely an original weld. If there was frequent inspection, the added stress applied during the disassembly process could contribute to the failed weld; therefore, maintenance history for the check valves was reviewed to determine how often they were inspected. The check valve nearest to the failure, 2T0-303, appears never to have been inspected. Another small check valve near the failure, 2T0-304, had only been inspected once. Therefore, inspection of these check valves did not likely contribute to this 1 EIIS System Code — BA
FACILITY NAME
2. DOCKET 6. LER NUMBER 2015 - 003 0 failure. Additionally, as there is no evidence that 2T0-303 has previously been disassembled, changes in gasket thickness from original configuration during reassembly is also not a probable cause for the failure.
The piping and welds for the section of piping from check valve 2T0-303 to the downstream orifice were replaced due to the poor quality of welds. Additional inspections will be performed on other welds in the turbine oil reservoir to ensure similar conditions do not exist.
SAFETY SIGNIFICANCE
This event did not challenge nuclear safety as all plant systems responded as designed. The automatic reactor trip occurred as designed because of the automatic turbine trip. There were no radiological, environmental, or industrial impacts associated with this event. The health and safety of the public and site personnel were not at risk at any time during this event.
CAUSE
Poor weld quality reduced the strength of the welded connection resulting in a broken weld at the connection of the main oil pump discharge to the high pressure seal oil supply. This resulted in a diversion of oil flow from the bearing oil supply piping. The poor weld quality combined with the normal stresses that occur in an operating system over the life of the plant led to the failure of the weld.
CORRECTIVE ACTION
- Perform weld repair, internal Unit 2 turbine oil reservoir piping. Complete (Work Order 524416-06).
O Inspect piping connections in the Unit 2 Turbine Oil Reservoir to determine if the fit up is appropriate and inspect welds for cracks (CA 1482090-11).
- Inspect piping connections in the Unit 1 Turbine Oil Reservoir to determine if the fit up is appropriate and inspect welds for cracks (CA 1482090-12).
PREVIOUS SIMILAR EVENTS
None.