05000287/LER-2013-001
| Oconee Nuclear Station, Unit 3 | |
| Event date: | 10-24-2013 |
|---|---|
| Report date: | 06-03-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2872013001R01 - NRC Website | |
EVALUATION:
Background
This LER revision provides supplemental information from Duke Energy's cause evaluation for a similar Oconee Unit 3 manual reactor trip that occurred on January 31, 2015, (LER 287/2015-001, Revision 0, ML15098A472) that was determined to be applicable to the 2013 event.
Main Feedwater Control Valve (MFCV) [EIIS: FCV] 3FDW-32 is a pneumatically operated valve, that regulates feedwater flow to the 3A steam generator when reactor power is between approximately 15% and 100%. (3FDW-41 is the corresponding MFCV for the 3B steam generator.) The analog control signal for this valve is generated by the Integrated Control System (ICS) [EIIS: JA] and transformed to a pneumatic signal via a voltage to pneumatic (E/P) converter [EIIS: CNV].
Independent of the Main Feedwater control function, an Automatic Feedwater Isolation System (AFIS) circuit will automatically close the Feedwater control valves when a Main Steam Line Break (MSLB) is detected on the associated header.
Technical Specification (TS) 3.7.3 "Main Feedwater Control Valves.(MFCVs), and Startup Feedwater Control Valves (SFCVs)" Limiting Condition for Operation (LCO) 3.7.3 requires "Two MFCVs and two SFCVs shall be OPERABLE," while in MODEs 1, 2 and 3, except when all MFCVs and SFCVs are closed and deactivated or isolated by a closed manual valve. The safety function associated with this TS is to isolate main feedwater to the steam generators upon a steam line break inside containment.
When the reactor trip was initiated, Oconee Nuclear Station (ONS) Units 1 and 3, were operating in MODE 1 at approximately 100% power. Unit 2 was defueled for a scheduled refueling outage. No significant structures, systems or components were out of service such that they contributed to this event.
This event was reported as a 4-hour notification to the NRC on October 24, 2013, in Event Notification (EN) number 49471 under 10 CFR 50.72(b)(2)(iv)(B) - RPS Actuation - Critical. This event is being reported under 10 CFR 50.73(a)(2)(iv)(A) as a manual actuation of the Reactor Protection System (RPS).
EVENT DESCRIPTION
On October 24, 2013, Oconee Unit 3 was operating at 100% power in Mode 1 when Control Room operators noticed that Unit 3 Main Feedwater flow indicators were oscillating beyond normal parameters. Control Room operators attempted to stabilize the oscillations by taking manual control of the Integrated Control System (ICS). When these efforts were recognized to be ineffective, the Control Room supervisor made the decision to manually trip Unit 3 at 0553 hours0.0064 days <br />0.154 hours <br />9.143518e-4 weeks <br />2.104165e-4 months <br />.
During plant response monitoring after the reactor trip, four main steam relief valves (MSRV) were identified as not being completely reseated. Procedural guidance was utilized to reseat the MSRVs by reducing main steam pressure. All other post trip conditions and system performance were normal and expected.
An investigation of the event determined that the feedwater flow oscillations were caused by failure of a bushing seal (o-ring) in the actuator for MFCV 3FDW-32 due to actuator piston rod and extension shaft linear misalignment. The investigation for a subsequent (2015) Unit 3 reactor trip revealed the cause for failure to properly control feedwater flow for MFCV 3FDW-32 was a latent defect in the E/P converter, 3FDW EP 0007. The latent defect produced an intermittent fault in the E/P converter. While I I I the o-ring failure was a plausible cause in 2013, given new information from the 2015 event, the faulted E/P converter was determined to be the cause of the 2013 event. The E/P converter was tested in 2013, but due to the intermittent nature of the fault the E/P was refuted after satisfactory testing of 3FDW-32 controls.
CAUSAL FACTORS
The 2015 cause investigation determined that the feedwater flow oscillations for the 2013 and 2015 events were caused by a failure of the current to pneumatic (I/P) converter (a subcomponent of the E/P converter) to properly control feedwater flow for MFCV 3FDW-32. The E/P converter (3FDW EP 0007) contains a voltage to current (E/I) converter and an VP converter. The cause of the failure was a manufacturing defect within the UP converter that resulted in intermittent malfunction of the E/P converter.
The condition that caused the failure of 3FDW-32 control function was determined to potentially apply to the MFCVs, SFCVs and other components on Oconee Units 1, 2 and 3 that use the same model E/P converter. The extent of condition was addressed in the final root cause evaluation.
The 2015 root cause evaluation concluded the following:
Root Cause:
An intermittent failure of the UP converter (a subcomponent of the E/P converter) due to a manufacturing defect, resulted in failure to properly control feedwater flow for MFCV 3FDW-32 for the 2013 and 2015 events.
Corrective actions are from the 2015 cause evaluation.
The components of the E/P converter are not safety related; therefore, 10 CFR Part 21 reportability does not apply.
CORRECTIVE ACTIONS
Immediate:
1. Prior to restarting Unit 3 from the 2015 event, the E/P converter, the Positioner and Filter Regulators for the E/P converter were replaced on 3FDW-32, the E/P converter, and Filter Regulators for the E/P converter were replaced on 3FDW-41, and successful checks and calibrations were performed on 3FDW-32 and 3FDW-41.
Completed:
1. Replaced the existing MFCV E/P converters with upgraded converters on all 3 units.
2. Increased the replacement frequency for the E/P converters to 4 years.
Planned:
1. Modify the MFCV controls to provide a more fault tolerant design.
- 1 2013 - 001 - 01
SAFETY ANALYSIS
The Oconee Unit 3 trip on October 24, 2013, is considered to be an uncomplicated reactor trip event with no significant impact on public health and safety. Although oscillations in Main Feedwater flow forced the unit offline, the Main Feedwater system continued to provide flow to the steam generators through the startup header and allowed operators to conduct an otherwise normal controlled shutdown. It was noted that four main steam relief valves failed to fully reseat at the normal expected pressures; however, the steam leakage was relatively small, no safety limits were challenged and no safety system actuations occurred. The valves were subsequently reseated by following procedural guidance to lower steam generator pressure incrementally. All four MSRVs seated within allowed procedural limits. A post-trip review found no procedural or human performance issues with the operator response to the event. There were no maintenance or other safety significant activities being conducted on any plant systems or equipment important to Unit 3 at the time of the trip. Therefore it is concluded that the conditional core damage probability for the Unit 3 reactor trip was very low and did not cause a significant increase in risk to the public.
ADDITIONAL INFORMATION
A search of the Oconee Corrective Action Program (CAP) database was conducted for the preceding five (5) year period. Similarly, a review of industry Operating Experience (OE) databases was conducted using applicable keyword searches, i.e. "feedwater oscillations," etc., to ascertain other reported events. This LER revision supplements LER 287/2013-001, Revision 0 (NRC ADAMS - ML 13358A336) for the event that occurred on October 24, 2013. This revision incorporates information from the cause evaluation for the subsequent (2015) Unit 3 reactor trip (LER 287/2015-001, ML15098A472) that occurred on January 31, 2015. No other similar events were identified.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX]. This event is considered INPO Consolidated Events System (ICES) Reportable. There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.