05000287/LER-2015-001, Manual Reactor Trip Due to Unacceptable Main Feedwater Flow Control Valve Oscillations
| Oconee Nuclear Station, Unit 3 | |
| Event date: | 1-31-2015 |
|---|---|
| Report date: | 03-31-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 50781 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 2872015001R00 - NRC Website | |
On January 31, 2015, Oconee Unit 3 was operating at 100% power in MODE 1 when Control Room operators observed that Main Feedwater flow indicators were oscillating outside of normal parameters. The Control Room supervisor made the decision to manually trip Unit 3 at 1431 hours due to erratic feedwater operation and increasing RCS pressure. A subsequent investigation determined the feedwater flow oscillations were caused by a subcomponent failure of the electrical to pneumatic converter (E/P), This event was reported as a 4-hour notification to the NRC on January 31, 2015, in Event Notification (EN) number 50781 under 10 CFR 50.72(b)(2)(iv)(B) - Reactor Protection System (RPS) Actuation - Critical. The event is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an actuation of the RPS.
EVALUATION:
BACKGROUND
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%.
generated by the Integrated Control System (ICS) [EDS: JA] and transformed to a pneumatic signal via an 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 When the reactor trip was initiated, Oconee Nuclear Station (ONS) Units 1, 2 and 3, were operating in MODE 1 at approximately 100% power. No significant structures, systems or components were out of service such that they This event was reported as a 4-hour notification to the NRC on January 31, 2015, in Event Notification (EN) number 50781 under 10 CFR 50.72(b)(2)(iv)(B) - Reactor Protection System (RPS) Actuation - Critical. The event is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an actuation of the RPS.
EVENT DESCRIPTION
On January 31, 2015, Oconee Unit 3 (ONS-3) was operating at 100% power in MODE 1 when Control Room (CR) operators noticed that the ONS-3 Main Feedwater (MFW) flow indicators were oscillating beyond normal parameters. The CR supervisor made the decision to manually trip ONS-3 at 1431 hours due to the erratic MFW flow oscillations and increasing Reactor Coolant System (RCS) pressure. During plant response monitoring after the manual reactor trip, one main steam relief valve (MSRV) did not completely reseat, and a condensate booster pump mechanical seal leak was observed. Existing post-trip procedure guidance was used to reseat the MSRV by reducing main steam pressure. The MSRV seated within allowed procedural and safety analysis limits. The condensate booster pump was secured to stop the seal leakage. Other post trip conditions and system performance indications were normal.
CAUSAL FACTORS
A preliminary cause investigation of the event determined that the feedwater flow oscillations were caused by a failure of the current to pneumatic converter (I/P) [a subcomponent of the electrical to pneumatic converter (E/P)] manufacturing defect within the UP that resulted in intermittent malfunction of the E/P.
MFCVs, SFCVs and other components on Oconee Units 1, 2 and 3 that use the same model E/P. The extent of condition is being reviewed and evaluated further in the final root cause evaluation.
The preliminary root cause evaluation concluded the following:
Root Cause:
An intermittent failure of the UP (a subcomponent of the E/P) due to a manufacturing defect, resulted Duke Energy's root cause evaluation for this event was not complete as of the date of submission of this LER. If the final root cause evaluation conclusions or corrective actions differ significantly than the preliminary results herein, then a supplement to this LER will be submitted.
The components of the E/P are not safety related; therefore, Part 21 reportability does not apply.
CORRECTIVE ACTIONS
Immediate:
1. Prior to restarting Unit 3, the E/P, the Positioner and Filter Regulators for the E/P were replaced on 3FDW-32, the E/P, and Filter Regulators for the E/P were replaced on 3FDW-41, and successful checks and calibrations were performed on 3FDW-32 and 3FDW-41.
Planned:
1. Replace the existing MFCV E/P converters on the other units.
2. Increase the replacement frequency for the E/P converters.
SAFETY ANALYSIS
The ONS-3 trip on January 31, 2015, was uncomplicated and had no impact on public health and the reactor trip, 3FDW-32 closed fully and 3FDW-31 (Motor Operated Isolation Valve) also closed. Based on the fact that the Automatic Feedwater Isolation System (AFIS) function was maintained, Duke Energy concluded that event. It was previously noted that there were minor perturbations with the reseating of one (1) Main Steam Relief Valve (MSRV) 3MS-3 and a small leak from a condensate booster pump but these were appropriately addressed. Additionally, no Emergency Core Cooling System (ECCS) actuations occurred. Consequently, due to the uneventful nature of the shutdown, the ONS-3 reactor trip did not result in 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 review revealed a similar Duke Energy event that occurred at ONS-3 on October 24, 2013, when the unit was manually tripped due to feedwater oscillations. This previous event was reported in LER 287/2013-01, Revision 0 (NRC ADAMS - ML 13358A336). Although the cause reevaluation of the prior event is ongoing (as part of the cause evaluation for this event), there appear to be similarities between the causes of both events.
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.