05000482/LER-2010-006
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4822010006R00 - NRC Website | |
On 3/8/2010 the Wolf Creek Generating Station (WCGS) was in the process of power ascension at approximately 42 percent power. The "B" main feedwater (MFW) pump [EllS Code: SJ-P] was being warmed in the process of being placed in service, but was not providing flow to the steam generators (SG). At 0332 CST the "A" MFW pump automatically tripped. With no feedwater supply to the steam generators (SG) [EllS Code: SG], water level in the steam generators began decreasing and the Control Room staff manually tripped the reactor at 0333 CST.
The MFW pump tripped due to a failed servo valve [EllS Code: JK-FCV] in the main feedwater speed control system. The servo failed when it was in the open position, causing the low pressure and high pressure control valves to become full open. This resulted in more steam being admitted to the feedwater turbine when there was no demand for it. The speed of the feedwater turbine increased beyond its overspeed set point, resulting in a trip of the MFW pump.
The reactor was manually tripped due to the decreasing SG level, prior to the automatic trip set point being reached. All control rods fully inserted and all safety equipment performed as designed. The Reactor Trip System (RTS) and the Engineered Safety Feature Actuation System (ESFAS) [EllS Code: JE] performed as required. The motor-driven auxiliary feedwater (AFW) pumps [EllS Code: BA] started due to the loss of the MFW pump and the turbine-driven AFW pump started due to the loss of the resultant Low SG level.
Emergency boration was initiated at 0341 CST per procedure EMG ES-02, "Reactor Trip Response," when the reactor coolant system (RCS) temperature decreased below 550 degrees Fahrenheit. This was a result of the actuation of auxiliary feedwater (AFW) and the lack of decay heat with the reactor power at 42 percent. Emergency boration was secured at 0404 CST when RCS temperature was raised to greater than 550 degrees Fahrenheit, by throttling AFW flow. The lowest RCS temperature reached following the reactor trip was 544 degrees Fahrenheit.
BASIS FOR REPORTABILITY
The reactor trip and subsequent actuation of ESFAS instrumentation described in this event is reportable per 10 CFR 50.73(a)(2)(iv)(A), which requires reporting of "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section." Paragraph (B)(1) of 10 CFR 50.73(a)(2)(iv) includes "Reactor protection system (RPS) including: reactor scram or reactor trip." Paragraph (B)(6) of 10 CFR 50.73(a)(2)(iv) includes "PWR auxiliary or emergency feedwater.
The cause of the MFW pump tripping is a failed servo valve in the main feedwater speed control system. A Hardware Failure Analysis is investigating the exact cause for the servo failure. The servo valve is a device that takes an electrical signal and converts it to mechanical signal. The servo ports hydraulic oil to the bottom side of the pilot valve that controls the position of the secondary operating cylinder, which then controls the position of the low pressure (LP) and high pressure (HP) control valves for the feedwater turbine. The servo failed when it was in the open position, causing the pilot valve to travel full open, and the LP and HP control valves to become full open. This resulted in more steam being admitted to the feedwater turbine when there was no demand for it. The speed of the feedwater turbine increased beyond its overspeed set point, resulting in the trip of the MFW pump.
CORRECTIVE ACTIONS
A Hardware Failure Analysis is in process to determine the the most probable cause of the servo failure.
The evaluation of the results of the analysis will be completed by 6/30/2010. This evaluation will determine if additional actions are needed to prevent recurrence.
The servo valve was replaced and retested satisfactorily on 3/9/2010.
Action has been taken to replace the servo valve on a 3-year frequency.
SAFETY SIGNIFICANCE
The safety significance of this event is low. This event is analyzed as reported in WCGS Updated Safety Analysis Report (USAR) Section 15.2.7, "Loss of Normal Feedwater Flow." Results of the analysis show that a loss of normal feedwater does not adversely affect the core, the reactor coolant system, or the steam system, since the auxiliary feedwater capacity is such that reactor coolant water is not relieved from the pressurizer relief or safety valves.
There were no adverse effects on the health and safety of the public.
OPERATING EXPERIENCE/PREVIOUS EVENTS
transfer of an invertor to its alternate power supply.
in response to failures of the primary and secondary fuses for the Westinghouse 7300 control card frame that contained the associated control cards for the MFRV.
MFRV separating from the valve stem.