05000414/LER-2001-001
| Catawba Nuclear Station, Unit 2 | |
| Event date: | 09-14-2001 |
|---|---|
| Report date: | 11-8-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4142001001R00 - NRC Website | |
Background
This event is being reported under 10CFR50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's Technical Specifications.
NUREG-1022 Revision 2, Event Reporting Guidelines 10 CFR 50.72 and 50.73, states a Licensee Event Report is required if there is firm evidence a condition existed for a time longer than permitted by the Technical Specifications even if the condition was not discovered until after the allowable time had elapsed and the condition was rectified immediately upon discovery.
The purposes of the main feedwater [EIIS:SJ] isolation valves [EIIS:ISV] are to restrict reactor coolant [EIIS:AB] system thermal transients following a secondary system high energy line break inside or outside containment, isolate forward feedwater flow to the steam generators on a high-high level to prevent overfill, and prevent flooding of the main steam/feedwater penetration room and the consequential disablement of safety related equipment due to a pipe rupture in the main steam/feedwater penetration room. The main feedwater isolation valves are arranged with 2CF33 aligned to the steam generator A, 2CF42 aligned to the steam generator B, 2CF51 aligned to the steam generator C, and 2CF60 aligned to the steam generator D.
The main feedwater isolation valves are flex wedge gate valves with pneumatic-hydraulic piston cylinder operators. Hydraulic fluid is pumped into the opening side of the piston cylinder to open the main feedwater isolation valves. At the same time, nitrogen from the closing side of the piston cylinder is compressed into a nitrogen accumulator as the piston retracts into the cylinder. Hydraulic solenoid valves control application of hydraulic pressure to the piston. The solenoids are energized to block fluid flow to the reservoir and allow the hydraulic pump to force the main feedwater isolation valve open. The hydraulic solenoid valves are de- energized to allow hydraulic fluid flow to the reservoir. Nitrogen from the accumulator works against the piston to force the hydraulic fluid to the reservoir and close the main feedwater isolation valve.
The main feedwater isolation valves are included in the Catawba preventive maintenance program. Main feedwater isolation valve actuators, including both hydraulic solenoid valves, are replaced on a six refueling outage frequency. This practice is to improve system reliability and prevent unexpected failures during plant operation. This practice has been in effect for many years.
Main feedwater isolation is accomplished by one of three redundant features - a) main feedwater isolation valve closure, b) main feedwater control valve and bypass valve closure, and c) main feedwater pump trip.
Each redundant feature consists of an A and B train isolation. The main feedwater control valves and bypass valves are air operated valves and operate in a throttled position during normal plant operations. The air operated valves are not susceptible to the same failure mechanism as the hydraulic solenoids for the main feedwater isolation valves.
The main feedwater isolation valves are required to be operable in Modes 1-4 for Technical Specification 3.6.3, Containment Isolation Valves and modes 1-3 for Technical Specification 3.7.3, Main Feedwater Isolation Valves, Main Feedwater Control Valves, Associated Bypass Valves, and Tempering Valves. Both Technical Specifications action statements require the inoperable main feedwater isolation valve to be closed or isolated within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Contrary to the action statement, valve 2CF60 was suspected to be inoperable and open for longer than the allowed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
At the time of the discovery of the failed valve, Catawba was conducting a controlled unit shutdown for the .nd-of-Cycle 11 refueling outage and was operating at approximately 17 percent power. Catawba remained at 17 percent power until the valve was closed locally. This event did not result in any plant transients. With the exception of the valve 2CF60 failure, no systems, structures, or components were out of service that had any significant effect on the event.
Event Description (dates and approximate times) 9-14-01 � Unit shutdown initiated for the start of the 1400 � scheduled End-of-Cycle 11 refueling outage.
2245 � Reactor power was approximately 17 percent.
Main feedwater isolation valve 2CF60 failed to close during the steam generator nozzle transfer operation. The transfer of feedwater flow from the main feedwater nozzles to auxiliary feedwater nozzles is a normal activity for unit shutdown.
Unit operators entered Limiting Conditions for Operations for Technical Specifications 3.6.3, Containment Isolation Valves, and 3.7.3, Main Feedwater Isolation Valves, Main Feedwater Control Valves, Associated Bypass Valves and Tempering Valves. The Technical Specifications action statements require inoperable valves to be closed within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
9-15-01 � Technicians locally closed the main feedwater 0407 � isolation valve 2CF60.
9-17-01 � Unit operators exited Limiting Conditions for 1653 � Operations for Technical Specifications 3.6.3 and 3.7,3 upon entry into Mode 5.
9-18-01 Engineering evaluation of the failed valve 2CF60 could not conclusively determine the time of failure. The valve 2CF60 failure most likely occurred earlier than the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Technical Specification::: action time limit.
Causal Factors The valve 2CF60 failure was due to the hydraulic solenoid valves being stuck in their energized position. The solenoid valves failure was due to hydraulic fluid unintentionally entering the coil section of the solenoids and the normally energized coil baking the hydraulic fluid to cause a sticky adhesive bond between the coil rod and coil housing.
The hydraulic fluid was able to surround the solenoid plunger due to seepage past the sealing surface !quad ring) which occurred during the manufacturer's proof testing at an elevated hydraulic pressure of > 6000 psig. In 1997, the manufacturer modified their test procedures to perform a post-test visual inspection to ensure no hydraulic fluid entered the coil area.
Another contributor to the hydraulic oil seepage past the sealing surface was a manufacturer initiated seal design change from an 0-ring to quad ring in the late 1980's. Valves with the earlier 0-ring design did not experience hydraulic fluid leakage. Therefore, the population of solenoid valves susceptible to this failure mechanism is restricted to after the quad ring design change and prior to the improved inspection procedure.
The valve failure is an EPIX reportable equipment failure. Reference Catawba Nuclear Station Unit 2 EPIX report 297.
Corrective Actions
Immediate 1. Valve 2CF60 was placed in the closed position. Operations personnel complied with Technical Specifications 3.6.3 and 3.7.3.
Subsequent 1. Valve 2CF60 actuator and hydraulic solenoids were replaced and tested during the End-of-Cycle 11 refueling outage. Valve 2CF60 is operable.
2. Engineering previously reviewed all main feedwater isolation valves and concluded another valve, 2CF51, may be susceptible to the same failure as valve 2CF60. The hydraulic solenoids for suspect main feedwater isolation valve 2CF51 were replaced and tested during the End-of-Cycle 11 refueling outage. Valve 2CF51 is operable.
Engineering previously reviewed the remaining Unit 1 and 2 feedwater isolation valves. It was determined that the remaining Unit 2 solenoids were manufactured following the manufacturer improved inspection practices. The Unit 1 valves were evaluated as not susceptible to the same failure mechanism, were inspected, or were replaced during the previous refueling outage.
Planned 1. The procurement description will include the requirement to inspect the hydraulic solenoids after proof testing even though this is the normal practice.
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Safety Analysis
There was no safety significance to this event. In the event of a transient that would have required feedwater isolation, the redundant isolation features of closing the main feedwater control valves, closing the bypass control valves, and tripping the main feedpumps would have fulfilled the safety feature of terminating feedwater flow to each steam generator.
The main and bypass feedwater control valves are high quality valves designed to fail closed on loss of air or signal. The isolation logic for the valves consists of both train A and train B protection grade signals.
Surveillance testing was performed on the feedwater control valves several days after the valve 2CF60 failure. The testing verified the main and bypass feedwater control valves isolated on both train A and train B actuation logic. Also, the remaining three main feedwater isolation valves isolated on both train A and train B actuation logic.
The feedwater isolation function was judged to be of low significance and overall high reliability and therefore is not explicitly included in the Catawba Probabilistic Risk Assessment. No quantitative evaluation of the core damage frequency impact of a failure of 2CF60 is available.
Qualitatively, the redundant isolation features for terminating feedwater flow assures that the overall failure probability for this function is very low. The failure of valve 2CF60 has an insignificant impact on the Catawba core damage frequency estimate.
This event was not a Safety System Functional Failure due to the operability of the redundant isolation features. The health and safety of the public were not affected by this event. There were no radiological events or consequences associated with this event.
Additional Information
Operating experience was reviewed for the past 36 months to determine if this event was similar to previous events.
In December 1999, the main feedwater isolation valve 2CF51 failed due to the nitrogen solenoid spool shuttling to the energized position and the nitrogen pressure holding the spool piece in place. This event was not similar to the valve 2CF60 failure described in this report.
In January 2000, while performing post modification testing, the main feedwater isolation valve 2CF33 successfully closed but experienced a delayed stroke time for the B-train actuation. After the initial slow valve stoke, all subsequent valve stroke times met the acceptance criterion and the valve was operable. A decision was made to gag all the nitrogen solenoid valves to assure that no inadvertent shuttling of the valves could occur as previously experienced by valve 2CF51. All feedwater isolation valves passed post modification stroke time testing. While entering the refueling outage in March 2000, valve 2CF33 again experienced a slow B-train stroke time. The actuator, including hydraulic solenoids, for 2CF33 was replaced during the refueling outage as a scheduled preventive maintenance activity. None of the other feedwater isolation valves exhibited abnormal stroke times.
In August 2000, as part of the root cause investigation of valve 2CF51, a failure of the hydraulic solenoids was identified at another station. This failure mode was confirmed by disassembly of the B-train hydraulic solenoid valve previously installed on valve 2CF33. Engineering performed an operability evaluation for all feedwater isolation valves and concluded that any suspect valves could be inspected or replaced during the next refueling outage. This decision was based on the different hydraulic configuration at the other station that experienced the failure, none of the other Catawba valves exhibited abnormal valve operation, no previously identified problems with the hydraulic solenoid valves in the plant lifetime, additional benchmarking of successful valve operation at other stations, and the letter from the manufacturer stating, "The possibility of additional failures is minimal." Based on these inputs, immediate corrective actions were not warranted but actions were scheduled for the next opportunity. The valve 2CF33 B-train intermittent slow stroke times was similar to the valve 2CF60 failure in September 2001.
The manufacturer has informed Catawba that following valve proof testing, the manufacturer now performs a partial disassembly to inspect for and remove hydraulic fluid in the coil.
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].