05000382/LER-2001-002
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii)(C), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3822001002R00 - NRC Website | |
REPORTABLE OCCURRENCE
On January 28, 2001, the inside (PSL-303) and outside (PSL-304) Containment isolation valves on the Pressurizer steam space primary sample line failed to close when attempts were made from the Control Room at their respective control switches. These valves are required to automatically close upon receipt of a Containment Isolation Actuation Signal (CIAS). Failure of both valves constituted a loss of safety function. This condition is being reported pursuant to 10CFR50.73(a)(2)(v) based on potential for the condition to have prevented the fulfillment of the safety function of valves which are needed to control the release of radioactive material under accident conditions. The condition is also reportable pursuant to 10CFR50.73(a)(2)(vii)(C) based on the single condition causing two independent trains to become inoperable in a single system designed to control the release of radioactive material. The condition was also referred to the vendor on Friday, March 16, 2001 for consideration for reporting under 10CFR21 in keeping with guidance provided in NIJREG-0302.
The vendor (Flowserve Engineering) has agreed that this condition is reportable under 10CFR21 and will initiate the required report.
INITIAL CONDITIONS
Just prior to the event, Waterford 3 was operating in Mode 1 at 100% Reactor power. The subject valves (PSL-303 and PSL-304) had been open for approximately eight hours while degassing the Pressurizer.
EVENT DESCRIPTION
On January 28, 2001 at 2245, it was discovered that PSL-303 and PSL-304, Pressurizer steam space sample inside and outside Containment isolation valves [JM] would not close when their respective control switches at Control Panel CP-8 in the Control Room were taken to the "Close" position. The valves had been open since 1455 that same day to perform degassing of the Pressurizer. The Nuclear Plant Operator first took the switch for PSL-304 to the "Close" position and observed that the valve continued to indicate "Open". He then took the switch for PSL-303 to the "Close" position, and observed that the position of PSL-303 also continued to indicate "Open".
Technical Specification 3.0.3, Technical Specification 3.6.3, and Technical Specification 3.6.1.1 were entered upon discovering that the valves did not close. A Nuclear Auxiliary Operator was dispatched to verify the position of PSL-304 locally. PSL-304 was found to be in the open position and the on-shift Chemistry Technician confirmed that there were still indications of flow through the sample line. Another attempt to close PSL-304 was made using the switch on CP-8 in the Control Room. The Nuclear Auxiliary Operator observed that the valve moved approximately one quarter- inch in the closed direction. At 2257 PSL-304 was closed using the valve manual gagging device.
At 2310 PSL-304 was de-energized to comply with the action requirements of Technical Specification 3.6.3.b. At 0007 on January 29, 2001, another attempt was made to close PSL-303 using the control switch at CP-8. This attempt was successful.
Initial corrective actions modified the valve actuators to increase their closing thrust. These actions were inadequate and the valves failed to close on February 21, 2001 during post maintenance testing. A formalized root cause determination was conducted.
The subject valves are 1/2 inch, 2500 lb, WKM model 70-18-9 DRTS cage-guided globe valves.
CAUSAL FACTORS
The root cause was determined to be inadequate design clearances to allow for thermal growth of internal valve components. Several tests and internal inspections identified evidence of valve binding. Scarring (galling) marks were found on the valve plug. This was evidence of binding between the plug and the cage. Testing and evaluations results indicate that the valve binding occurred at temperatures in the range of >600 ° F, thus constituting thermal binding. It was determined that the valve was assembled correctly and passed post maintenance testing. While in service, heating occurred from the pressurizer such that the internal valve parts expanded proportionally to their thermal coefficient of expansion. The resulting expansion of the parts caused valve binding. The as found clearances were marginally within the lower end of the (valve specification) allowable clearance range. The temperature range wherein binding is believed to have occurred was within the range specified in the valve specification.
CORRECTIVE ACTIONS
Immediate actions included declaring the failed valves (PSL-303 and PSL-304) inoperable. PSL- 304 (outside Containment isolation valve) was manually closed using its gagging device and its breaker was opened to comply with the action requirements of Technical Specification 3.6.3.b. The breaker was danger tagged to ensure Technical Specification compliance was maintained. The failures were entered into the plant Corrective Actions Program, where testing and evaluations were conducted. This included studying and evaluating the subject valve maintenance histories. The Kepner-Tregoe problem solving methodology was utilized to determine the root cause.
Independent evaluations were performed by an external engineering company (Kalsi Engineering).
The valves were disassembled and inspected and internal measurements were taken. New parts were installed, with wider tolerances provided by the vendor (Flowserve Engineering). Retests were performed and the valves passed the tests.
Other safety related applications of WKM caged globe valves have been tested to determine if they are subject to thermal binding under flow conditions. They were found not to bind. A generic review of other safety related cage globe valves is in progress.
Procedures are being reviewed and will be addressed appropriately.
These corrective actions have been entered, and are being tracked, in the plant's corrective action program.
SAFETY SIGNIFICANCE
The potential impact of concurrently having Containment isolation valves PSL-303 and PSL-304 not go closed upon receipt of a Containment Isolation signal during applicable plant events was reviewed and assessed. The loss of both valves constituted a loss of safety function. This constitutes a Safety System Functional Failure (SSFF). The potential safety significance of the component failures with respect to applicable events is discussed below.
Radiological Events The inability of PSL 303/304 to close gives an additional potential radiological release pathway outside containment for all of the UFSAR dose events with the exception of the sample line break which will be discussed below. The PSL-3031304 valves are connected to the pressurizer steam space sample line. A simplified pathway description begins at the pressurizer and runs through RC- 319 valve, PSL-301 valve, PSL-303 valve, containment, PSL-304 valve, PSL-306, sample cooler, then back to the Volume Control Tank (VCT). The VCT can be vented manually to the Gaseous Waste Management System or, if the VCT relief valve pressure is exceeded, to the hold up tanks.
The potential radiological release into the sample line would be maintained within a closed system and would not be available to the environment prior to being filtered. Thus, the dose consequences would remain acceptable for an event that occurred while these valves were open. This analysis takes credit for a portion of Non-Nuclear Safety line that is assumed to remain intact for the duration of the event.
Sample Line Break Event The inability of PSL 303/304 to close has the potential to release primary coolant outside of containment as a result of a sample line break. Sample line breaks are described in UFSAR Section 15.6.3.1.1 and the limiting sample or instrument line break is analyzed for the letdown line. The letdown line is a two-inch schedule 160 pipe and the pressurizer sample line is a 1/2-inch line with a flow-restricting orifice immediately off the pressurizer_ Waterford Steam Electric Station, Unit 3 05000-382 YEAR I SEQUENTIAL I REVISION NUMBER � NUMBER 6 OF 7 01 -- 002 — 00 During a sample line break, the failure of PSL 303/304 would allow sample line blowdown until operator action is taken to either close RC319 or PSL301. The two hour Exclusion Area Boundary (EAB) dose for the letdown line break will continue to bound the pressurizer steam space sample line break even with the valve failures because the overall primary mass release of the letdown line break exceeds that of the sample line.
The letdown line break duration Low Population Zone (LPZ) dose will also bound the sample line break provided that operator action isolates the break or depressurizes the unit to Mode 5 within a reasonable time frame (as described next). The design basis letdown line break dose calculation uses an RCS activity of 1 micro Ci/gm, atmospheric dispersion factors that are conservative 95% of the time, and no credit for activity plate-out, deposition, or decay. These factors when compared to normal operation would allow the operator response time to be approximately 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> and still maintain the radiological consequences within the current acceptance criteria.
Thus, the PSL 303/304 failure remains bounded by the letdown line break dose consequences.
Additional Event Criteria The leakage could also cause additional depressurization during a transient, thus reducing the Departure from Nucleate Boiling Ratio (DNBR) for those DNBR events. This has the potential to increase fuel failure and/or push non-fuel failure events closer to their Specified Acceptable Fuel Design Limit (SAFDL). All of the fuel failure and DNBR events are analyzed with a 2% multiplier on power, with maximum 3-D peaking factors, and from a Power Operating Limit (POL). During the valves inoperable time, the plant power, peaking, and COLSS POL were conservative enough that sufficient thermal margin was available that all the DNBR events would remain bounded by the current analyses.
Safety Significance Conclusion Based on considerations described above, the inability of PSL 303/304 to close had a low safety significance due to the available valves, operator actions, closed system, and available thermal margin. The current licensing and design bases would have remained bounding.
10CFR Part 21 Considerations Entergy has sent results of tests and evaluations (root cause analysis) associated with the subject valve failures to the vendor (Flowserve Engineering) for consideration for reporting under 1OCFR21 regulatory requirements. The vendor agreed that the failures meet the criteria for 10CFR21 reporting and further has agreed to file the required report
SIMILAR EVENTS
Record searches performed have identified no other Waterford 3 reported events involving WKM caged globe valve binding due to thermal expansion.
ADDITIONAL INFORMATION
Energy Industry Identification System (EliS) codes are identified in the text within brackets [].