05000255/LER-2013-001
| Palisades Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation |
| 2552013001R00 - NRC Website | |
In Mode 1 at 100% power, on February 10, 2013, due to a slowly lowering level trend in the component cooling water [CC] (CCW) surge tank, investigations began to identify a potential CCW leakage path. On February 14, 2013, at 2030 hours0.0235 days <br />0.564 hours <br />0.00336 weeks <br />7.72415e-4 months <br />, the right train of the CCW system was declared inoperable due to the identification of an approximate 40 gallons per hour CCW leak into the service water (SW) system [BI] section of the "A" CCW heat exchanger [FIX]. The majority of CCW leakage was attributed to one tube, located in the center of the tubesheet, within the CCW heat exchanger. Very minor leakage was identified from the tube plug area of seven previously plugged tubes. No additional structures, components, or systems were inoperable or contributed to the event at the time of discovery.
Technical Specification (TS) 3.7.7.A.1 requires restoration of an inoperable CCW system train within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Due to the inability to repair the leak within the required 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> time frame, a plant shutdown was initiated at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br /> on February 15, 2013. The plant entered Mode 3 at 1649 hours0.0191 days <br />0.458 hours <br />0.00273 weeks <br />6.274445e-4 months <br /> on February 15, 2013. At 1111 hours0.0129 days <br />0.309 hours <br />0.00184 weeks <br />4.227355e-4 months <br /> on February 16, 2013, the plant entered Mode 5 to execute repairs.
This condition is reportable in accordance with 10 CFR 50.73(a)(2)(i)(A) for the completion of a plant shutdown required by TS.
The right train of the CCW system was subsequently determined to have been inoperable since January 30, 2013, when the CCW surge tank first exhibited a lowering level trend. Therefore, this condition is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) for an operation or condition which was prohibited by TS.
BACKGROUND
The CCW System provides a heat sink for the removal of process and operating heat from safety related components during a design basis accident or transient. During normal operation, the CCW system also provides this function for various nonessential components. The "A" CCW heat exchanger is one of two essentially identical ASME Section III class 3 heat exchangers installed in the CCW system. The heat exchangers are straight tube, fixed tube-sheet, and arranged for single pass flow in both shell and tube side passages with CCW on the shell side and SW on the tube side. The CCW heat exchangers are original equipment and have been in service since the original plant start-up.
In 1990, in response to Generic Letter 89-13, the inspection frequency was aligned such that both heat exchangers would be inspected every refueling outage for three consecutive outages, alternating inspections of the heat exchangers each refueling outage thereafter. Inspections and maintenance consisted of 100% cleaning, eddy current testing of all the tubes that are susceptible to flow induced vibrational damage (outside 8 columns of the tubesheet), eddy current testing a sample of the rest of the tubes, thermal performance testing, and sampling/analyzing for biofouling.
Prior to 1996, the CCW heat exchangers were subjected to high CCW flow rates. Consequently, tube failures resulted from vibration and fretting in the region near the shell side inlet. Failed tubes in this region (in the corners of the support plates) were plugged and the CCW flow rate through the heat exchangers was restricted to reduce the potential for this failure mechanism. As a result, the flow induced vibration damage stabilized.
CCW heat exchanger inspections continued as scheduled until 2010. In 2010, the "A" CCW heat exchanger was not inspected as scheduled. The inspection was deferred to the 2012 refueling outage based on the results of past performance and eddy current testing. Again in 2012, the "A" CCW heat exchanger was not inspected as scheduled. The inspection was deferred to the 2013 refueling outage NRC FORM 3615/, I0-21TIn? U.S. NUCLEAR REGULATORY COMMISSION
CONTINUATION SHEET
based on the results of past performance and eddy current testing. The most recent inspection and maintenance of the "K CCW heat exchanger was in 2007. During the 2007 inspection, 784 tubes (38.8%) were eddy current tested and 100% of the tubes were cleaned. One tube was plugged due to the indication of some outside diameter damage, approximately 60% wall loss, caused by fretting at the support plate.
Review of the data from the previous inspection, performed in 2004, showed the indication was not present at that time. Even though the indication was determined to be less than the 70% wall loss criteria for tube plugging, the tube was plugged as a precautionary measure. This was the first CCW heat exchanger tube that required plugging since 1996 when CCW flow rates were reduced following the discovery of vibrational damage.
CAUSE OF THE EVENT
Due to the inability to fully isolate CCW system flow into the heat exchanger, the failed tube could not be inspected to determine a definitive failure mechanism. A failure modes analysis determined probable failure mechanisms, but none could be verified. The results of an evaluation for organization and programmatic issues determined the heat exchanger program, developed in part as a response to Generic Letter 89-13, was not designed to discover random failures of CCW heat exchanger tubes. Additionally, periodic assessments of the heat exchanger program did not use adequate rigor when looking for best practices within the industry. Operating experience was not utilized to identify new potential failure modes.
CORRECTIVE ACTIONS TAKEN
Mechanical type tube plugs were installed in the tubes where leakage was identified.
CORRECTIVE ACTIONS TO BE TAKEN
The inspection scope for both CCW system heat exchangers will be changed to include eddy current testing 100% of the tubes each inspection. The frequency of inspections will be modified to ensure both CCW system heat exchangers are appropriately maintained.
ASSESSMENT OF SAFETY CONSEQUENCES
The CCW leak into the SW system section of the "A" CCW heat exchanger did not challenge radiological or nuclear safety. Even though the leak necessitated a TS required plant shutdown due to a right train of the CCW system being declared inoperable, the CCW system maintained the ability to provide 100% post-accident cooling, as required by TS 3.7.7, during the period between January 30, 2013, and February 14, 2013. Therefore, no event or condition existed that could have prevented fulfillment of a safety function.
PREVIOUS SIMILAR EVENTS
None