05000255/LER-2012-001
| Palisades Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2552012001R00 - NRC Website | |
A slightly increasing trend in primary coolant system (PCS) [AB] unidentified leakage was realized soon after plant start-up on July 14, 2012. Extensive efforts to determine the source of the unidentified leakage during power operations were unsuccessful. Unidentified leakage had trended from approximately 0.2 gallons per minute (gpm) to 0.35 gpm. At approximately 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br /> on August 11, 2012, a plant shutdown began from full power operation in accordance with plans to confirm the source of the leakage and make repairs. At approximately 0400 hours0.00463 days <br />0.111 hours <br />6.613757e-4 weeks <br />1.522e-4 months <br /> on August 12, 2012, with the plant in Mode 3 at normal operating temperature and pressure, a PCS pressure boundary leak was identified in the upper housing assembly for control rod drive mechanism (CRDM) number 24 [DRIV;AA]. No structures, components, or systems were inoperable or contributed to the event at the time of discovery. The plant was placed in Mode 5 at approximately 2000 hours0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> on August 12, 2012, to effect repairs.
Initial examination, using liquid penetrant testing in the area of the leak, identified a 1/8" x 1/16" L-shaped crack indication on the outside surface of the type 316L stainless steel pipe section of the CRDM-24 upper housing assembly. Subsequent non-destructive and destructive examinations revealed a total of nine axially oriented crack indications, located in the proximity of an inside surface weld onlay. One of the nine crack indications was a through-wall crack at the leak point approximately 3" in length.
CAUSE OF THE EVENT
The direct cause for the crack indications in the CRDM-24 upper housing assembly was transgranular stress corrosion cracking (TGSCC). TGSCC was the result of stresses applied in the proximity of the inside surface weld onlay caused by manufacturing irregularities and misalignments between CRDM-24 upper housing assembly and supporting components. Based on the lack of crack indications in the additional eight upper housing assemblies examined, the failed CRDM-24 upper housing assembly was subject to an additional stress that has not yet been identified.
CORRECTIVE ACTIONS TAKEN
The CRDM-24 upper housing assembly was removed and replaced with an upper housing assembly of modified design. Examinations using ultrasonic testing of eight additional CRDM upper housing assemblies were performed on an area 1" below to 1-1/2" above the area of interest. No deficiencies were noted.
CORRECTIVE ACTIONS TO BE TAKEN
An inspection plan for CRDM upper housings assemblies will be developed and implemented for future refueling outages. There are several ongoing analyses of the examination data