Nuclear Plant Journal Outage Management ... - Digital Versions

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Dual Methodology... Continued from page 41 thermal sleeves are assumed to contain degradation, only nine (9) of the nozzles contained degradation that prevented compliance with the examination requirements.) Proposals were obtained that detailed the cost of the thermal sleeve ‘replacement’. Material and contracted labor costs for the first thermal sleeve replacement would be $480,000.00 and each subsequent thermal sleeve would cost $150,000.00 each. Additionally, the timeline for the thermal sleeve replacement was estimated to be five (5) days for the initial deployment of personnel and equipment, and eleven (11) days to perform the thermal sleeve replacement and nozzle examinations, for a total of 16 days. The total cost of the thermal sleeve replacement and inspection was estimated to be $7,102,500.00; ($1,680,000.00 for material and labor to replace the thermal sleeves, $622,500.00 for keeping the examination crew for the additional duration required to complete the inspection, and $4,800,000.00 for 16 additional critical path days if $300,000.00 per day is assumed) without factoring in the costs for DNC personnel supporting the project. The examination technique developed during the outage only required one (1) additional day of examination without the removal of any thermal sleeves, for a savings of $6,802,500.00. Innovation: The existence of thermal sleeve wear had been reported at several other PWR stations; however, the fact that the movement of the thermal sleeve had degraded the nozzle base material had previously gone undetected. No other station had reported the detection of the wear and neither of the vendors that perform these examinations had previously reported the detection of this phenomenon. The primary inspection probe utilized for the ultrasonic examination was an axial blade probe. The ultrasonic examination is performed by inserting a thin strip of stainless steel material (the blade) between the thermal sleeve and the nozzle ID surface. At the end of the blade are separate 5MHz transducer elements operated in a pitch-catch configuration mounted on one end. These elements are oriented circumferentially with respect to the nozzle being examined. A thin stream of water is delivered to the surface of the nozzle at the transducers to provide coupling. The distance between the transducers and the material to be examined must be filled with a medium capable of transferring the sound energy with a minimum amount of attenuation to facilitate coupling. The incomplete ultrasonic coverage was obtained on 9 of the 78 nozzles, (nozzle numbers 1 through 9). The area where complete coverage was not obtained with the axial blade probe is consistent with the location of the thermal sleeve centering tabs. Analysis of the ultrasonic data indicated that the surface of the nozzle was diverting away from the transducer elements just prior to the loss of coverage. This conclusion was reached due to the water-path, (the distance that the ultrasound is traveling through the water), increased just before no more data could be obtained. Since wear was also observed on the thermal sleeves in a circumferential area on the same lateral plane as the end of the nozzles, Engineering personnel theorize that the centering tabs had worn away some of the nozzle material, creating a surface irregularity that prevented the ultrasonic transducer elements from coupling with the nozzles. By removing the thermal sleeves, AREVA could use an open housing scanner that would fill the entire nozzle with a column of water and ultrasonically interrogate the nozzle in the area of the geometric distortion. Although data on the additional penetration nozzles could not be obtained at the lower centering tab locations, it must be assumed that signs of similar wear are also present for all nozzles that contain thermal sleeves. (Note that no data was obtained at the upper centering tabs of any nozzles due to blade length restrictions.) DNC Engineering personnel and AREVA ultrasonic examination technicians separately came up with examination techniques that provided additional examination coverage but individually fell just short of complying with the inspection requirements and determining the extent of condition. But collectively the two techniques provided enough quantitative and qualitative information to completely characterize the nozzle base material in the area of interest. This first of a kind approach provided the solution. The 9 nozzles where this condition existed were rescanned with a circumferential blade probe in the area of interest to obtain the required volumetric coverage and to assess the area for discontinuities. The circumferential blade probe has transducer elements oriented in the axial direction with respect to the nozzle, and allowed the transducers to be coupled on either side of the surface irregularity. With the transducer elements oriented axially, the circumferential probe was able to bridge the ‘gap’ created by the material loss and provide the necessary coverage by successfully transmitting and receiving the lateral, (surface wave), and back-wall signals, (angle beam). From this examination, Engineering obtained confidence that circumferential cracks did not exist in the nozzle base material in the area of interest. In addition to the ultrasonic data collected, DNC Engineering personnel developed a procedure for performing a quantitative and qualitative examination of the wear region utilizing an (Continued on page 44) 42 www.nuclearplantjournal.com Nuclear Plant Journal, May-June 2009
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