EDDY CURRENT MAGNETIC BIAS X-PROBE QUALIFICATION AND INSPECTION OF STEAM GENERATOR MONEL 400 TUBING IN PICKERING NUCLEAR GENERATION STATION

摘要

The X-probe MB 350 qualification procedure utilized a statistical approach to assessing the performance with flaw detection based on S/N ratios. This method compared eddy current signals from flaws in the laboratory with noise measured from eddy current scans of in-service tubes. The POD calculations were based on the number of detectable and non-detectable flaws in a sample set, where the detection criterion requires that a flaw have a S/N ratio greater than 2. A conservative approach was maintained throughout the qualification process; this is primarily important when dealing with Monel 400 because of the added complexity of saturation issues, especially in the presence of structures and lift-off. Although only freespan axial cracks have been detected to date, all possible locations were taken into account during qualification. In addition, the field noise assessment methods were not always performed in the same manner as recommended by the EPRI guidelines since the variety of noise sources and their resemblance to flaws at times warranted more conservative approaches. The use of EDM notches also called for conservatism. The signal amplitudes were taken as scaled probe responses to a set of tube specimens containing axially oriented EDM with depths ranging from 30 percent to 80 percent deep, by 6.35 and 12.7 mm long. The comparison between the probe responses to the crack found in Tube R25 C52 in PNGS 2 SG 11 compared to that from a 13 mm long, 80 percent EDM notch provided the scaling factors at each frequency and multi-frequency mix channels to account for the variations between the actual field crack and the EDM notches. The X-probe MB 350 is now qualified for detection of open axial cracks similar to that found in PNGS A Unit 2 SG 11 Monel 400 tubing. The POD capability was assessed at the freespan, sludge pile, U-bend, TTS and support locations for both PNGS A an B SGs. In these cases, the detection threshold ranges as follows: - 30 percent TW at freespan and U-bend locations, - 40 percent TW under the broach plates, - 45 percent TW under lattice bars, and - 50 percent at the TTS.