In service inspections of French nuclear pressure water reactor (PWR) vessels are carried out automatically in complete immersion from the inside by means of ultrasonic focused probes working in the pulse-echo mode. Concern has been expressed about the capabilities of performing non destructive evaluation of Outer Surface Defects (OSD's) i.e., defects located in the vicinity of the outer surface, in nuclear PWR vessels. The OSD's are insonified by both a "direct" field that passes through the inner surface of the inspected component containing the defect and a "secondary" field feflected from its outer surface. Consequently, Bscan images containing the signature of such defects are complicated and their interpretation is a difficult task.
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