Application of Computed Tomography for the Examination of Pressure Retaining Nuclear Plant Components

Abstract

Pressure retaining nuclear plant components require high levels of quality assurance to support the rigorous requirements of the modern safety case and the ASME Boiler and Pressure Vessel Code. Part NB-2500 of the ASME BPVC requires 100% volumetric examination of pressure retaining components and attachment welds utilising suitably demonstrated ultrasonic and radiographic examination techniques respectively. The capability of the inspection technique is particularly important for thin-walled components where the typical size of manufacturing defects tend to make up a greater proportion of the wall thickness. Computed Tomography (CT) is a development of conventional radiography and makes use of computer-processed combinations of many X-ray images taken from different angles to produce cross-sectional images of specific areas of a scanned object. This technique can be used to compliment and add strength to the standard volumetric examinations; ie the detection capability for conventional radiography is often dependent on alignment of the source to defect and ultrasonic methods are more suited to targeting planar flaws. Rather than the technical challenges placed on the Non-Destructive Examination (NDE) practitioner, this paper focuses on the benefits of application of CT to thin-walled nuclear plant components from the viewpoint of the nuclear plant component Design Engineer. The CT examples presented in this paper demonstrate reliable detection of voids measuring 4% of thickness including those located at features which are difficult to examine using standard techniques. These CT examinations compliment the standard volumetric inspections and provide high confidence that the component is free of structurally significant defects.