DURING THE FABRICATION of a boiler, pressure vessel, and such related components as boiling water reactor piping or steam generator tubes, various types of nondestructive inspection (NDI) are performed at several stages of processing,mainly for the purpose of controlling the quality of fabrication. In-service inspection is used to detect the growth of existing flaws or the formation of new flaws.
This can be done while the vessel is in operation or down for servicing. The inspection methods used include visual, radiographic, ultrasonic, liquid penetrant, magnetic particle, eddy current, and acoustic emission inspection, as well as replication microscopy and leak testing. The assurance of component quality depends largely on the adequacy of NDI equipment and procedures and on the qualification of personnel conducting the inspection. In many cases, nondestructive inspection, both prior to and during fabrication, must be done to sensitivities more stringent than those required by specifications. The use of timely inspection and rigid construction standards results in the reduction of both the costs and delays due to rework.
Quality planning starts during the design stage. For inspections to be meaningful, consideration must be given to the condition of the material, the location and shape of welded joints, and the stages of production at which the inspection is to be conducted. During fabrication, quality plans must be integrated with the manufacturing sequence to ensure that the inspections are performed at the proper time and to the requirements of the applicable standard. In the newest nuclear plants, quality design planning includes:
· Avoidance of complex weld geometries to facilitate attachment of ultrasonic transducers to the surface at the best positions
· The increased use of ring forgings for pressure vessel components; this means that there are no longitudinal welds that have to be inspected in service. The result is a reduction in the amount of inservice inspection and man-rem exposures
· Incorporating large numbers of access points for introducing mechanized inspection equipment, which can be operated remotely, thus avoiding exposures to operators and enabling more accurate processing than is possible with handheld inspection equipment
· The elimination of welds between cast austenitic components; inspection of welds through cast welds is difficult because they are opaque to ultrasonic inspection to a large degree