International round robin test block trials such as PISC2 and PISC3 have provided a demonstration of capability in practice. Valuable though they are, such trials also need to be integrated with theoretical modeling and laboratory studies to provide a comprehensive view of inspection capability and reliability. Theoretical models may be expensive to develop, but once validated they can provide statistical evidence of inspection reliability without undue cost. Magnets Electric has been involved with development of theoretical models for ultrasonics, eddy currents and radiography. The ultrasonic models cover detection of both smooth and rough defects whilst the eddy current models are applicable to defects in nonferrous and ferromagnetic metals. Feedback from experience on plant is also important to avoid overlooking any practical limitations of the inspections, and to ensure that the metallurgical characteristics of potential defects have been properly taken into account when designing and qualifying the inspections. Various studies have been carried out to estimate the probability of occurrence of various types of defects and to quantify the defect parameters relevant to the inspection techniques proposed.

For critical applications, inspection techniques are often supported by a Technical Justification which draws on all the relevant theoretical and experimental evidence, as well as experience of inspections on plant. Much of the key information for such Technical Justifications has been assembled into a Handbook of Evidence on Inspection Capability which now extends to about 12 Chapters. This approach, which has been pioneered by the UK nuclear industry is significantly influencing the development of the European Network on Inspection Qualification (ENIQ).

This paper summarises some recent advances in demonstrating the capability of ultrasonics, eddy currents and radiography both theoretically and in practice, and indicates where further evidence is desirable. It reviews the way inspection qualification has developed and gives examples of the support provided for safety assessments based on fracture mechanics.