Hybrid simulation model of ultrasonic inspection of pressure tubes in nuclear industry

Pressure tube inspection within CANDU nuclear reactors is a critical maintenance operation to identify and track the growth of defects within the tube. Current inspection approaches utilising ultrasonic techniques are technically challenging, which cause the whole inspection process to be resource intensive and expensive to implement. This paper will describe the initial stages in the development of a simulation approach for the ultrasonic inspection methodology to research advanced solutions with the objective of improving the inspection accuracy. Zirconium tubes with a thickness of 4.3mm and a required measurement accuracy of defect depth of 0.1mm require the use of high frequency ultrasonic transducers. The finite element modelling of high frequencies is challenging due to the increased mesh requirements to resolve the small wavelengths and the large propagation distance which can cause numerical dispersion. Hence, a 2D finite element hybrid model is developed in PZFlex software to overcome this difficulty with five subsequent components containing both finite element models and analytical solutions: ultrasound transmission; transmission extrapolation (wave propagation); target interaction; echo wave extrapolation and ultrasonic reception. To test the capability of defect inspection using the hybrid model, a slot with a depth of 1mm is introduced in the model. The depth information was calculated from the time-of-flight between the reflections of the tube surface and the slot. The predicted modelled depth estimates produces errors of less than 20micron for both 10MHz and 20MHz probe configurations validating the hybrid modelling approach. Moreover, experimental validation of the hybrid modelling approach is demonstrated.