Assessment of ultrasonic inspection techniques and models for nuclear power plant components

Abstract

Reliable detection and sizing of small and complex flaws is paramount in safety critical Nuclear Steam Raising Plant (NSRP) components and the pressure vessels of Pressurised Water Reactors (PWR). Also accurate modelling approaches are fundamental to understand the propagation and the scattering of ultrasonic waves from defects to aid interpretation of their ultrasonic responses. The objectives of this work are twofold: firstly, to assess the reliability of existing ultrasonic inspection techniques in detection and sizing of small and complex defects in ferritic steel. Secondly, to validate the finite element (FE) and CIVA simulation methods in modelling propagation of ultrasonic waves and scattering from geometrically simple defects. Experimental results have shown that the Time-of-Flight Diffraction (TOFD) technique is effective in locating and sizing linear cracks in welded ferritic components, though not arrays of small pores, and under-clad carbon cracks. It has also been shown that the pulse-echo phased array technique is efficient for defect detection. The TFM post-processed FMC data produces images of defects with a superior resolution than the pulse-echo phased array technique. Finite element simulations of waves in isotropic and transversely isotropic steels have predicted wave speeds agreeing well with theory. Similarly, the bulk elastic velocities in isotropic steel using the commercial simulation software CIVA have been found to be in close agreement with theory, however, significant discrepancies were found in the shear velocities in the transversely isotropic steel. The validations of the FE and CIVA modelling techniques for scattering from a small circular hole and a semi-infinite crack were performed. The FE predictions of the scattered and diffracted fields were in good agreement with analytical solutions. The corresponding CIVA predictions also agreed well with theory, though not close to the specular direction and at large receiving angles. Finally, FE and CIVA simulations of the TOFD technique were carried out on an ultrasonically smooth crack and a pore. The results were presented as B-scan images; the simulated B-scan images were in harmony with the experimental studies.