Laser ultrasonics evaluation and testing of coated HTR nuclear fuel

Abstract

Laser ultrasonics was applied to the manufacturing control of the quality and integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). The coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The quality assessment of the coating layers is of major importance. Using laser ultrasonics, we determined the vibration eigenmodes of dummy HTR particles. The vibration spectrum of a HTR particle provides a non-destructive method of evaluating some important mechanical parameters of the coating. Moreover, without damaging the particle, the laser ultrasonics technique allows to test the presence of a crack in the SiC layer, through the observation of the particle vibration spectrum, which is significantly changed, compared to that of a defect-free particle. We applied nanosecond acoustic pulses, i.e., high frequency laser-generated ultrasound, to measure the acoustic velocity of longitudinal waves the SiC layer. This technique provides an alternative method of evaluation of the Young modulus of the SiC layer. We measure the velocity of surface acoustic waves (SAW) on a pyrocarbon layer cross-section and we demonstrated that the anisotropy of the internal pyrocarbon layer can be evaluated by laser ultrasonics.