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.

Remote Thermoelastic Characterization of Candidate Structural and Protective Coatings for Lead-Bismuth Eutectic Cooled Nuclear Reactors

“…Two distinct GILBD scanning approaches were taken: (i) "local detailed grating spectroscopy" for a set of 10 grating spacings varying between 7 and 200 µm (range in which the signal quality was ok for the used laser spot sizes) and (ii) "topographic mapping" of a sample surface area (2 mm × 2 mm), by performing an XY scan for a set of four different grating spacings (15,25,45 and 200 µm with a scanning step of 50 µm (and 1 µm for signal optimization) in the x-direction and 100 µm in the y-direction. To limit the total measurement time to a reasonable amount, the combined experiment, i.e., scanning the grating mask for a set of gratings while the surface area is scanned was not implemented.…”

“…Quite some work has already been done on the use of laser ultrasonics to characterize the properties of optically smooth layers and coatings, with several applications in non-destructive testing [4][5][6][7]. In typical optical configurations, the pump laser light is focused onto a point or line [8], resulting in the photoacoustic generation of wideband surface acoustic waves, from which different parameters can be extracted, such as the thickness [9,10] and elastic and structural information of thin coatings [11][12][13][14][15]. In an alternative optical configuration, impulsive stimulated scattering of probe laser light by a laser generated transient thermal grating [16], detected by heterodyne diffraction [17,18], has shown to be a very efficient way to detect narrowband surface acoustic waves and extract the thickness [19] and elastic and structural parameters of coatings [20][21][22][23][24] and their bonding to a substrate [25], with important applications in electronics [26].…”