Remote Thermoelastic Characterization of Candidate Structural and Protective Coatings for Lead-Bismuth Eutectic Cooled Nuclear Reactors  
[PERSON]  
1 Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Heeverlee, Belgium; [PERSON](B.V.)  
[PERSON]  
1 Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Heeverlee, Belgium; [PERSON](B.V.)  
[PERSON]  
2 Department of Materials Engineering, KU Leuven, W. de Croylaan 2, B-3001 Leuven, Belgium;  
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[PERSON]  
3 Belgian Nuclear Research Centre SCK-CEN, Boeretang 200, B-2400 Mol, Belgium;  
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roughness RRMS \(<\) 3 \(\upmu\)m). Moreover, since the surface alloy is remelted by the electron beam, an improved bonding between the coating and substrate material can be achieved. Given the critical importance of protecting components in a nuclear reactor, and the harsh circumstances in terms of temperature and radiation to which protective coatings are subjected, regular and reliable quality control of their functional and structural properties is crucial for the viability of nuclear power stations. Once fuel claddings have been in operation, their radioactivity impedes the use of contact methods to characterize their condition. Remote inspection is therefore needed. The aim of this work was to show the feasibility of laser ultrasonic inspection [3] to remotely extract both microscopic and macroscopic structural and functional information on quite rough fuel cladding GESA coatings, by determining the elastic and thermal properties of the coating layer and of the substrate, which are known to be strongly dependent on the microstructure. In many implementations of laser ultrasonics, the photoacoustic effect is exploited to convert an optical energy pulse into impulsive thermal expansion, which in turn launches acoustic waves. Typically, the propagation of guided acoustic waves is optically mapped. From the wave velocity (dispersion), damping and/or mode conversion, information on the elastic properties can be extracted. 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]. Analysis of the dispersion of optically detected laser generated surface acoustic wave also allows characterization of sub-surface layers [27]. Since also the thermal expansion that goes along with transient thermal gratings can be detected by heterodyne diffraction of impulsive stimulated scattering, the technique also allows determination of the thermal diffusivity of coatings and or substrates [28; 29; 30; 31]. In the following, we report on the investigation of a steel substrate coated (T91 steel) with GESA modified FeCrAlY using laser ultrasonic excitation of surface acoustic waves (SAWs) and thermal waves in a narrowband grating geometry. Since the roughness of the sample surface and the resulting optical scattering of the reflected probe light made it difficult to apply heterodyne diffraction detection, a more robust beam deflection approach (LBD) [8] was used to detect the associated surface ripple.