Kennas, Miltiadis Demetrios (2020-01). Investigating Fuel Cladding Coating Materials for Accident Tolerant Fuels. Doctoral Dissertation.
This study is a systematic investigation on the interface reactions of various coating materials for nuclear fuel cladding. In particular this study focused on metallic interdiffusion between these coatings and Zircaloy-4. The coatings used were Fe70Cr20Fe10, Ti2AlC, and pure Cr, applied via cold spraying. This study utilized various testing methods, including accelerator irradiation, SEM imaging, TEM imaging, micropillar compression, nanoindentation, and cantilever bending. The research performed is necessary for confirming phase diagram predictions, creating kinetic data to predict intermetallic growth rates outside of experimental conditions, and determining the impact, if any, to the long-term structural integrity of coated cladding. It was found in the case of Fe70Cr20Fe10 (FeCrAl) coating that the intermetallics Zr3Fe and (Fe,Cr)2Zr/ Fe2Zr formed. Mechanical properties of these intermetallics indicated that they are harder and less ductile than the FeCrAl coating. Under ion irradiation, all intermetallics showed good void and swelling resistance, but the intermetallic (Fe,Cr)2Zr amorphized, potentially significantly impacting structural integrity of the coating-cladding interface. In the Ti2AlC-Zircaloy-4 system, the intermetallics Zr3Al, ZrAl, and ZrAl2 formed. In addition, precipitates of (Fe,Cr)2Zr were found in the Zr3Al intermetallic. All intermetallics were tested to be less hard than Ti2AlC. Under ion irradiation, Ti2AlC, Zr3Al, and Zircaloy-4 hardened. In addition, partial amorphization was noted in every intermetallic, and the (Fe,Cr)2Zr precipitates fully amorphized, consistent with the material's behavior in the FeCrAl-Zircaloy-4 system. Investigating the materials property changes of the pure Cr-coated Zircaloy-4 proved to be challenging, as there was no discernible intermetallic formation and only a ~1.1 um thick on average Cr diffusion zone into Zircaloy-4 was observed. The thickness of this diffusion zone saturated rapidly, making kinetic analysis inconsequential. The thin width of this diffusion zone combined with the lack of intermetallic formation made conventional micromechanical testing methods impossible or provide poor reproducibility. Ultimately this thin diffusion zone is unlikely to severely impact the structural stability of the Cr-Zircaloy-4 system.