Detailed Analysis of the Debris-Fretting Damage Areas on Coated Fuel Cladding

Fuel failure caused by fretting damage to cladding remains a relevant issue despite decades of research and development aimed at enhancing the physical parameters of fuel. This paper presents the results of experiments conducted at the Research Centre Řež on Zr-1%Nb alloy tube specimens covered with protective coatings made of chromium (Cr) and nitrogen (N) compounds. The experiments involved debris-fretting tests under dry conditions at room temperature as well as microscopic measurements of groove depths. A detailed analysis was performed using the Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy, Electron Backscatter Diffraction, and Focused Ion Beam techniques. The objectives of the tests were (1) to compare the debris-fretting resistance between the reference Zr-1%Nb specimens and those of the same alloy coated with various compositions, and (2) to demonstrate the positive effects of coating applications on the endurance of fuel cladding. The conducted analysis revealed a significant advantage in using cladding with a thin, wear-resistant layer compared to standard cladding material, with the CrN-coated specimens exhibiting 36 times better fretting resistance.