| Accident tolerant fuel(ATF),in comparison with the standard UO2-Zr system,is proposed to improve the margin of safety under extreme conditions and maintain good performance under normal operating conditions for light water reactors.One of the ATF strategies is applying a protective coating on the zircaloy cladding to delay the onset of severe core degradation.In the present study,the synthesis,microstructure and properties of Zr-Me-C(Me=Al,Si and Y)and Cr2AlC coatings were systematically investigated.The aim of the present work is to evaluate the potential application of candidate coatings in ATFs.The work includes the following aspects:The phase composition,microstructure,mechanical property and oxidation resistance of Zr-Me-C coatings were systematically studied.Up to a certain concentration,there would be a solubility of Me in ZrC0.55,forming a solid solution phase.At higher Me concentration,alloying resulted in a nanocomposite film.The maximum solubility was strongly dependent on the ability of the dissolved element to form carbides.Y was strong carbide forming element,and the amount of metal dissolved into the carbide could be higher than 28.2 at.%.While for 17.7 at.%Al and 13.6 at.%Si coating,amorphous phases occurred at the grain boundary.Compared with binary carbide coating,Zr-Me-C coating exhibited higher hardness and modulus,which is comparable with bulk ceramics.Improved air oxidation resistance was achieved,and the oxide scale retained adhesion to the substrate until the transition of t-ZrO2.Zr-Me-C coatings demonstrated enhanced oxidation resistance compared to Zircaloy-4(Zry-4)alloy under simulated LOCA environment,and 17.7 at.%Al coating exhibited the lowest weight gain at 800 and 1200?,which decreased by 38%and 11%,respectively.The gradient carbide composite coating ZrC0.55/[nc-(Zr3Al)Cx/a-C]was designed.During a hypothetical LOCA scenario,ZrC0.55/[nc-(Zr3Al)Cx/a-C]coating protected Zircaloy-4 alloy from fast oxidation at 800?,and the weight gain of coated sample decreased by 75%.The fretting wear depth and wear coefficient of Zry-4 alloy could be effective reduced with ZrC0.55/[nc-(Zr3Al)Cx/a-C]coating,which decreased by 50%and 59%,respectively.From the view of water vapor oxidation and fretting wear performance,ZrC0.55/[nc-(Zr3Al)Cx/a-C]was proposed as potential coating candidate for zircaloy cladding tube for ATF applications.Cr2AlC coating exhibited excellent oxidation resistance in both normal operating and simulated LOCA conditions.In 360 ?/18.7 MPa pure water,a thin and protective Cr2O3 scale was formed on Cr2AlC coating,which inhibited further oxidation of coating.Zry-4 alloy substrate was prevented from corrosion by the use of equiaxial Cr2AlC coating,in which the penetration of hydroxyl ions through grain boundaries was eliminated.The high-temperature oxidation resistance of Zry-4 alloys in water vapor was improved with Cr2AlC coating.Oxide scales(Al2O3 scale at 900 and 1000 ?,Al2O3 and Cr2O3 mixed scales at 1100 and 1200 ?)effectively inhibited the inward diffusion of hydroxyl ions,and then decomposition of Cr2AlC coating occurred owing to the fast inward diffusion of Al atoms.A barrier layer needed to be applied between Zry-4 alloy and C r2AlC coating.Among Cr,Nb and Mo candidate coatings,Cr-coated Zry-4 alloy exhibited the thinnest inter-diffusion layer after annealed at 800? for 1 h.Accordingly,bilayer Cr/Cr2AlC coating was synthesized.At 900?,Cr/Cr2AlC coating displayed excellent oxidation resistance,and the parabolic rate constant was two orders of magnitude lower than that of Zry-4 alloy.Excellent fretting wear resistance of Cr/Cr2AlC coating was also detected.The maximum wear depth decreased significantly for Cr/Cr2AlC coating,which is only 1/20 wear depth of Zry-4 alloy.In conclusion,Cr/Cr2AlC coating was regarded as one of the most promising coating candidates,owing to the good oxidation resistance and fretting wear resistance. |
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