Experimental Investigation And Mechanistic Modelling For The Fretting Wear Of Cr-coated Zircaloy Cladding Induced By Turbulence Excitation

The Fukushima nuclear accident indicated that the UO₂-Zr fuel system used currently was unreliable under limiting accidents accompanied with other incidents,and consequently resulting in severe accidents.Accident tolerant fuels which have better performance in normal operations and accidents are becoming the research focus of nuclear engineering.Besides,the fuel system with Cr-coated zircaloy cladding is regarded as one of the most promising candidate of accident tolerant fuels.Various investigations on oxidation and corrosion of Cr-coated zircaloy cladding have been conducted,while only few studies for fretting wear behaviors of Cr-coated zircaloy cladding were conducted.Therefore,in this study,the influence of different fretting parameters was studied by fretting wear experiments in room-temperature water environment.Then,the numerical simulation method was applied to obtain turbulence excitation of a typical channel in the PWR fuel assembly.The mechanisms of wear caused by turbulence excitation and effects of key factors including frequency,normal load,amplitude and cycles,were summarized,and a theoretical calculation during three fuel cycles of Cr-coated zircaloy cladding was performed based on a multi-steps wear model established in this research.The main jobs and conclusions of this study include:（1）The fretting wear test facility was built.The fretting wear experiments,in which fretting amplitude ranged from 50 to 125μm,fretting cycles ranged from 0.5～4.32×10⁵,fretting frequency ranged from 2.5 to 7.5Hz,normal load ranged from 5 to 20N,were performed.Consequently,an empirical relation of wear volume to fretting parameters was derived.The comparison revealed that the difference between 68%of experimental results and calculation data of the empirical relation is less than 50%.（2）A numerical analysis model of a typical channel in the PWR fuel assembly was developed based on computational fluid dynamics and finite element analysis.And the fluid dynamic analysis,modal analysis and transient structure analysis was conducted in this model.The results shows that the first order natural frequency of a fuel rod is around27Hz,and the maximum root mean square vibration amplitude is about 11μm.Subsequently,relations of vibration amplitude and fretting velocity to support force of a fuel rod was established.（3）Wear mechanisms of Cr-coated zircaloy cladding was analyzed based on local wear morphology of Cr-coated cladding.A relation of wear volume to maximum wear depth was derived based on the contact state of rods and grids.A multi-steps wear model based on fretting induced by turbulence-induced vibration was proposed.Then,the wear volume of 0.451mm³ and maximum wear depth of 122.8μm of the cladding after three fuel cycles was calculated by the model.Based on the above work,the wear model and wear mechanisms of Cr-coated zircaloy cladding were analyzed,which could not only provide technical supports for optimizing of the Cr coating,but also provide guidance for the development of fretting wear analysis code.