Chromium hardening and Zr-Cr interface stability of irradiated chromium-coated Zircaloy-4 alloy

International audienceIn Light Water Reactors, under hypothetical LOss of Coolant Accident (LOCA) scenario, zirconium alloy nuclear fuel claddings undergo significant steam oxidation at high temperatures. In such accidental situation, decreasing the oxidation rate of nuclear fuel claddings is a key issue to improve the accident tolerance of the nuclear fuel sub-assembly. Hence, CEA has engaged specific studies on chromium-coated zirconium alloys in the framework of CEA-FRAMATOME-EDF collaborative program on Enhanced Accident Tolerant Fuel (E-ATF) cladding materials. Beyond the LOCA issues, the adhesion strength of the Cr coating has to be assessed under irradiation. Thus, the purpose of the study is to determine the Cr hardening and the stability of the Zr/Cr interface of a Cr-coated Zircaloy-4 alloy after irradiations by means of Transmission and Scanning Electron Microscopy.On a first type of Zr/Cr interface, it is reported the formation of a few tenths nanometers thick Zr(Fe,Cr)2 polytype structured Laves phase displaying both C14 and C15 lattice symmetry. After ion irradiation up to 10 dpa at 400°C, only the C14 phase is observed while the destabilization of the C15 phase could be attributed to some segregation of iron at the interface. For a second interface, obtained under different deposition conditions, only the C15 phase is observed at the interface. The in-situ ion irradiation at 400°C up to 20 dpa showed a partial dissolution of the C15 phase accompanied with a sharpened distribution of the Zr and Cr atoms. In all cases, the semi-coherent atomic structure of the interface observed before and after irradiation may explain the good residual adhesion properties of the coating to the substrate.Additionally, some tensile tests were performed at 350°C on neutron irradiated chromium-coated Zircaloy-4 alloys. Fractograph analysis revealed no peeling of the chromium coating irradiated up to ~2 dpa and confirms the good residual adhesion properties