Role of noble metal fission products on the microstrutural evolution and corrosion behaviour of ferritic steel-Zr based metal waste form alloys in the simulated repository environment

In the present work, the noble metal fission products (NMFPs) alloyed ferritic steel-Zr based metal waste form (MWF) alloys and its influence on microstructure and corrosion behavior in simulated ground water environment was investigated for pyrochemical nuclear spent fuel reprocessing applications. In-gots of 9Cr1Mo steel-12Zr-NMFPs MWF alloy with different NMFP (Ru, Rh and Pd) concentrations (1-4 wt%) were fabricated via vacuum arc melting. Microstructural examination revealed the eutectic layer microstructure consisting of alpha-Fe and Fe-Zr phases with the preferential distribution of NMFP along with Fe-Zr phases. XRD analysis confirmed that the noble metal does not show any discrete noble metal phases but as intermetallics phases. Electrochemical corrosion studies of the MWF alloys in the simulated Rajasthan Ground Water (RGW) and Kalpakkam Ground Water (KGW) media showed improved corrosion resistance with increase in noble metal content of the alloy due to cathodic modification. Higher noble metal-containing MWF alloys exhibited more protective passive film properties is attributed to the enhancement of Ru, Rh and Pd noble metals in the alloy which influences on the formation of more stable Fe3+ state than unstable Fe2+ state and vice versa with less noble metal-containing MWF alloy confirmed by XPS. Thus, the current investigation addressed the immobilization of noble metals fission product in the alloy microstructure and their beneficial impact on corrosion performance of ferritic steel-Zr based MWF alloy which are of primary concern in the high-level radioactive waste disposal in a geological repository. (C) 2019 Elsevier B.V. All rights reserved.