Nitridation and dissolution studies of U-Zr metallic alloys

Aqueous based PUREX reprocessing is an alternate to pyro processing of spent metallic fuel. In the present study, a nitridation route is proposed to overcome the issues associated with oxidation based route of metal fuel. In this context, nitridation of U-Zr metal alloy followed by its dissolution has been investigated. The metallic nitrides can be converted by two methods 1) Direct Nitridation 2) Hydridation followed by Nitridation. The first method demands use of high temperature and longer reaction time compared to the latter. In the second method, initially, the metallic fuel reacts with hydrogen to form hydrides, which increases the porosity and surface area of the pellets facilitating the formation of nitrides at lower temperature. Thus, hydridation-dehydridation-nitridation is a preferred method to convert the U-Zr metallic fuel into its nitrides and product characterised using XRD technique. The U-Zr alloy was converted into hydrides at 400-500℃ using hydrogen from UH₃ getters followed by dehydriding at 700℃. The hydriding-dehydring cycle is performed several times to ensure complete conversion of metallic hydrides. Then nitridation of the metallic hydrides was carried out in N₂ atmosphere at 400℃. To ensure complete reaction, the temperature was gradually raised up to 1100℃ with rate of 10-12℃/min. After nitridation, for the complete conversion, nitrides were heated at 1000℃ under vacuum. The formation of nitride was confirmed by comparing the XRD pattern with standard JCPDF. Furthermore, the metallic nitrides were dissolved in 12 M nitric acid under reflux conditions at 120℃ for 8 h. The U and Zr contents of the dissolver solution were analysed using modified Davies-Gray method and Spectrophotometric methods respectively. Near complete dissolution of U (>99%) and partial dissolution of Zr (>90%) was established