The Other Metallic Phase in Spent Nuclear Fuel: A Complete Thermodynamic Evaluation of the U–Pd–Rh–Ru System

During burnup of nuclear fuel, fission products accumulate. Post-irradiation examination of burned up nuclear fuel has revealed the presence of several phases, namely, the fuel matrix of UO₂, with dissolved oxides present; a white metallic phase consisting of the so-called “noble metals” (i.e., Mo–Ru–Pd–Rh–Tc); a gray oxide phase consisting of alkali or alkaline earth oxides (e.g., BaZrO₃ or Cs₂UO₄); and an another metallic inclusion containing a mixture of UPd₃–URh₃–URu₃, which is not completely assessed due to the lack of phase diagrams of the UPd₃–URh₃, URh₃–URu₃, and UPd₃–URu₃. Understanding how these phases behave becomes especially important from a safety perspective, if one considers a potential accident scenario. The quaternary system U–Pd–Rh–Ru has been evaluated and a thermodynamic model has been developed by first considering the six binary subsystems and the four ternary subsystems. A critical examination of the U–Pd, U–Rh, and U–Ru experimental phase diagrams has been made, with attention placed on both the solution phases, generally present on the uranium side of the diagrams and the UPd₃–URh₃–URu₃ compounds. Finally, the implications of this new model and its potential refinements of the Royal Military College of Canada nuclear fuel treatment developed by previous authors (notably the RMCC group under Thompson and Lewis) will be explored.