[en] The quaternary U-Pu-Zr-Fe system was assessed using thermodynamic and phase diagram data in order to evaluate fuel-cladding chemical interactions (FCCI) of metallic fuel in a fast reactor. The Gibbs energy of mixing for solution phases and the Gibbs energy of formation of compounds in the binary sub-systems were calculated using an optimization procedure. The use of such data in optimizing the binary sub-systems enabled appropriate calculations for the thermodynamic properties of the systems, which were also important when extrapolating to higher-order systems. Isotherms of ternary sub-systems were calculated by using the optimized parameters of the binary sub-systems. Based on the phase relation data measured in regions of the ternary systems, the isotherms were then modified by adding ternary interaction parameters. The calculation results agreed well with the experimental data points. Finally, the quaternary system was assessed. The phase relationship observed experimentally in the diffusion couple of U-Pu-Zr-Fe was in reasonable agreement with the calculated phase diagrams. (authors)