- Thermodynamic models are constructed for the U–Al–Si and U–A–Zr ternary alloy systems using the CALPHAD (CALculation of PHAse Diagrams) method. For the U–Al–Zr system the modeling covers only the aluminum-rich corner (from 100 at% to 67 at% Al) and is based only on literature data. For the U–Al–Si system, the whole range of compositions is covered and new key experiments were done in the uranium-poor region of the U–Al–Si system. These experiments have shown that under conditions of equilibrium with Al and Si, the Si-content of the U(Al,Si)3 is significantly higher than reported by earlier works. Different extrapolation methods were tried for the Gibbs energy of the liquid phase. However, it was found that for the U–Al–Si and U–Al–Zr systems, symmetric Muggianu method and the asymmetric method by Hillert give similar predictions. The constructed thermodynamic database was investigated by calculating isothermal sections, vertical sections and the liquidus projection. The calculated diagrams are in reasonable agreement with experimental data. Finally, solidification simulation (Scheil simulation) was done in order to assess the phases obtained in solidification as a function of the silicon addition to U–Al alloys.