The major components of a numerical model (SPECKLE-III) for the irradiation behavior of mixed-carbide Sphere-Pac fuel are described. Based on user-supplied power and outer cladding temperature history, radial profiles for the pin temperature, restructuring, fission gas behavior, and stress distribution are calculated at several axial locations as a function of irradiation time. Finite difference and finite element methods are used to solve the quasi-steady, axisymmetric governing equations. Axial slip at the fuel cladding interface is also modeled. The effects of temperature and restructuring on the fuel thermal and mechanical properties are included. Representative results are presented for a Sphere-Pac fuel irradiation experiment.