A promising fuel concept for fast breeder reactor applications is sphere-pac mixed carbide fuel. To support this development, it is necessary to devise analytical methods to assess and predict the behavior of fuel pins under irradiation conditions. This Note describes the modeling of the thermal aspect of this behavior. As a first step in treating a sphere-pac mixed carbide fuel pin, models for various physical phenomena have been developed and integrated to give the overall fuel pin response. Included are descriptions for the thermal conductivity of the packed particle bed in its initial configuration and during restructuring, the sintering of fuel spheres leading to the restructuring within the pin, the temperature distribution, grain growth and porosity redistribution, gas release and free swelling, and the effect of the gas in the free volume of the pin. The models describing the various thermal components were incorporated into the computer program SPECKLE-I. In the absence of a detailed mechanism analysis, restrictive assumptions were made. While the code is a limited first step in the analysis, results from SPECKLE-I were compared with several pin irradiations. Calculations of gas pressure and composition, fractional gas release, and the extent of initial-stage restructuring within the pin were compared to measurements. Initial results generally agree to within 20% or better for the parameters investigated.