In the feasibility study of a FBR cycle system, alternative nuclear fuels are considered. One option is the "sphere-pac" fuel, which consists of a spherical particle bed vibro-packed into the cladding tube. Potential advantages of this concept are an almost powder less production and lower mechanical interaction between fuel and cladding. JNC has developed a new fuel irradiation behavior analysis code for sphere-pac fuel. It is based on a pellet type fuel code because of its analogous irradiation behavior. However, the macrostructure of the sphere-pac fuel affects some properties. Therefore, some modifications have to be applied. The PSI SPACON model is used to calculate the effective thermal conductivity. This theoretical calculation model takes into account the discontinuous structure with voids between the particles. It also accounts the increased thermal conductivity due to the sintering of the bed. A mechanistic sintering model addresses this structural change. Finally, the mechanical interaction between particle bed and cladding is studied. A three-dimensional Distinct Element Method is applied to investigate mechanical properties of the bed. Together with the theory of radial expansion pressure, these results then built a new model for the sphere-pac mechanical interaction.