Nuclear fuel can be fabricated and used in the form of microspheres (sphere-pac fuel). The heat transport mechanisms in fuel pins containing sphere-pac fuel are however very different from those in pellet pins. They are controlled not only by the thermal conductivity of fuel, cladding, and fill gas but also by particle sizes and packing density, by their state of sintering, and by radiation and gas pressure. A theoretical model is presented accounting for all these parameters, but still simple and fast enough to be implemented into a fuel pin modeling code. The basic geometrical element for this model is derived from the orthorhombic packing. For calculation of a binary package, four different radial zones within the basic element are distinguished, i.e., neck zone, gas zone, infiltrated zone, and bypass zone. The method presented here combines an analytical one-dimensional treatment with a radial heat flow relaxation procedure simulating the second (radial) dimension. Results are compared with experimental and theoretical data from the literature. With the model presented here, sophisticated modeling of sphere-pac fuel pins is possible.