The report presents refined methods for calculation of fuel temperatures in PuO/sub 2/-UO/sub 2/ fuel in Fast Breeder Reactor (FBR) fuel pins. Of primary concern is the calculation of the temperature changes across the fuel-to-cladding gap of pins with fuel burnups that range from 60 to 10,900 MWd/MTM (0.006 to 1.12 at.%). Described in particular are: (1) a proposed set of heat transfer formulations and corresponding material properties for modeling radial heat transfer through the fuel and cladding; and (2) the calibration of a fuel-to-cladding gap conductance model, as part of a thermal performance computer code (SIEX-M1) which incorporates the proposed heat transfer expressions, using integral thermal performance data from two unique in-reactor experiments. The test data used are from the HEDL P-19 and P-20 experiments which were irradiated in the Experimental Breeder Reactor Number Two (EBR-II), for the Hanford Engineering Development Laboratory (HEDL).