Thermodynamic and transport property predictions for UF6-He gas mixtures are presented covering the operating range of conceptual, circulating gas core nuclear systems. The gas mixtures of interest contain 10 to 20% helium by mass, which corresponds to helium mole fractions of 0.9 and higher. For UF6 partial pressure <10 atm and temperatures in the range of 500 to 2000 K, mixture density can be determined from the ideal gas equation of state with an uncertainty of <10%. Compared to pure UF6, the thermal conductivity of the mixtures is an order of magnitude greater; specific heat is doubled while viscosity is changed very little. For identical systems, it is shown that heat transfer rates for UF6-He mixtures are five to six times greater than for pure UF6.