[en] Two 6-rod, instrumented test assemblies, designated IFA-431 and IFA-432, were designed to study the effects of fabricated fuel - cladding gap size, fuel density and stability, fill gas composition, linear heat rate, and burnup. Rod 4 of each assembly was designed to study the effects of fuel - cladding geometry and minimum gas thermal conductivity upon heat transfer across the gap between the fuel and cladding. To accomplish this, mechanical restraint was used to form a concentric fuel - cladding geometry in the upper end of the rod, and an eccentric geometry in the lower end of the rod; the rod was then backfilled with xenon gas. The eccentric fuel region of the xenon rod was observed to have lower fuel temperatures, for equal power, than the concentric fuel region. It is concluded that fuel - cladding eccentricity enhances the azimuthal average gap conductance, thus reducing the fuel centerline temperature. The xenon-filled rods were compared to rods backfilled with helium. As expected, the xenon rods had higher fuel temperatures than the helium-filled rods in the assemblies, although not as high as was initially predicted. It is concluded that the reduced temperatures (relative to the predicted temperatures) were the result of a greater decrease in fuel - cladding gap size than is predicted to occur from thermal expansion alone. The ratio of the gap conductances for a xenon rod and a helium rod with equal operating thermal radial gaps, is equal to the ratio of the gas thermal conductivities for the two rods