In-Pile Measurement of Fuel-Cladding Conductance for Pelleted and Vipac Zircaloy-2 Sheathed Fuel Pins

The heat transfer coefficient between fuel and cladding has been measured in-pile for different types of Zircaloy-2 sheathed fuels: UO₂ and UO₂-PuO₂ pellets, UO₂ Vipac, and UO₂-PuO₂ Sphere-Pac sol-gel. The samples were tested in (a) instrumented rigs, in which fuel and cladding temperatures and nuclear and gamma-ray generated power were directly and continuously measured up to a heat rating of 500 W/cm, and in (b) a hydraulic rabbit at very high rating (over 1000 W/cm). Semiempirical correlations starting from known models of fuel/clad heat transfer, based on strong or weak fuel/clad interaction, fuel cracking, and decrease in gap conductance due to oxide micro layer formation on the cladding internal surface, have been set up to fit the experimental data.