UO₂/Sheath Heat Transfer During Irradiation

UO₂ fuel elements, sheathed in Zircaloy or stainless steel, were irradiated under controlled conditions to study the transfer of heat across the fuel-to-sheath interface. Variables studied were diametral clearance, heat-transfer medium, duration of irradiation, and power rating. After irradiation, fractured and polished cross sections and β autoradiographs were examined to determine the temperature distribution in the UO₂. The heat-transfer coefficient h increases with increasing power per unit length. For a specified power, h increased with lower initial clearances. The use of helium instead of argon increased h especially with large clearances, but by a factor much less than the ratio of the thermal conductivities of the gases. Values of h varied widely with lead-bonding; in some positions, h was very large, whereas in others its values were less than for the argon-filled elements. Metallographic examination showed that the lead had moved from some areas of the interface, leaving gaps with poor heat transfer. In the loop elements the grain-growth pattern indicated that some of the heat passed through the lead that had flowed between the pellets. Elements evacuated just before final sealing had values of h equal to or higher than those for argon-filled elements. This is tentatively attributed to the release of natural gases (mainly hydrogen) from the U02 pellets during irradiation, as observed in auxiliary experiments.