The evaluation of temperature jump distances and thermal accommodation coefficients from measurements of the thermal conductivity of UO2 packed sphere beds

Abstract The results of thermal conductivity measurements on UO 2 80 μm diameter microsphere beds in helium, argon and krypton over a range of temperatures and pressures are reported. These and previous results on UO 2 microsphere beds in helium, krypton and nitrogen obtained at Oak Ridge National Laboratory (ORNL) were analysed in terms of a previously developed model which enables thermal accommodation coefficients (TAC) and temperature jump distances to be evaluated. Values derived from ORNL results tend to be in reasonable agreement with the corresponding, rather sparse data reported previously. In particular, in the case of helium, TAC's are in reasonable agreement with the direct measurements of Ullman et al. and exhibit a similar decrease with increasing temperature. However the TAC and temperature jump distance values derived from the Harwell measurements are respectively larger and smaller than the comparable ORNL figures. Also, in the case of helium, the TAC increases with pressure in contrast with values obtained from the ORNL data which are pressure independent. These differences are attributed to the differing surface finishes that are achieved during the manufacturing processes employed in the two laboratories.