Measurements of the thermal conductivity of uranium dioxide at 670 1270 k

Abstract Thermal conductivity measurements have been made on porous stoichiometric and hyperstoichiometric uranium dioxide in the temperature range 670–1270 K using the laser-flash method. The pore volume fraction varied between 0.014 and 0.096 and the O/U ratios between 2.00 and 2.11. The variation of thermal conductivity ( k ) with pore volume fraction ( P ) at O/U ratios of 2.00 and 2.015 followed an equation of the form k = k 0 (1 − βP) , where k 0 is the thermal conductivity of fully-dense material, at all temperatures in the range covered. The value of β was independent of the temperature in both the stoichiometric and hyperstoichiometric material but had different values, 2.8 and 1.5 respectively, in the two materials. No explanation can at present be offered for this difference. The individual effects of three kinds of phonon scattering centres, viz. other phonons, impurities in the lattice and excess oxygen ions, have been measured. The average phonon scattering cross-section for excess oxygen ions has been evaluated as 6.3 × 10 −19 m 2 The thermal resistivity ( R ) of material having 96% of the maximum theoretical density, has been shown to conform to an equation of the type R = A + BT where both A and B are parameters which depend on the O/U ratio, and T is the absolute temperature. On the basis of our observations we suggest that the thermal conductivity of uranium dioxide might well become independent of both temperature and O/U ratio at temperatures above about 850 K and at O/U ratios in excess of about 2.13.