Thermal Conductivity of Nearly Stoichiometric Single‐Crystal and Polycrystalline UO2

The thermal conductivities, λ, of single-crystal and polycrystalline UO2 were measured from 80° to 420°K. The results indicate no observable difference in λ between single-crystal and polycrystalline UO2, and both materials have broad peaks in λ at ∼220°K. The results were used with literature values to determine the effect of closed porosity on λ. The thermal conductivity of theoretically dense UO2 is described phenomenologically from 80° to 1400°K, where conduction is dominated by the phonon component. The phonon conduction is analyzed by comparison with ThO2. This analysis indicates that the high-temperature λ is limited by 3-phonon Umklapp scattering processes. Scattering by the disordered spins associated with the paramagnetic U ions contributes a large temperature-independent phonon scattering term. This mechanism has a mean free path of about 51 A, which implies that grain boundaries and impurities have a relatively insignificant effect on the phonon conduction far above the antiferromagnetic-para-magnetic transition at ∼ 30°K. This implication agrees with the experimental results.

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