Effect of ion irradiation on the thermal conductivity of UO₂ and U₃O₈ epitaxial layers

The effect of 2 MeV Ar⁺-ion irradiation on the thermal conductivity of epitaxial layers of UO₂ and U₃O ₈ was studied with time-domain thermoreflectance (TDTR) at temperatures between 323 and 658 K. The thermal conductivity of unirradiated UO₂ is 10.2 W m^-1 K^-1 at 323 K and scales with temperature like 1/T. Upon irradiation with doses >0.7 × 10 ¹⁵ Ar⁺ cm^-2, the thermal conductivity decreases to a nearly constant value <4 W m^-1 K. By comparing the data to an analytic model, we report the point-scattering strength of irradiation induced defects in UO₂. Unirradiated U₃O₈ has a thermal conductivity of 1.6 W m^-1 K^-1 at 333 K. Low and medium doses of Ar-ion irradiation decrease the thermal conductivity of U ₃O₈ to <1.0 W m^-1 K^-1. Unexpectedly, it is found that higher doses of Ar⁺ irradiation increased the conductivity of U₃O₈ to ∼2.0 W m ^-1 K^-1.