The effect of 2 MeV Ar+-ion irradiation on the thermal conductivity of epitaxial layers of UO2 and U3O8 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.K at 323 K and scales with temperature like 1/T. Upon irradiation with doses >0.7 x10^15 Ar+ cm², the thermal conductivity decreases to a nearly constant value <4 W/m.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.K at 333 K. Low and medium doses of Ar-ion irradiation decrease the thermal conductivity of U3O8 to <1.0 W/m.K. Unexpectedly, it is found that higher doses of Ar+ irradiation increased the conductivity of U3O8 to 2.0 W/m.K.