The high-temperature thermal conductivity of ceramic materials that are candidates for an application as RF transmission windows in a fusion reactor was examined. Materials of interest are aluminum oxide (Al²O³), aluminum nitride (Al²O³), and magnesium aluminum oxide (MgAl²O⁴). In order to simulate the fast neutron interaction within the RF windows in an operating fusion reactor, different neutron sources with different energies were used. Additionally a-particle irradiation was performed in the KfK cyclotron with E^max = 100 MeV. Our results show that the thermal conductivity of all irradiated samples was reduced due to the scattering of lattice thermal waves by radiation-induced defects. The magnitude of the reduction depends on the concentration and type of defects, i.e., point defects or three-dimensional defect aggregates. This effect is discussed within a microscopic model.