Thermal and electrical conductivity degradation can occur over a wide range of irradiation temperatures. For pure metals, there are two primary contributions: electron scattering from point defects (and associated defect clusters) and nuclear transmutation solute atoms. The conductivity degradation associated with radiation defects usually amounts to less than ~1% change except in the case of high void swelling conditions.233-236 Conversely, the conductivity deg­radation associated with neutron-induced transmuta­tion products tends to monotonically increase with increasing dose and typically becomes larger than the radiation defect contribution for doses above ~1 dpa. Thermal conductivity degradation much greater than 10% can occur in high-conductivity metals and ceramics.235,237 The conductivity degradation in irradiated alloys can be complex due to short — range ordering and precipitation phenomena,238 with the possibility for either increased or decreased conductivity compared to the unirradiated condition. For nonmetallic irradiated materials, the electrical conductivity during irradiation typically experi­ences a transient increase due to excitation of valence electrons into the valence band by ionizing radiation.239-243 The thermal conductivity of irra­diated nonmetals is typically degraded by displace­ment damage due to phonon scattering by point

defects and defect clusters.237,243-246