Irradiation performance data available to date indicate that the behavior of Zircaloy-clad thermal reactor mixed-oxide fuels is entirely satisfactory. There do not appear to be any inherent performance limitations created by the addition of small amounts of PuO2 to UO2. The wide range of test conditions include peak linear heat ratings sufficient to cause fuel melting and peak burnups to over 50 000 MWd/MTM. Because small PuO2 additions to UO2 have an insignificant effect on those fuel properties which could affect irradiation behavior, thermal reactor mixed-oxide fuel performs similarly and equally as well as UO2-only fuel. The evidence indicates that the PuO2-rich regions that persist in nonrestructured mechanically mixed, mixed-oxide fuel have an insignificant effect on performance. The extent of plutonium migration which can occur during irradiation also has an insignificant effect on mixed-oxide fuel behavior. Satisfactory defect behavior of Zircaloy-clad UO2-PuO2 fuel under normal operating conditions is indicated. Transient behavior can be affected by the presence of abnormally large PuO2 particles in mixed-oxide fuel; however, the effect appears to be of little practical significance. PuO2 additions to UO2 do introduce special considerations related to fuel design and fabrication. However, the successful development of a sound base technology assures the timely utilization of plutonium in water-cooled power reactors on a reload basis beginning in the mid-1970s.