[en] Single-crystal forms of MgAl₂O₄, Y₃/Al₅O₁₂, and Al₂O₃ were irradiated in the EBR-II fast fission reactor to a fluence of 0.3 x 10²⁶ n/m² at 1015⁰K, and to ^-1 to 2 x 10²⁶ n/m² at 925 and 1100⁰K. Fracture toughness was subsequently measured at room temperature by an indentation technique, and radiation-induced defect aggregates were characterized by transmission electron microscopy. A slight increase in toughness of MgAl₂O₄ was observed, and attributed to interaction of cracks with strain fields around dislocation loops. No significant change was noted for Y₃Al₅O₁₂, despite the presence of a high concentration of unresolved defect clusters. Fracture toughness of Al₂O₃ was markedly increased, with the enhancement apparently attributable in large part to impedance of crack propagation by interaction with the irradiation-induced void lattice