[en] The effects of specimen size, temperature and irradiation on fracture toughness were studied in unirradiated compact tension specimens of alloy A286 and HT-9 and irradiated specimens of 20 percent cold worked Type 316 stainless steel. Techniques for specimen preparation and test procedures were developed. Fracture toughness tests on these precracked specimens were conducted at various temperatures and test results were analyzed using the multiple specimen R-curve method. Due to irradiation hardening and postirradiation toughness degradation, sufficient constraint existed in thin irradiated specimens to permit the application of the J-integral approach. Analysis showed the absence of a significant size effect and that the size requirements for a valid J/sub 1c/ test were satisfied, indicating that small specimens may be used for post-irradiation fracture toughness testing. Results are discussed in terms of size criteria for valid J/sub 1c/ determination and conditions for J-controlled crack growth