Fracture toughness testing in the elastic-plastic temperature range of material behavior is described in the Master Curve standard ASTM 1921-97. The standard allows measurement of valid fracture toughness values up to K^JC= 100 MPa√m with clearly smaller than standard CVN-size specimens for the irradiated material condition. However, a relatively small amount of data measured with miniature specimens of cross-sections 5 mm × 5 mm or 3 mm × 4 mm have been published and hence the utilization of such small specimens is generally not accepted even if the universal specimen size criteria are fulfilled. In this study, the fracture toughness transition temperature T⁰ has been measured for a high phosphorus content WWER 440 weld coded as W502 both in the non-irradiated and in the irradiated and annealed material conditions. Three-point bend specimens of cross-sections 10 mm × 10 mm, 5 mm × 5 mm and 3 mm × 4mm were used in the study.