[en] An evaluation is made of the radiation stability of 15Kh2MFA and ASTM-A543 steels and their welded joints. The courses are given of the temperature dependence of tensile strength and shear strength (-140 degC to 250 degC) and of notch impact strength (-60 degC to 150 degC; in case of microsamples -100 degC to 100 degC) for irradiated and non-irradiated materials. The fracture toughness was evaluated by means of Ksub(IC), COD(deltasub(c)) and Jsub(IC). Evaluated was the transition temperature shift following irradiation with an integral dose of the order of 10¹⁹ n/cm² with neutrons of above 1 MeV using the accepted criteria deltasub(c)=0.1 mm for the COD technique and 50 N/mm for the approach using integral J (a value approximately equivalent to deltasub(c)=0.1 mm). For the Ksub(IC) technique the shift was evaluated from maximal values at which valid values of Ksub(IC) were obtained. The transition temperatures of notch impact strength were determined applying the criterion 50 J.cm^-2 (ΔT₁) and by one half of the value of maximum absorbed energy 1/2 RVsub(max) (ΔT₂) and finally applying the criterion of the 50% proportion of the plastic appearance of the fracture surface (50% PL). (B.S.)