[en] Tensile specimens and miniature impact specimens of the low alloyed pressure vessel steel Fortiweld have been irradiated at 265 deg C in R2 to two neutron doses, 6.5 x 10¹⁸ n/cm² (> 1 MeV) and 4 x 10¹⁹ n/cm² (thermal) and also 9.0 x 10¹⁸ n/cm² (> 1 MeV) and 6 x 10¹⁹ n/cm² (thermal). Material from three laboratory melts, in which the boron consisted of ¹⁰B, ¹¹B and natural boron respectively, were investigated. The results both of tensile tests and impact tests with miniature impact specimens show that the ¹⁰B-alloyed material was changed more and the ¹¹B-alloyed material was changed less than the material containing natural boron. At the higher neutron dose the increase in yield strength (0.2 % offset yield strength) was 11 kg/mm in the ¹⁰B containing material compared to 5 kg/mm in the ¹¹B-containing material. The decrease in total elongation was 5 and 0 percentage units respectively. The transition temperature was increased 190 deg C at the higher neutron dose in the ¹⁰B-alloyed material, 40 deg C in the ¹¹B-alloyed material and 80 deg C in the material containing natural boron