Small disk tensile specimens of the reduced activation tempered martensitic steel Eurofer97 were tested from −100 °C up to 370 °C before and after irradiation in the spallation source SINQ at Paul Scherrer Institute. The calculated irradiation dose and helium content are respectively 11 dpa and 540 appm. Even for such a large dose, the tensile curves showed a significant total elongation at all testing temperatures. Fractographic observations revealed a predominance of cleavage facets at −100 °C for unirradiated and irradiated specimens while at higher temperature the fracture surfaces were dominated by ductile dimples. True stress/strain curves were derived using a finite element based inverse method for simulating small disk tensile tests. The finite element simulations indicated that the local fracture stress in the neck is consistent with that calibrated based on fracture tests and that the micro-mechanisms of fracture were not affected by helium up to 540 appm.