Irradiation experiments were conducted in order to determine the effect of radiation damage on the elastic-plastic ductile fracture toughness of pressure vessel steels. Curves for the J-integral versus crack growth resistance (Δa) were established on specimens taken out of two nuclear reactor pressure vessel steels before and after neutron irradiation at 290°C to a fluence of 3.10¹⁹ n/cm² (work function energy E > 1 MeV). The J-Δa tests were performed at room temperature (25°C) and service temperature (290°C). The main results obtained are as follows:

• For J-Δa testing the tearing modulus appears to be the most sensitive parameter.

• The two steels have significant differences in toughness, depending on their sulfur content.

• The testing temperature strongly affects the J-Δa resistance curve.

• Irradiation affects J-Δa curves only slightly for one steel but more significantly for the other at 290°C.

• Changes in the J-Δa curves with irradiation and temperature seem difficult to predict from Charpy V results.