Modelling of Irradiation Embrittlement in the Ductile to Brittle Transition Range for an A508 Pressure Vessel Steel

The aging behavior of structural steels used to manufacture nuclear pressure vessels is surveyed using Charpy V-notch specimens located in capsules inside the pressure vessel. The Charpy data are then used to assess the safety integrity of the structures based on semi-empirical relations relating Charpy impact transition curve shifts and the fracture toughness shifts due to irradiation. Using a computational strategy proposed in [1] which combines a deterministic model for ductile fracture and a statistical description of brittle fracture, this work aims at the prediction of the whole Charpy transition curve of irradiated steels. The actual strain hardening behavior of an A508 Cl.3 steel from the french surveillance program is considered in the simulations, contrarily to a previous work where a shift of the un-irradiated stress-strain curve to higher stress values was considered. Comparison with Charpy energy data for two levels of irradiation shows that irradiation possibly also affect brittle fracture. It is also shown that if a low increase of the yield stress is considered, the ductile fracture energy can decrease as a result of a compensation between the increase of dissipated energy due to a higher yield stress and a decrease of dissipated energy due to a faster ductile crack propagation.