Physical and Mechanical Aspects of Radiation Embrittlement of RPV Steels

A multi-scale approach to brittle fracture of irradiated RPV steels has been developed within the EURATOM FP6 Integrated Project PERFECT, and the EC-sponsored ISTC Project 3072: “Modelling of Brittle and Ductile Fracture and Prediction of Irradiation Damage Effects on Fracture Toughness Properties of Steels for Reactor Pressure Vessels on the Basis of Local Approach”. In the present paper, the physical and mechanical aspects of the irradiation embrittlement phenomenon are analysed on the basis of a multi-scale approach that includes the physical mechanisms of irradiation damage, the physical mechanisms of nucleation and propagation of cleavage microcracks, formulation of a local approach criterion and calculation of fracture toughness. It has become clear from the study performed that irradiation embrittlement is not only due to the hardening of material as traditionally considered. Damage features such as irradiation-induced lattice defects, element precipitation and impurity segregation also affect cleavage microcrack nucleation, although they do not appreciably change the critical stress for microcrack propagation. It is shown that irradiation embrittlement of RPV steels is connected both with the increase of yield stress (mechanical factor) and the decrease of the critical stress for microcrack nucleation (physical factor). This finding allows an interpretation of the experimental observations, showing that for some cases there is no simple one-to-one relationship between the brittle-to-ductile transition temperature shift ΔT_tr and the corresponding increase in yield stress Δσ_Y. For example, this explains why irradiation-induced phosphorus segregation may significantly increase the transition temperature shift ΔT_tr but does not result in a significant increase in yield stress. Calculation of the K_JC(T) curves has shown that the contributions of mechanical and physical factors in irradiation embrittlement may differ for various steels. So, for low-alloy, low-strength steel the mechanical factor predominates, while for RPV steels both of these factors control irradiation embrittlement. The results obtained have been interpreted with reference to a local cleavage fracture criterion.