On the Mechanism of Irradiation Embrittlement Enhancement in Reactor Pressure Vessel Steels at High Neutron Fluences

Abstract

The DBTT shift in irradiated VVER pressure vessel steels is represented by standard guidelines with a power law dependence on fast neutron fluence multiplied by a material constant. However, from recent data on surveillance specimens for several units of VVER-440, an irradiation embrittlement enhancement has been observed at neutron fluences above (2–3)×1020 n/cm2 (E>0.5 MeV) with a significant deviation from the standard guideline. This suggests an additional mechanism of irradiation embrittlement at high fluences. In the present work it was supposed that such a mechanism could be intergranular embrittlement caused by grain boundary phosphorus segregation. A threshold dose was used that accounts for intergranular embrittlement, that occurs after sufficient phosphorus segregation, when the critical stress for intergranular fracture falls below that for cleavage fracture. The modeling of phosphorus accumulation on grain boundaries is performed taking into account the radiation-enhanced phosphorus diffusion by both vacancy and interstitial mechanisms, the radiation-induced segregation in the matrix near grain boundaries, and the Gibbsian adsorption on grain boundaries. Predictions of such a composite model are in agreement with surveillance data for VVER-440.