Modeling of Helium Embrittlement of Nickel-based Alloys by Neutron Irradiation

Abstract

Annulus spacers in CANDU fuel channels are manufactured from Ni-based alloys, which are subjected to high neutron flux during the reactor operation. It is known that Ni-alloys irradiated by neutrons lose their ductility due to the formation of gas bubbles along the grain boundaries. A microstructure observation of ex-service spacers revealed the presence of dense bubbles with several thousand appm of helium gas. In this study, we present a theoretical model for predicting the irradiation creep parameter, that is, the time to rupture. This model is based on the growth of grain boundary bubbles under the influence of the production of helium and vacancy. The amount of radiation damage was calculated for annulus spacers in Wolsong Unit 2 reactors, and the size of the helium bubbles was estimated. It is suggested that application of the helium embrittlement model has potential to predict the integrity of annulus spacers, provided the initial bubble distribution is appropriately defined.