State of knowledge of radiation effects on environmental cracking in light water reactor core materials

Laboratory and field data amply demonstrate that accelerated intergranular stress corrosion cracking of stainless steels and nickel-base alloys can result from long term exposure to the high energy neutron and gamma radiation that exists in the core of light water reactors (LWRs). Radiation can exacerbate specific aspects of cracking susceptibility via its effects on material micro-chemistry (e.g., radiation induced segregation), water chemistry (e.g., radiolysis), and stress (e.g., radiation induced creep and hardening). Characterization of irradiation assisted stress corrosion cracking (IASCC) as an entirely unique phenomenon is not consistent with the evidence that many of the same sensitivities, e.g., to water chemistry and stress, apply to cracking under both irradiated and unirradiated conditions. Following an overview of service history data on IASCC, the current extent of (1) fundamental understanding and quantitative modeling of individual radiation effects and (2) their influence on environmental cracking are discussed, with particular emphasis placed on radiation water chemistry and radiation induced segregation (RIS), which are viewed as key contributors. For example, experimental and theoretical RIS studies on fast neutron effects in austenitic alloys has resulted in considerable insight into the physical processes which give rise to lattice displacement damage, diffusion of vacancies and atoms, and consequent grain boundary segregation of major elements and impurities. Nonetheless, predictions of RIS (e.g., vs. temperature and alloy chemistry) and the contributions to IASCC of segregation of specific elements (Cr, Si, Ni, S, P, etc.) requires further elucidation. Finally, the status of overall modeling of environmental crack growth rates under irradiation conditions is reviewed