1.06.3.1 Background

Clearly, it is not possible to cite, let alone describe in detail, the extensive literature on He effects in irradiated alloys. This literature encompasses both mechanical properties, especially HTHE, and the effects of He on microstructural evolutions, particu­larly void swelling. There is also a more limited literature on fundamental processes and properties related to He in solids, like desorption measurements and He solution, binding, and diffusion activation energies. Much of previous work pertains to fcc (face-centered cubic) AuSS, which is one of interest for fast reactor cladding applications. However, stan­dard AuSS, like AISI 316 («Fe-0.17Cr-0.12Ni-bal Mo, Si, Mn, …) are highly prone to both HTHE and void swelling. Thus, advanced AuSS and bcc FMS have supplanted conventional AuSS as the leading candidates for nuclear applications. Never­theless, conventional AuSS alloys nicely illustrate the damaging effects of He (see Section 1.06.3.2 and following), which are both subtle and signifi­cantly mitigated in advanced steels. Swelling and HTHE resistance are largely due to microstructural designs that manage He.

Particular emphasis in this section is placed on the critical bubble model (CBM) concept of the transition of stable He bubbles to unstably growing voids, both under irradiation-driven displacement damage, and stress-driven growth of grain boundary creep cavities. We believe this focus is appropriate, since it seems that many current modeling efforts have lost connection with the basic thermody­namic-kinetic foundation for understanding He effects provided by the CBM concept and the large body of earlier related research.

The organization of this section is as follows. Section 1.06.3.2 outlines the historical motivation for concern about He effects in structural alloys, including examples of HTHE and void swelling. Section 1.06.3.3 describes the mechanisms of swelling and its relation to He and He bubbles, especially in AuSS. Section 1.06.3.4 presents a quantitative CBM for void nucleation and a simple rate theory (RT) model of swelling. Section 1.06.3.5 summarizes the implications of the experimental observations and models, and the development of irradiation-resistant alloys. Sections 1.06.3.6 and 1.06.3.7 discuss the application of the CBM to HTHE and corresponding experimental observations, respectively.