Research into the effects of irradiation on nickel — based alloys peaked during the fast reactor develop­ment programs carried out in the 1970s and 1980s. Interest in these materials focused on their high resis­tance to radiation-induced void swelling compared to austenitic steels, though a perceived susceptibility to irradiation embrittlement limited their application to some extent. Nevertheless, the Nimonic alloy PE16 was successfully used for fuel element cladding and subassembly wrappers in the United Kingdom, and Inconel 706 was utilized for cladding in France. Both of these materials are precipitation hardened and consequently have high creep strength, and much research and development of alternative alloys was directed toward maintaining swelling resistance and creep strength while aiming to alleviate, or at least understand, irradiation embrittlement effects. There has been some revival of interest in nickel-based alloys for nuclear applications, and various aspects of radiation damage in such materials have recently been reviewed by Rowcliffe et al‘ in the context of Gener­ation IV reactors, and by Angeliu et at2 in consider­ation of their use for the pressure vessel of the Prometheus space reactor. Nickel-based alloys are also candidate structural materials for molten salt reactors, for which resistance to corrosion by molten fluoride salts and high-temperature creep strength are prime requirements, though intergranular attack by the fission product tellurium and irradiation embrittlement due to helium production are poten­tially limiting factors for this application.3

This chapter focuses on the void swelling behav­ior, irradiation creep, microstructural stability, and irradiation embrittlement of precipitation-hardened nickel-based alloys. Fundamental to all of these effects are the basic processes of damage production
by the creation of vacancies and interstitial atoms in displacement cascades, and the ways in which these point defects migrate and interact with, causing the redistribution of, solute atoms. Detailed discus­sions of damage processes and radiation-induced segregation are beyond the scope of this chapter but these topics will be introduced where necessary, particularly in relation to void swelling models. More detailed reviews are given in Chapter 1.01, Fundamental Properties of Defects in Metals; Chapter 1.03, Radiation-Induced Effects on Microstructure; Chapter 1.11, Primary Radiation Damage Formation; Chapter 1.12, Atomic-Level Level Dislocation Dynamics in Irradiated Metals and Chapter 1.18, Radiation-Induced Segregation.

Typical compositions of nickel-based alloys and some precipitation-hardened steels, which are con­sidered in this chapter, are shown in Table 1. Alloy compositions are generally given in weight percent throughout this chapter unless stated otherwise. Precipitation-hardened alloys may be utilized in a number of different heat-treated conditions, which are generally abbreviated here as ST (solution trea­ted), STA (solution treated and aged), and OA (over­aged). Further information on the material properties of nickel alloys is given in Chapter 2.08, Nickel Alloys: Properties and Characteristics.

Neutron fluences are generally given for E > 0.1 MeV unless indicated otherwise. Atomic dis­placement doses (dpa) are generally given in NRT (Fe) units, although the half-Nelson (N/2) model was
widely used particularly in the United Kingdom in the 1970s6. The exact relationship between these units will vary depending on the neutron spectrum (which may differ, not only from one reactor to another, but also depending on location within a reactor), but approximate conversion factors for fast reactor core irradiations are

1026n m~2(E > 0.1MeV)

= 5dpa NRT(Fe) = 6.25dpa (N/2)