It is important to note that material modification by radiation arises from two primary spectral-related processes. In addition to the neutron-induced dis­placement of atoms there can be a chemical and/or isotopic alteration of the steel via transmutation. With the exception of helium production, transmuta­tion in general has been ignored as being a significant contributor to property changes ofstainless steels and nickel-base alloys. In this chapter, transmutation is shown to be sometimes much more important than previously assumed.

Both the displacement and transmutation pro­cesses are sensitive to the details of the neutron flux-spectra, and under some conditions each can synergistically and strongly impact the properties of the steel during irradiation. In addition to the brief summary presented below on flux-spectra issues rel­evant to stainless steels, the reader is referred to various papers on transmutation and its consequences in different reactor spectra.5- ’21-23

Transmutation may be subdivided into four cate­gories of transmutants. Three of these are relevant to fission-derived or fusion-derived spectra, and the fourth is associated with spallation-derived spectra. The first three are solid transmutants, gaseous trans­mutants, and ‘isotope shifts,’ the latter involving pro­duction of other isotopes of the same element. While the latter does not change the chemical composition of stainless steels, it is an underappreciated effect that is particularly relevant to nickel-containing alloys such as stainless steels and nickel-base alloys when irradiated in highly thermalized neutron spectra.

Whereas the first three categories arise from dis­crete nuclear reactions to produce discrete isotopes of specific elements, the spallation-induced transmuta­tion arising in accelerator-driven devices involves a continuous distribution ofevery conceivable fragment of the spalled atom, producing every element below that of the target atom across a wide range of isotopes for each element. While individual solid transmutants in spallation spectra are usually produced at levels that do not change the alloy composition significantly, the very wide range of elements produced allows the possibility that deleterious impurities not normally found in the original steel may impact its continued viability. This possibility has not received sufficient attention and should be examined further ifspallation devices continue to be developed.

Another consequence of spallation-relevant trans­mutation is that the induced radioactivity per unit mass is correspondingly much higher than that pro­duced per dpa in other spectra. The majority of the spalled fragments and their daughters/granddaugh- ters are radioactive with relatively short half-lives, leading to materials that are often much more diffi­cult to examine than materials irradiated in fission spectra.

Most importantly, there is a very strong produc­tion of hydrogen and helium in spallation spectra at levels that are one or two orders of magnitude greater than produced in most fission or fusion spectra.5,6,21

While there is a tendency to view displacement and transmutation processes as separate processes, it will be shown later that under some circum­stances the two processes are strongly linked and therefore inseparable in their action to change alloy behavior.