Numerous additional ceramics have been either used or proposed for nuclear reactor materials applica­tions. These include graphite (discussed in other chapters in this volume) as well as carbides and nitrides, such as ZrC and ZrN, which have higher thermal conductivities than their sister oxide com­pound, ZrO2. Research into the radiation damage properties of these materials is in its infancy, and therefore, these compounds are not described in further detail here.

1.05.2. Summary

The response of ceramic materials to radiation is espe­cially complex because ceramics (with the exception of graphite) are made up of anions and cations (some­times several different cations) such that the atomic defects that initiate radiation damage are different in their size, chemistry, charge, mobility, and so on. Thus, it is difficult to predict how the microstructure of a ceramic will evolve under irradiation and, in turn, how properties such as structural stability will change in response to the radiation-induced microstructural alterations. Nevertheless, we present a case study (described below) wherein researchers have succeeded in explaining the extraordinary differences between the radiation responses of two important engineering ceramics.

We devoted much of this chapter to comparing and contrasting the high-temperature radiation dam­age response of two quite similar refractory, dielectric ceramics: a-alumina (Al2O3) and magnesio-aluminate spinel (MgAl2O4). Al2O3 is highly susceptible to radiation-induced swelling, whereas MgAl2O4 is not. The swelling of Al2O3 is due to excessive void forma­tion in the crystal lattice. We considered in detail in this chapter the atomic and microstructural mechan­isms that help to explain why voids nucleate and grow in Al2O3 to a very significant degree, whereas in MgAl2O4, this problem is much less pronounced. We showed that the reasons for the great differences between the radiation damage behavior of Al2O3 and MgAl2O4 have mainly to do with differences in the way interstitial loops nucleate and grow in these two oxides. The hope is that by understanding these dif­ferences, we will by analogy be able to understand the radiation damage behavior of other ceramic materials.

In this chapter, we also examined two different phenomena that lead to degradation in the mechani­cal properties of ceramics: (1) nucleation and growth of interstitial dislocation loops and voids and (2) crystal-to-amorphous phase transformations. Both these phenomena cause macroscopic swelling of materials. This ultimately leads to the failure of materials because of unacceptable dimensional changes, microcracking, excessive increases in hard­ness (or alternatively, softening in the case of amor- phization), and so on.

We concluded this chapter with brief discussions of a few ceramics additionally important for nuclear energy applications, namely silicon carbide (SiC), uranium dioxide (UO2), and graphite (C).