In this section, we discuss briefly some important radiation effects in a few ceramics that are used in nuclear reactor applications. We will consider three representative ceramic materials: (1) urania (UO2); (2) silicon carbide (SiC); and (3) graphite (C). Unfor­tunately, we cannot provide a thorough review of the radiation damage studies that have been performed on many hundreds of other nonmetallic solids.

1.05.3.1 Radiation Effects in Uranium Dioxide

UO2 is an important nuclear material because it is the fuel form of choice for conventional light water reac­tors. Unfortunately, our knowledge of pure radiation effects in UO2 is somewhat limited. This is because 235U undergoes fission in a thermal neutron radiation environment, and consequently, the radiation response of UO2 is dictated by chemical evolutionary effects rather than by conventional, point defect condensation effects. Swelling in UO2 during service as a nuclear fuel can be significant; several percent for some percent burnup of heavy metal.29 This swelling is primarily due to the accumulation of gaseous fission products as well as to some degree, solid fission pro­ducts. It is not believed that UO2 is susceptible to void swelling (described in previous sections).

Under ballistic radiation damage conditions, UO2 exhibits polygonization, that is, a grain subdivision process in which UO2 grains initially ~10 pm diame­ter subdivide into 104 to 105 new small grains of ^0.2 pm size.30 These authors demonstrated that polygonization is initiated at a critical ballistic dam­age dose, apparently independent of temperature. In particular, irradiation of single crystal UO2 with 300 keV Xe ions at 77 K, 300 K, and 773 K, to a fluence of 4 x 102°Xem~2 or higher, produces the polygonization transformation.30 However, these authors concluded that this transformation cannot be due to radiation damage alone but is probably also related to the implanted impurity atoms (Xe), which reach a concentration of 5-7% at the critical fluence described above. Despite the polygonization transformation in UO2, no amorphization transfor­mation, induced by ballistic damage conditions, has ever been observed.

Under SHI (electronic stopping) irradiation dam­age conditions, once again amorphization was not observed (even with overlapped ion tracks), and the SHI-induced swelling is negligible.3 These experi­ments included numerous ion species (Zn, Mo, Cd, Sn, Xe, I, Pb, Au, and U) and energies ranging from 72MeV to 2.7 GeV. Latent tracks were visible by TEM for electronic stopping powers greater than 29 keV nm~ , but all tracks were crystalline. Lattice parameter expansion and polygonization were also observed.