Two main models for the mechanism of irradiation creep have been put forward but neither has any microstructural observations to support them. The first suggestion is that a model by Roberts and Cottrell97 for a-uranium may be appropriate. This model proposes that the graphite crystallite struc­tures will yield and shear because of the generation

8 10 12 14 16 18 20

Fluence (GWdte 1)

Figure 66 The effect of stress and irradiation on the interlayer spacing of graphite. Modified from Francis, E. L. Progress Report for the JNPC-Materials Working Party: Graphite Physics Study Group; UKAEA, TRG-M-2854 (AB 7/17604); 1965.

the need for present plants to extrapolate beyond current data and to predict the behavior of new graphite grades operating for longer lifetimes at higher temperatures than before means there is still a substantial amount of work for the graphite special­ist. Future understanding and validation of property/ microstructural change relationships that enable the prediction and interpolation of existing databases and the development of new graphite grades is now pos­sible using new characterization, modeling, and com­putation techniques. These allow the investigation of mechanisms and graphite behavior that were previ­ously impossible or impractical to conduct. Areas of particular interest are obtaining a better understand­ing of the mechanism of dimensional change and irradiation creep, and the development of a validated graphite failure