As discussed in Chapter 4.10, Radiation Effects in Graphite, irradiation-induced dimensional changes in graphite are highly anisotropic, and a strong function ofirradiation temperature and neutron dose (dpa). The temperature range of interest for fusion applications varies from 100 °C in areas well removed from the plasma of experimental devices, to over 1000 °C for the surface of PFCs, which experience appreciable plasma flux, and for future power-producing machines. As described in detail in Chapter 4.10, Radiation Effects in Graphite, the mechanism of graphite irradiation-induced dimensional change is a combi­nation of intra — and intercrystallite effects. Within the crystallites, displacement damage causes an (a)-axis shrinkage (within the basal plane) and a (c)-axis growth (perpendicular to the basal plane.)

The upcoming ITER reactor will be the first fusion reactor to provide a flux of neutrons to pro­duce measurable thermophysical effects to fusion structural materials. Even so, this will be a relatively modest fluence machine, with the maximum fast dose accumulating less than 1 x 1025nm~2 (E > 0.1 MeV), or less than a displacement per atom, over its lifetime. The work of Bonal provides data on the dimensional changes in CFCs, which are expected in this dose range. Specifically, his work11 irradiated 2D and 3D
composites to doses approaching 1 dpa in the tempera­ture range of 610-1030 °C. Figure 7 shows the dimen­sional instability that occurs in these materials in the sub-dpa region, specifically indicating a shrinkage.

The work of Burchell12 in Figure 8 shows the dimensional change behavior of 1, 2, and 3 direc­tional composites for doses somewhat in excess of the ITER lifetime. In this example, solid cylinders were irradiated at 600 °C to doses ranging to 5 dpa and the resulting diameter and length measured. The behavior of each material can be explained by the accepted theory for dimensional change in graphite (Chapter 4.10, Radiation Effects in Graphite) after taking into account the individual fiber architec­tures, and by the observation that a model for fibers describes them as graphite fiber, filaments of circum­ferential or radial basal planes running parallel to the fiber axis. The irradiation-induced dimensional change of such a fiber is therefore a shrinkage in length and a growth in diameter. However, at doses

less than 1 dpa the dimensional change is relatively minor (Figure 7). As the dose is increased, the direc­tion perpendicular to the fiber axis is more or less unchanged while a significant shrinkage along the direction parallel to the fiber axis occurs. At about 2-3 dpa, swelling in the composite occurs in the perpendicular direction. The random fiber composite of Figure 8 has a random orientation of chopped PAN fibers in the plane of the composite. The speci­men diameter shows practically no change perpen­dicular to the fiber axis to about 4.5 dpa, though it exhibits ^2% shrinkage parallel to the fiber axis. The 3D balanced PAN weave fiber has essentially isotropic shrinkage to a dose of ^2 dpa, at which point the diameter of the fibers, and hence the sam­ple, begins to swell.

Also given in the 3D composite plot in Figure 8 is the radiation-induced dimensional change parallel to the fiber axis of an Amoco P55 pitch fiber com­posite. This material was processed in an identical manner to the PAN fiber composite. From the plot, it appears that the pitch fibers, and thus the compos­ite, undergo slightly less shrinkage, possibly due to the higher fiber crystallinity. This hypothesis is also supported by the observation that fibers with higher final heat treatment temperatures tend to exhibit less dimension change13 and it is also consistent with the observation that elevating the heat treatment temper­ature of graphite reduces the irradiation-induced

shrinkage.14

The irradiation-induced dimensional changes are of fundamental importance to the design and perfor­mance of the fusion structure, and even more so of the PFCs. This is due to the need to precisely define the plasma edge. For this reason, it is instructive to look at the irradiation effects at the higher dose and temperature conditions representative of the next — generation fusion power devices. The data shown in Figures 9 and 10 provide higher temperature dimen­sional swelling data for the FMI-222 3D CFC and MKC-1PH 1D CFC, which were model, high ther­mal conductivity CFCs studied in the early phases of the ITER composite development program.15 In Figure 10, the dimensional change of the 1D composite yields substantial swelling perpendicular to the fiber axis and equally impressive shrinkage parallel to the fiber. The FMI-222 of Figure 10, a nearly isotropic orthogonal weave pitch-fiber com­posite with equivalent fiber volume fraction in the x, y, and z directions, undergoes a positive dimensional change (swelling) parallel to the cylindrical axis of the sample, which increased with increasing temper­ature. The magnitude of swelling was in excess of 10% at the highest temperatures studied at the 2 dpa dose level. This is in contrast to the FMI-222 swelling data reported by Burchell12 and Snead,16 also for HFIR irradiation, though at a lower irradiation tempera­ture. Specifically, a contraction of 0.6% is interpolated from the data of Burchell for FMI-222 irradiated

at 600 ° C to an equivalent fluence as the data of Figure 9. Snead16 reports on an 800 °C irradia­tion to a substantially higher dose (7.7 x 1025nm~2) than the Figure 8 dose (~2.4 x 1025nm~2). In this case, the material underwent a contraction of 3.6%

along the length of a bend-bar (2.3 x 6 x 30 mm). It was also noted in this work that the width and thickness direction exhibited swelling. Specifically, swelling parallel to the width direction (6 mm) was 1.4% and swelling parallel to the thickness direc­tion (2.3 mm) was 5.9%. The overall dimensional effects were related to the effect of (measured) gross changes in the dimension of fiber bundles noting that gaps were evident on the surface of the bend bars. Figure 11 shows an example of the top surface of an FMI-222 composite irradiated in the present work to 980 °C, 2.4 dpa. This composite underwent very low swelling. By inspection of the figure the contraction of the fiber tows below the free surface of the sample is evident. However, there is evidence from this micro­graph that some of the fibers (particularly at the tow edge) have not withdrawn into the sample. This is evidence of shear within the fiber bundle as opposed to the tow-matrix interface. This observation is evi­dence of the large stresses that must be building in the composite under irradiation. The fact that the bundles are not failing at the tow-matrix interface also sup­ports the previous finding that, at least in the initial period of gross dimensional change, the load-carrying capacity of the composite has not been degraded. In fact, previous measurement of FMI-222 irradiated to a dose of ^7.7 x 1025nm~2 (E> 0.1 MeV) at 800 °C described a 54% increase in strength.16

Figure 12 shows an scanning electron microscopy (SEM) image comparing the 2 dpa surface of the FMI-222 composite of Figure 11 with cylindrical samples of the same size, also irradiated near 1000 ° C, though at progressively higher doses. Clearly the dimensional instability continues with dose leading to gross changes in the composite.