As mentioned earlier, the first wall materials in next — generation machines will receive many tens of dpa. At low doses (<0.01 dpa), there are essentially no mechanical property changes expected in graphite materials (see Chapter 4.10, Radiation Effects in Graphite). However, even at these low doses, thermal conductivity and stored energy are of concern, spe­cially for low irradiation temperatures (<400 °C). For displacement levels >0.01 dpa, significant property changes occur, including strength, elastic modulus, specific heat, coefficient of thermal expansion (CTE), Poisson’s ratio (v), and thermal conductivity. In addi­tion, the dimensional stability under irradiation is important because the induced stresses may be sig­nificant and may need very tight tolerances at the plasma edge. It has been shown in fission neutron experiments that specific heat Cp6 and v7 are not greatly affected by irradiation. Moreover, only mod­erate changes in the CTE occur, but the magnitude and nature of the CTE change is highly dependent on the type of graphite.6,8-10

The irradiation-induced property changes for graphite and composites that have received the most study by the fusion community deal with dimensional stability, strength, elastic modulus, thermal conduc­tivity, and hydrogen retention. A large body of data exists on the thermophysical changes in graphites, coming mainly from graphite-moderated nuclear reac­tor development programs. A smaller body of research
exists on CFCs, mainly from the same source, but with some additional data from fusion research. These data suggest that CFCs have very similar irradiation behav­ior compared to graphite. In Chapter 4.10, Radiation Effects in Graphite, Burchell discusses radiation dam­age mechanisms in graphite, and some of the specific property changes that occur in fission reactor appli­cations. Because they are of special significance to fusion energy, the radiation effects in CFCs in gen­eral and the radiation-induced degradation in thermal conductivity in graphite and CFCs in particular will be focused on in the remainder of this section. How­ever, it is first important to contrast nuclear graphite (essentially a form of purified structural graphite) with that of graphite composites. For the purposes of dis­cussing graphite materials for fusion applications, the term composites is applied specifically to continuous fiber composites, typically woven, and infiltrated with pitch or some other resin that is graphitized to form a highly crystalline graphite matrix. The fibers compris­ing these composites are, as compared with most forms of graphite, highly crystalline and of comparatively high strength, elastic modulus, and thermal conduc­tivity. The fibers themselves are typically either poly­acrylonitrile (PAN) or Pitch derived. In general, one would select the PAN-based fiber, which is some­what less expensive, if the application required higher strength while the Pitch-based fibers would produce a product with superior elastic modulus and thermal conductivity.

As observed in Sections 4.18.2.2 and 4.18.2.3,

the composite materials, due to their typically higher strength and elastic modulus, have a superior performance in terms of thermal stress and thermal shock. Another key advantage of these materials stems from the fact that they tend to fail in a less abrupt manner than seen for graphite or ceramics in general due to the presence of the reinforcing fibers, which bridge evolving crack fronts. This can be seen by casual inspection of Figure 6, which compares the nuclear graphite Poco AXF-5Q(historically used in TFTR and for other nuclear applications) and the FMI-222 balanced weave, 3D CFC. From Figure 6, and by a comparison of the graphite and composite data of Table 2, it is clear that the FMI-222 CFC material has both higher bend strength and higher elastic modulus (greater slope) as compared to this Poco graphite. Moreover, it is clear from Table 2 that other engineering properties of importance, such as strength and thermal conductivity, are superior for the CFC. These superior properties are primarily attributable to the exceptional quality of graphite

fiber. Unlike nuclear graphite, which is on the order of 20% porosity with a relatively imperfect, heavily faulted, inhomogeneous amalgam of filler particles (such as coke) and graphitized binder (such as pitch; see Section 4.10.2 in Chapter 4.10, Radiation Effects in Graphite for a discussion of graphite man­ufacture), graphite fibers, while somewhat different depending on the starting material (PAN, Pitch, Rayon, etc.), are extremely uniform, and highly crys­talline with density that can approach theoretical den­sity. This leads to exceptional properties. For example, the PAN-based T-300 fiber has a tensile strength of 3.66 GPa, slightly higher than the 2.41 GPa strength of the P120 fiber of the FMI-222 composite of Table 2, or more than 40 times that of the Poco AXF-5Q graphite. Similarly, the elastic moduli of T-300 and P-120 fibers are 21 and 75 times the elastic modulus of the Poco AXF-5Qgraphite. In the case of the P-120 fiber, which has been graphitized at a very high temperature, very long, defect-free basal planes oriented along the axis of the fiber result in excep­tional 1D thermal conductivity (640 W m-1 K, twice that of copper). This property is the primary reason for the twofold increase in ambient thermal conduc­tivity of the FMI-222 composite as compared to the Poco AXF-5Qgraphite. Clearly from this example of thermal conductivity, the architecture (fiber weave or loading) will determine the composite properties.

Examples of practical fusion CFCs are the mate­rials chosen for consideration and application by the ITER project. Table 3 provides the nonirradiated thermophysical property data for selected CFCs

of the ITER project. A review of these properties emphasizes the anisotropic nature of the composite system, which is engineered through selection of the fiber type and route to matrix infiltration, fiber architecture, and final heat treatment of the system. All the materials for this application have been

engineered with a preferred thermal conductivity direction (the x direction in the table), and in order to maximize thermal conductivity, the composites will tend to have a higher volume fraction offibers in that direction and the fibers will be ofthe higher conduc­tivity pitch-based type. In the directions normal to

this preferred thermal conductivity direction, for strength, cost, and fabricability reasons PAN-based fibers are typically chosen. The composite INOX Sepcarb NS31 underwent a final processing step of 10 ± 2% liquid silicon infiltration. This silicon reacted with carbon-producing SiC, which is thought to mitigate chemical erosion and tritium retention while enhancing oxidation resistance.

Also observed from Figure 6 is the clear differ­ence in the shape of the load-displacement curves for the two materials. Clearly, the composite material has significant nonelastic behavior, which is attributed to the progressive load transfer from the composite to the high-strength fiber as the matrix becomes exten­sively microcracked. This contrasts with the graphite material, which undergoes abrupt failure when the load exceeds some critical stress adequate to propa­gate a crack through the test article. This added toughness of the composite is another key attribute to the systems that make it particularly attractive for fusion applications where disruption (shock) loading tends to produce interconnected cracking in materi­als leading to loss of material mass.