Significant increases in both strength and elastic modulus occur in graphite at dose levels as low as 0.01 dpa.8 This increase continues to high displace­ment levels until volumetric expansion and extensive micro cracking occurs. For graphite, the reversal to property degradation typically occurs at tens of dpa depending on the graphite type and irradiation tem­perature. The increase in modulus is a result of dislo­cation pinning by lattice defects produced by neutron irradiation. The magnitude of the increase is depen­dent on the perfection of the graphite. For most graph­ite types, a maximum modulus increase of 2-2.5 times the nonirradiated value is typical for irradiation tem­peratures less than 300 °C, with the change becoming less pronounced at higher irradiation temperatures. Irradiation-induced increase in strength occurs in a similar fashion as in the elastic modulus.

Several authors17-23 report the effect of neutron irradiation on the elastic modulus of CFC. For example, Sato18 reports an increase of 42% and 30% in modulus following neutron irradiation to 1.2 x 1025nm~2,

E > 0.18 MeV in the temperature range of 750-810 °C for a 2D pitch fiber and PAN fiber composite, respec­tively. Similar to the irradiation-induced increase in strength, the absolute increase and percent increase in elastic modulus is highly dependent on starting material and irradiation condition. The irradiated and nonirradiated mechanical properties of some can­didate ITER PFC materials are shown in Table 4 for ITER relevant temperatures and doses somewhat higher than ITER neutron doses. Specifically, these materials were irradiated at approximately 1000 °C to a dose of about 2 dpa.20 The change in properties is relatively small because of the high irradiation tem­perature and the relatively low dose.

As with elastic modulus, reported data on the effect of irradiation on the strength of CFCs are somewhat sparse.16-22 Snead23 has reported the strength and elastic modulus of the 3D pitch fiber composite FMI-222 for doses higher than expected for ITER, or more consistent with a fusion power reactor. Figure 13 gives the modulus as a function of dose to 32 dpa at 800 ° C, exhibiting a marked increase to at least 10 dpa followed by a degradation by the 32 dpa

Dose (dpa)

value. The same samples, as seen in Figure 14, exhibit more than a 50% increase in strength, which is retained even at the 32 dpa value. This is particularly remarkable given that the composite had undergone significant dimensional change in this dose range.