The damage accumulation is independent of dose rate at very low temperatures, where point defect migration does not occur. However, at elevated temperatures (above recovery Stage I) the damage rate can have a significant influence on the damage accumulation. Simple elevated temperature kinetic models for defect accumulation72,142-144 predict a transition from linear to square root dependence on the irradiation fluence when the radiation-induced defect cluster density becomes comparable to the density of preexisting point defect sinks such as line dislocations, precipitates, and grain boundaries. Similar square root flux depen­dence is predicted from more comprehensive kinetic rate theory models6,70,71,145 for irradiation tempera­tures between recovery Stage II and IV. Electron microscopy analyses of electron5 and neutron146 irra­diation experiments performed above recovery Stage I have reported defect cluster densities that exhibit square root dependence on irradiation flux or fluence.

Figure 18 summarizes the square root dose rate dependence for dislocation loop densities at inter­mediate temperatures in several electron-irradiated pure metals.5

Similarly, the predicted critical dose to achieve amorphization is independent of dose rate below

recovery Stage I and depends on the inverse square root of dose rate for temperatures above recovery Stage I.147 Experimental studies have confirmed that the threshold dose to achieve amorphization in ion-irradiated SiC is nearly independent of dose rate below ~-350 K (corresponding to recovery Stage I) and approaches an inverse square root flux dependence for irradiation temperatures above 380 K, as shown in Figure 19.148

In the void swelling149-151 and high temperature helium embrittlement119,152,153 regimes, damage rate effects are very important considerations due to the competition between defect production and ther­mal annealing processes. Experimental studies using ion irradiation (^10-3 dpas-1) and neutron irradia­tion (^10-6 dpa s-1) damage rates have observed that the peak void swelling regime is typically shifted to higher temperatures by about 100-150 °C for the high-dose rate irradiations compared to test reactor neutron irradiation conditions.114,154-158 Similarly, the minimum and maximum temperature for measureable void swelling increase with increasing dose rate. For example, recent low dose rate neutron irradiation studies111-113 performed near 10-9-10-8dpas-1 have observed void swelling in austenitic stainless steel at temperatures as low as 280-300 °C, which is signifi­cantly lower than the ^400 ° C lower limit for void swelling observed during fission reactor irradiations near 10-6dpas-1 (cf. Figure 12).