A typical plot of void swelling as a function of temperature in an irradiated 304 austenitic stainless steel is shown in Figure 6.15. It is clear from this figure that the peak in swelling happens at the intermediate temperature. Lower defect mobility limits void growth at lower temperatures, whereas at higher tempera­tures the irradiation-produced defect concentration reaches the value close to thermal equilibrium value and loses the level of vacancy supersaturation (i. e., a lack of excess vacancy concentration) that is required to sustain void growth. These two opposing effects give the shape of the swelling-temperature curve as shown in Figure 6.15.

With increasing temperature, the void number density falls and the average size increases, which is a typical behavior for a process that is dominated by nucleation at lower temperatures (i. e., the void growth is slow) and by growth at higher

temperatures where the driving force for void growth is small. Figure 6.16 shows the void distribution function as a function of void diameter at different irradiation temperatures in a 316-type stainless steel irradiated to a fluence of 6 x 1022 ncm~2. The void size distribution function is narrow and average void size small at low temperatures. However, with increasing temperature, the distribution becomes wider and the average void size also increases. This occurs due to the enhanced mobility of point defects at elevated temperatures.