At the time of the thorough review by Northwood,51 no (c) component loops had been observed yet. The ‘round robin’ work45 also established that up to an irradiation fluence of 1 x 1025 n m~2 no (c) component dislocation loop is observed. As highly irradiated Zircaloy samples became available, for fluence higher than 5 x 1025 nm~2, evidence of (c) component loops arose.46’54’70-73’189 The (c) component loops have been analyzed as being faulted and of the vacancy type. They are located in the basal plane with a Burgers vector
1 /6(2023) having a component parallel to the (c) axis (Figure 6). The (c) component loops are much larger than the (a) loops but their density is much lower. For instance, for recrystallized Zy-2 and Zy-4 irradiated at 300 °C, after5.4 x 1025nm~2, (c) component loops are found with a diameter of 120 nm and with a density between 3 and 6 x 1020 m~ .

Whatever the irradiation conditions, these (c) component loops are always present in conjunction with more numerous and finer (a) loops. The (c) component loops can therefore only be observed edge-on by TEM by using the g = 0002 diffraction vector, which leads to invisible (a) type defects. The (c) loops thus appear as straight-line segments.

There is considerable evidence to show that their formation is dependent on the purity of the zirconium used (Figure б).46,74-76,190 It is also observed that at the beginning of their formation, these dislocation loops appear to be located close to the intermetallic precipi­tates present in the Zircaloy samples46,76 (Figure 7). By using an HVEM on iron-doped samples, it has been possible to prove that iron enhances the nucle — ation of the (c) loops, the loop density increasing as a function ofthe iron content. Moreover, iron was found to have segregated in the plane of the loops.76