In the case of zirconium alloys, many authors have studied the postirradiation microstructure by using transmission electron microscopy (TEM). In 1979, an international ‘round robin’ was undertaken consisting of TEM observations of neutron-irradiated recrys­tallized zirconium alloys45 in order to determine the nature of the point-defect clusters. A more recent compilation of observations is given by Griffiths.46 It has been now proved by numerous authors that in zirconium alloys mainly dislocation loops with (a) Burgers vector can be found. Only for high fluence, the (c) component dislocation loops appear. Cavities are observed only in very specific cases.

4.01.1.3.1 (a) Dislocation loops

It is now clearly established by numerous authors45-57 that for commercial neutron-irradiated zirconium alloys (e. g., annealed Zircaloy-2 described in Northwood eta/.45) at temperatures between 250 and 400°C and for irradiation dose lower than 5 x 1025 nm~2, the point-defect clusters that can be observed by TEM (>2 nm) consist of perfect dislocation loops, either of vacancy or interstitial nature, with Burgers vector (a) = 1/3(1120), situated in the prismatic planes with typical diameter from 5 to 20 nm, depending on the irradiation temperature (Figures 3 and 4). These loops are found in very high density, typically between 5 x 1021 and 5 x 1022 m~3 depending on the irradiation temperature (Figure 5). , 1 The three (a) Burgers vectors are equally represented. Thorough studies of neutron damage in zirconium using the high-voltage electron microscope (HVEM) have also been given.53,58,59

The proportion of vacancy loops to interstitial loops depends on the irradiation temperature. Indeed, it is observed that for an irradiation temper­ature of 350 °C approximately 50% of observed loops are vacancy loops, whereas for an irradiation temper­ature of 400 °C, 70% of loops are vacancy loops.45,46 For a low irradiation temperature (below 300 ° C), the majority of loops present in the material are of the interstitial type.

The loop habit plane is close to the prismatic plane, but accurate determination proves that the loops are not pure edge but their habit plane is usually closer to the first-order prismatic plane {10Ї0}. The authors have also observed that for loop diameters lower than 40 nm the loops are circu­lar but for diameters larger than 40 nm the vacancy loops become elliptical with the great axis along the (c) axis, the interstitial loops remaining circular. The (a) loops also appear to be aligned in rows parallel to

the trace of the basal plane.46,50

For an irradiation temperature of 300 °C, no dislocation loop can be observed below a neutron fluence of 3 x 1023 nm~2 in the case of annealed Zy-2 (Zircaloy-2) irradiated at 300 °C.51 However, from this fluence, the loop density increases rapidly with increasing fluence but saturates at a density of 3 x 10 m, from a relatively low fluence of approx­imately 1 x 1024 nm~2 (Figure 5). The loop density saturation has been confirmed by X-ray analysis.60 The loop size exhibits a parabolic increase with fluence but no clear saturation in the evolution of the loop size is seen even after a fluence of 1 x 1026 n m~2.51,67

Increasing the irradiation temperature leads to a decrease in the loop density and to an increase of the loop size.45,55,61 Indeed, it was shown by Northwood etal.45 that neutron irradiation performed at 350 °Cof annealed Zy-2 up to a fluence of 1 x 1025 n m~2 leads to a mean loop diameter between 8 and 10 nm and a loop density between 8 x 1021 and 5 x 1022 m~ ; whereas a neutron irradiation of the same alloy per — formedat400 ° Cup to a fluence of 1 x 1025nm~2 leads to a mean loop diameter between 16 to 23 nm and a loop density between 4 x 1021 and 2 x 1022 m~3.45 Above 500 °C, no irradiation damage is formed.52 The (a) loop microstructure is found to be very sensitive to alloying elements such as oxygen. Indeed, for high — purity zirconium with very low oxygen content, the (a) loops are large and in low density, whereas for commercial zirconium alloys (with oxygen content between 1000 and 1500 ppm) the growth speed of loops is considerably reduced yielding smaller loops

in much higher density.45,55

It was also reported from TEM observations that a particular band contrast of alternative black and white was superimposed on the usual radiation damage nor­mally visible on thin foils of irradiated materials. This phenomenon has been connected to the alignment of the loops in the same direction and is believed to be a thin-foil artifact. It has been named ‘corduroy’ contrast by Bell.62 The commonly accepted explanation of this artefact is based on the local elastic relaxation of the internal stresses in TEM thin foils, in areas where pronounced alignment of (a) loops is present.63