## 4 Discussion
### Influence of Alloy Elements  
A comparison of loop formation in Zircaloy-2 and pure Zr clarifies the role of alloying and impurity elements in defect nucleation and growth. At 773 K, the higher DY in Zircaloy-2 as compared to pure Zr, starting from the low dose, suggests that there has been more defect clustering and nucleation in Zircaloy-2, perhaps due to the presence of a larger number of nucleation sites in the form of solute atoms/clusters. Once a defect cluster nucleates in Zircaloy-2, its growth rate is much smaller than in pure Zr. This suggests that dislocation loops are receiving a lower flux of point defects in Zircaloy-2, leading to much finer dislocation loops.  
It is well understood that atoms of alloying elements act as preferential sites for loop nucleation. A larger number of solute alloying elements in Zircaloy-2 provide more sites for defect nucleation resulting in a higher DY than in pure Zr. There is a significant amount of oxygen in Zircaloy-2 (1000 wppm) as shown by the STEM-EDS analysis presented in Figure 2. Oxygen is present in the alpha phase (uniform distribution), while the secondary phase precipitates are rich in oxygen as well. [PERSON] et al. [PERSON] et al. (2008) have shown that the presence of oxygen in Zirconium can decrease loop growth by increasing the vacancy migration energies due to segregation of oxygen to the dislocation loops. However, there is a similar amount of O present in \"pure\" Zr discussed in [PERSON] et al. (2008), approximately 900 wppm.  
An increase in defect density and a decrease in loop size by the addition of substitutional solutes in Zr has been reported, e.g., due to the presence of Nb by [PERSON] and [PERSON] [PERSON] and [PERSON] (1998). More relevant to the present study, loop growth rate is decreased significantly due to the presence of Sn, attributed to the change of the point defect mobility and to enhanced vacancy-interstitial recombination by [PERSON] (2000). Indeed, it has been reported that Sn is a very important element in zirconium, affecting radiation damage significantly: [PERSON] suggests that Sn at low concentrations (0.1-0.2%) has little influence, whereas the high-Sn (1.5%) alloys show the strong effects of Sn on radiation damage by increasing vacancy trapping and recombination possibilities ([PERSON] et al., 2011). Looking at the chemistry of Zircaloy-2, the only major alloying elements present in the alpha solid solution are Sn and O, whereas other elements are present in the intermetallic precipitates. Compared to the pure Zr composition, it is reasonable to assume that Sn is the most important alloying element causing the differences observed between the defect nucleation in pure Zr and Zircaloy-2 at lower doses where irradiation induced dissolution of Fe and Cr from the intermetallics will be negligible. At higher doses, both Fe and Cr diffuse from the intermetallic precipitates into the alpha Zr matrix through an interstitial diffusion mechanism at similar diffusion rates and can alter the microstructural evolution mechanisms significantly ([PERSON] and [PERSON], 1998; [PERSON] et al., 2000).  
It should be noted that the influence of alloy elements on defect nucleation also varies with temperature. Compared to irradiation at 573 K, the alloy elements appear to be more available to offer nucleation sites for defects at higher temperatures, which might be related to the enhanced mutual trapping between those alloy atoms and point defects at elevated temperatures.