## 4 Discussion  
### Prismatic Loop Formation  
The above presented results show the formation of small prismatic dislocation loops in the form of small defect clusters. The small size and low formation dose of these defect clusters suggest that these loops may be nucleated directly from the cascade collapse in the prism plane. The small white features, which show very weak contrast in the TEM sample, indicate defect cluster formation. The clusters then develop into larger prismatic loops, with a slightly larger size as the dose is increased to 0.016 dpa. Indeed, these loops were reported to form in MD computations in \(\alpha\)-Zirconium before [26], but were not commonly experimentally observed/reported due to the availability and inspection mostly of neutron irradiated Zr in the literature, with most samples at relatively high doses.  
MD simulations have provided a great insight into the formation of prismatic loops as it has been reported that most of the small interstitial clusters produced in the cascade have the form of a dislocation loop with Burgers vector \(\frac{1}{3}\langle 1\bar{1}\bar{2}0\rangle\). Point defect clusters in the prismatic plane always relax to form perfect dislocation loops with \(<\)a\(>\)-type Burgers vector [27]. Small interstitial clusters in  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Cluster Size**} & \multicolumn{2}{c}{**600 K**} & \multicolumn{2}{c}{**800 K**} \\ \cline{2-5}  & **SIA** & **V** & **SIA** & **V** \\ \hline
5–10 & 11 & 14 & 8 & 7 \\
11–20 & 5 & 1 & 5 & 6 \\
21–40 & 4 & 8 & 1 & 1 \\
41–50 & 1 & 2 & 1 & 2 \\ Summary & 21 & 25 & 15 & 16 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Total number of interstitial and vacancy clusters in 20 simulations at 600 K and 800 K.  
Figure 7: Vacancy cluster distribution at 600 K (white column) and 800 K (gray column).  
the form of dislocations and the collapse of 24-vacancy clusters to a prismatic dislocation loop with Burgers vector \(\frac{1}{3}\langle 11\overline{2}0\rangle\) were reported by [PERSON] et al. [PERSON] et al. (1998) and [PERSON] et al. [PERSON] et al. (2000). These small clusters have not been observed previously in the TEM. We see that these small clusters can be resolved and observed in the TEM, if they exist, by using in situ irradiation experiments. The current results provide, to our knowledge, the first experimental confirmation of the formation of these small clusters in Zr. The lower DY observed in the TEM at higher temperature can be directly correlated to lower defect cluster production at higher temperature. At 773 K, a smaller number of both vacancy and SIA clusters are produced after the cascade collapse, as compared to 573 K. At the higher irradiation temperature, vacancies and SIAs can diffuse across larger distances and therefore enhance the recombination of vacancies and SIAs. This also results in the growth of a limited number of clusters, and hence a lower DY and a larger loop size. The similar trends of DY vs. temperature and loop size vs. temperature were also recently found in other materials regardless of their different crystal structures, such as in bcc pure Fe ([PERSON] et al., 2008; [PERSON] et al., 2010; [PERSON] et al., 2011) and pure W ([PERSON] et al., 2011; [PERSON] et al., 2011), and in fcc pure Ni ([PERSON] et al., 2011) and Ni alloys ([PERSON] et al., 2011).