## 2 Materials and Methods  
The pure zirconium used is the same as the previous paper [15]; is is 99.8% pure with Hf, Sn, Ni, Fe, C, and O present as impurities. The microstructure of pure zirconium is composed of near equiaxed grains of \(\sim\)5 \(\upmu\)m diameter. The zirconium alloy used is Zircaloy-2 with a grain size of \(>\)5 \(\upmu\)m (many with an elongated shape shown in Figure 1), which was mechanically ground to thin foils and subsequently electropolished using a solution of 5% HCl\({}_{4}\) in methanol at \(-\)40 \({}^{\circ}\)C. Optical micrographs, TEM (Company, FEI, Hillsboro, OR, USA) analysis, and scanning transmission electron microscopy associated with energy-dispersive X-ray spectroscopy (STEM-EDS) (FEI, Hillsboro, OR, USA) mapping done on an FEI Technai Osiris (FEI, Hillsboro, OR, USA) are presented in Figure 1, which shows the distribution of alloying elements in the secondary phase precipitates present in Zircaloy-2 used in the irradiation experiments. Alloying elements Zircaloy-2 contains 1.5 wt % Sn, 0.15% Fe, 0.1% Cr, and 0.05% Ni and 1000 wppm oxygen. From EDS element maps (Cr, Fe, Ni, O, Sn, Zr, and Cr + Ni) in Figure 1, Sn mostly remains in solid solution in the \(\alpha\)-phase, O in the \(\alpha\)-phase, and slightly enriched in intermetallics, whereas the other alloying elements are mostly contained in intermetallic precipitates: Zr(Fe,Cr)\({}_{2}\) and Zr\({}_{2}\)(Fe,Ni). Figure 1 indicates these precipitates mostly with irregular shape, in sizes from 100 nm to \(\sim\)1 \(\upmu\)m. Similarly, the detailed analysis on intermetallic precipitates in Figure 2 shows the presence of two types of intermetallics present in the microstructure: Zr(Fe,Cr)\({}_{2}\) with hexagonal close-packed (HCP) structure and Zr\({}_{2}\)(Fe,Ni) with body-centred tetragonal (BCT) structure, as confirmed by selected-aperture electron diffraction pattern (SAEDP).  
In situ irradiation experiments were done at the intermediate voltage electron microscope coupled with tandem accelerator (IVEM-Tandem) at Argonne National Laboratory. The 1 MeV Kr ion irradiation experiments were done at 573 and 773 K from 0.007 to 0.8 dpa. Details of damage calculations, experimental procedures, and data collection methods can be reviewed elsewhere [15]. The nucleation and clustering behaviour of vacancies and interstitials by cascade collapse can be studied by calculating the parameter Defect Yield (DY) first defined by [PERSON] [23; 24],  
\[\mathrm{DY}=\frac{N_{L}}{N_{c}} \tag{1}\]  
where \(N_{L}\) is the number of loops/unit area and \(N_{c}\) is the number of cascades/unit area. In the heavy ion irradiations commonly used for this type of investigation, \(N_{c}\) is taken as the ion dose per unit area.  
Figure 1: (**a**) Optical micrograph on Zircaloy-2, showing elongated grains. Inset shows the transmission electron microscopy (TEM) bright field micrograph of the grain structure. (**b**) Low magnification scanning transmission electron microscopy associated with energy-dispersive X-ray spectroscopy STEM-EDS mapping on Zircaloy-2 showing the distribution of intermetallic precipitates. The scale bar of those maps is 3 μm. The brightness of colors in element map indicates their concentration. The higher concentration shows in brighter contrast, and the darker area means depletion of this element.  
Figure 2: STEM-EDS elemental mapping on intermetallic precipitates present in Zircaloy-2. Corresponding selected-aperture electron diffraction pattern (SAEDP) and TEM bright field micrographs are given. Each micrograph of precipitates is labelled for it chemistry and shows crystallinity from SAEDP. The diffraction patterns well indicated that Zr(Fe,Cr)\({}_{2}\) particles are in hexagonal close-packed (HCP) structure (HCP) structure and Zr\({}_{2}\)(Fe,Ni) particles are in body-centred tetragonal (BCT) structure. The scale bar in element maps is 500 nm.