## 2 Materials and Methods
### Image Analysis of Zirconium Hydride Morphology at Volumetric Flaws  
Micrographs of the Zr-2.5%Nb alloy samples in the radial cross-section were taken to examine the hydride reorientation phenomenon that occurred when the samples were heated under constant mechanical stress. The Zr-2.5%Nb alloy's phenomena of hydride reorientation were caused by the requirement that the material's hydrogen concentration be higher than the solid solubility limits at a particular temperature. The hydrides appeared as tiny filaments in the radial-transverse plane with varying orientations. Hydride platelets with their orientation direction positioned at an angle different from the transverse direction (45-90\({}^{\circ}\)) are known as reoriented hydride platelets, as defined in Figure 4a. Examination of the micrographs of the zirconium hydride platelets showed that the hydride continuity coefficient (HCC) metric, a dimensionless measure, provides an estimate of the hydrides' reorientation [15]. The subsequent steps needed to obtain the HCC were as follows:  
1. A rectangular-shaped surface with dimensions of roughly 0.1 mm \(\times\) 1.0 mm was selected in the radial-transverse plane for analysis;
2. Radial projections, \(L_{k}\), of the hydride platelet lengths were constructed on this surface;
3. Together with the total length of the investigated rectangular surface, \(L_{total}\), the projected lengths, \(L_{k}\), were measured;
4. The following relationship was used to compute the HCC: \[HCC=\frac{\sum_{k=1}^{n}L_{k}}{L_{total}}\] (6)An example of HCC computation is presented in Figure 4b. At higher hydrogen concentrations and with a greater number of thermal cycles that induce hydride reorientation, the HCC became easier to calculate and increased significantly. From the Canadian scientific literature [7], the values of the tensile stress at which this reorientation phenomenon occurs are above 110 MPa if the stress state is biaxial or 120-155 MPa for uniaxial traction ([7; 21]). To meet the threshold stress criterion, some thermal cycles under continuous load were performed with the mechanical stress \(\sigma>120\) MPa at the notch tip. The hydride platelets were seen to be clearly orientated in the pressure tube's radial direction as the number of cycles increased. Their lengths extended in this direction as a result, and their HCC values increased. The reorientation of zirconium hydrides resulted in a particular shape in the tests using hydrided specimens manufactured from a CANDU pressure tube that included the V-type notch, as shown in Figure 5a. The hydride platelets were oriented normally, parallel to the direction of tensile stress, but the complex stress field around a tip flaw promoted the formation of some circles from hydride filaments. Starting from the tip flaw in the radial direction, we defined a \"reorientation zone\" (RZ), with a circular form, until its border was tangential to the un-orientated hydrides. From Figure 5, it was also estimated that the radius of the reorientation zone had a depth of approximately 2 times the depth of the V-type flaw. Also, from the finite element analysis, we found that the area with such higher mechanical stresses, approximately 120 MPa, had a depth of approximately 2 times the depth of the defect for normal circumferential stresses to which the CANDU 600 pressure tube wall was subjected. By intercomparison with the metallographic analyses, it was observed that the radius of the reorientation zone could be approximated by 2 times the depth of the defect, which was largely confirmed. The importance of characterizing the morphology of the hydrides in the reorientation zone lies in the fact that the initiation of a crack and its subsequent propagation occurred from the tip of the defect to the radially brittle reoriented platelets, as shown in Figure 5b. Also, when the stress conditions were met (fatigue, thermal cycling, etc. ), crack propagation extended through the reorientation zone, as demonstrated in Figure 5c.