To understand the mechanisms by which the re-solution of Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, the solubility and clustering of Fe and Cr in model binary Zr alloys was investigated using a combination of experimental and modelling techniques -atom probe tomography (APT), x-ray diffraction (XRD), thermoelectric power (TEP) and density functional theory (DFT). Cr occupies both interstitial and substitutional sites in the α-Zr lattice; Fe favours interstitial sites, and a low-symmetry site that was not previously modelled is found to be the most favourable for Fe. Lattice expansion as a function of Fe and Cr content in the α-Zr matrix deviates from Vegard's law and is strongly anisotropic for Fe additions, expanding the c-axis while contracting the a-axis. Matrix content of solutes cannot be reliably estimated from lattice parameter measurements, instead a combination of TEP and APT was employed. Defect clusters form at higher solution concentrations, which induce a smaller lattice strain compared to the dilute defects. In the presence of a Zr vacancy, all two-atom clusters are more soluble than individual point defects and as many as four Fe or three Cr atoms could be accommodated in a single Zr vacancy. The Zr vacancy is critical for the increased apparent solubility of defect clusters; the implications for irradiation induced microstructure changes in Zr alloys are discussed. (P.A. Burr) SPP dissolution [1,3,[20][21][22][23][24][25][26]. It is important to limit hydrogen uptake during reactor operation because hydrogen causes dimensional changes to the cladding [24], reduces its ductility [24] and reduces integrity performance in hypothetical accident scenarios [27,28], and potentially in the storage conditions relevant to spent nuclear fuel [27,29].Recent advanced transmission electron microscopy (TEM) [30] and atom probe tomography (APT) studies [31] have shown that clusters of Fe and Cr form at a and c dislocation loops following the re-solution process. This was previously suggested by TEM investigation [9-11, 17, 18, 32, 33] but not observed directly. It has been suggested that irradiation induced defects may also act as trapping sites for hydrogen, thereby increasing the terminal solid solubility of hydrogen in α-Zr [34,35].The solubility of Fe in Zr -and to a lesser extent also that of Cr in Zr -has also been investigated using atomic scale simulations, but so far, the clustering behaviour of alloying elements has hardly been con-

Commercial zirconium alloys contain second phase particles (SPPs) that are precipitated during processing. These particles not only influence mechanical properties but more crucially also have a profound influence on the corrosion performance. To understand how to control evolution and size distribution of SPPs, it is necessary to know how alloy composition and process variables influence the precipitation kinetics. In this work, a detailed study has been performed of the precipitation kinetics in binary Zr-1 wt % Nb, Zr-2.6 wt % Nb, and ternary variants with added iron and tin. A numerical model has also been developed to predict the precipitation kinetics of β-Nb in niobium containing zirconium alloys. Precipitation has been tracked by synchrotron X-ray diffraction measurement of lattice parameter change in the zirconium matrix as solute is removed into SPPs. The X-ray data has been complemented by thermoelectric power measurements. The combination of these two approaches is shown to be effective in quantifying the overall precipitation kinetics of SPPs. The results confirm previous observations that without prior deformation, precipitation kinetics is very sluggish in the binary Zr–Nb system. Deformation accelerates precipitation, and this effect is much stronger for the 1 wt % Nb alloy than for 2.6 wt % Nb, because the supersaturation is least. Ternary additions also have a profound effect on the overall precipitation kinetics. Iron accelerates the rate of niobium loss from solution, whereas tin additions appear to increase the incubation time for the onset of precipitation of niobium.

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