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-