Our latest paper on alloying additions in Zr is now available as a pre-print on arXiv:

• P.A. Burr, M.R. Wenman, B. Gault, M.P. Moody, M.Ivermark, M.J.D. Rushton, M. Preuss, L. Edwards and R.W. Grimes, “From solid solution to cluster formation of Fe and Cr in α-Zr”, arXiv preprint (2015). PDF

By combining ab-initio simulations with advanced experimental techniques we investigate the microstructural change of Zr alloys under irradiation Zr alloys are used as nuclear fuel cladding, and their integrity is crucial for the safe production of nuclear power. Of particular concern is the increase in corrosion that is observed when Fe and Cr particles (alloys additions) are dissolved. Dissolution of these particles is related to neutron irradiation, but the mechanisms by which this occurs is still unknown.

One of the main findings of this paper is that Fe and Cr are more soluble in a defective Zr matrix (e.g. one that has been irradiated) compared to a pristine Zr matrix, and that by forming nano-clusters their solubility is increased further. We propose that this increase in solubility, combined with the amorphisation of the particles due to irradiation, is the main driving force for particle dissolution, and, consequently, for the increase in corrosion rate.

Other findings include a remarkably anisotropic lattice expansion associated to the incorporation of Fe, the ability of Fe and Cr to occupy both interstitial and substitutional sites in the Zr lattice with marginal energy difference, and the fact the preference for one site over the other is strongly sensitive to applied strain and/or stress. Read the preprint here.

Abstract

To understand the mechanisms by which Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, a combination of experimental (atom probe tomography, x-ray diffraction and thermoelectric power measurements) and modelling (density functional theory) techniques are employed to investigate the non-equilibrium solubility and clustering of Fe and Cr in binary Zr alloys. 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 alloying concentration (in the dilute regime) is strongly anisotropic for Fe additions, expanding the c-axis while contracting the a-axis. Defect clusters are observed at higher solution concentrations, which induce a smaller amount of 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 solubility of defect clusters; the implications for irradiation induced microstructure changes in Zr alloys are discussed.