Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ZrO 2. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ZrO 2 while it binds strongly to oxygen vacancies in tetragonal ZrO 2. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3+ is the favourable oxidation state but with between a third and half of the iron, still in the metallic Fe 0 state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pickup of zirconium cladding are discussed.

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The linear prolongation flux update scheme is extended to both regular CMFD acceleration, as well as partial CMFD acceleration in 2D multi energy group Monte Carlo k-eigenvalue neutron transport problems. The acceleration performance of these CMFD variants were investigated in simple 2D slab geometries, first with a monoenergetic case and then with a three group problem on the same geometry based on the monoenergetic cross sections. Flux convergence was determined via an on-the-fly convergence diagnostics developed by Ueki and Brown. It is found that on top of providing better acceleration in general, the linear prolongation scheme is also able to correct for instabilities in the CMFD scheme. Overall, the lp-pCMFD scheme employing a maximum history length is found to have the best performance across the cases presented.

Atomic scale simulations were used to predict defect formation in tetragonal ZrO 2 doped with Cu and Sb. Both dopants form strong associations with oxygen vacancies impeding oxygen progression through the oxide. Sb suppresses the free oxygen vacancy population though Cu increases the concentration. Thus, while addition of Sb is predicted to be beneficial against corrosion, Cu will show a more complex behaviour. Previous simulations showed that Ni 0 promotes molecular hydrogen dissociation. Neither Cu nor Sb exhibit this behaviour despite Cu + having the same electronic configuration as Ni 0 . Both Cu and Sb show a favourable response to applied local space charges.

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