Atomic Scale Degradation of Zirconium Alloys for Nuclear Applications

Doctoral thesis, 2015

Due to their low thermal neutron capture cross-section, zirconium alloys are widely used in the nuclear industry as fuel cladding and for structural components. The lifetime of the fuel assemblies in the reactors is primarily dictated by the ability of the cladding to withstand oxidation and hydrogen pick-up from the coolant water and radiation damage induced by the neutron flux. In order to study the hydrogenation and irradiation damage of zirconium on the atomic scale, atom probe tomography (APT) is utilized. This technique offers some unique virtues for nanometer scale materials analysis, such as equal sensitivity to all elements and near-atomic resolution. However, as APT has rarely been used for hydrogen studies previously, methods for accurate qualitative and quantitative analysis need to be developed. In this thesis, methods to control adsorption of hydrogen onto the APT specimen are explored. Techniques for hydrogen measurement are further developed using deuterium, whereby it is shown that hydrogen enters the alloy through grain boundaries in the oxide scale. A model for mitigation of hydrogen pick-up is proposed, in which oxide grain boundaries are decorated with transition metal alloying elements such as Fe and Ni, which affects the probability of reducing ingressing protons to form inert H2 gas. Zr alloys incur irradiation damage when subjected to the neutron flux in the reactor core, dissolving secondary phase particles and generating dislocation loops that embrittle the material. By studying the microstructure on the atomic scale before and after prolonged in-rector exposure, it is shown how the alloying elements in Zr interact with the irradiation-induced lattice defects.