Zirconium alloys, widely used as the cladding material for fuel rods in pressurized-waterreactors,
undergo oxidation during service. The initial oxidation kinetics is very similar in
form to that of a classically passivating layer. However, as the oxide layer reaches a certain
critical thickness, a sudden increase in the rate of weight-gain is observed while the oxide
layer remains adhered to the substrate. This process repeats itself with an approximately
regular period in time. Microscopy images reveal a co-related periodicity in the oxide’s
microstructure as well. The acceleration in the oxidation kinetics associated with these
transitions leads to an increase in the amount of hydrogen entering the Zircaloy, limiting
the fuel burn-up in the reactors by causing hydrogen embrittlement. The aim of this thesis,
therefore, is to develop physics-based-models to understand the mechanisms which govern
this anomalous behavior. Specics of the three major parts of this work are outlined below.
Mechanistic model for oxide growth stresses: The stresses within the oxide layer
have been postulated to play an important role in aecting its protectiveness, for example, by
allowing crack formation. Therefore, a mechanistic model for the oxide stresses is presented,
showing that the oxide deforms primarily by dislocation glide for T < 900 K, while the creep
of the substrate only becomes signicant at higher temperatures. Model predictions also
suggest that the transitions in the oxidation kinetics cannot be attributed to a macroscopic
fracture of the oxide. Hence, a possible indirect in uence of the oxide stresses via a phase
transformation is studied next.