The life-limiting component of the fuel assembly in water-cooled fission reactors is the Zr cladding. One of the factors limiting the life of the cladding is hydride precipitation. Accurately predicting the performance of the cladding under the complex combination of thermo-mechanical loading in a corrosive and radioactive environment poses a major challenge. Although safety cases have typically deployed models at the component (mm) scale, a mechanistic understanding of the effect of precipitates in Zr-alloys can only come by investigating their precipitation, plastic relaxation and fracture at the micro-scale and below. Hence, this thesis investigated the important role that precipitates—namely Zr hydrides and second phase particles—have on the elastic and on the plastic deformation in the cladding. To do so, I coupled the theoretical modelling of stress-bearing particles and computational simulations using discrete dislocation plasticity, implemented in a
bespoke code that was written during this project. The objective was twofold. First, to propose original modelling studies, such as a discrete dislocation plasticity framework that couples edge and screw dislocations, or simulations of the plastic relaxation caused by the Zr/hydride interfacial stresses. Second, to provide possible mechanisms to explain experimental facts such as the presence
of a “memory effect” in the hydride precipitation, the Fe segregation to second phase particles,
the crack initiation inside hydrides found in strained and hydrided Zr-alloys. In all of these cases, the stress field of the particles proved to be an important ingredient. Every effort was devoted to validating quantitatively the predictions against the available experimental data. Thanks to some underlying approximations, and to the code being optimised for the specific problems at hand, the simulations were substantially faster than ordinary, more general discrete dislocation formulations.
This enabled me to analyse an unusually high number of simulations for each of the individual
studies, providing a statistical aspect that is novel in the field of discrete dislocation plasticity.