Date of Award
Doctor of Philosophy
Brian D. Wirth
Brian D. Wirth, Steven J. Zinkle, William J. Weber, Richard W. Smith, Jesse J. Carter
One of the issues concerning the long-term lifespan of Zr cladding tubes is an axial expansion and radial contraction that occurs in response to neutron irradiation. This volume-conservative response in the absence of an applied stress has been termed irradiation growth, a consequence of both the inherent anisotropy of alpha-Zr hexagonal close-packed crystal structure, and crystalline texture in tube fabrication. Irradiation growth strains generally saturate at low doses, but suddenly accelerate after an incubation dose. This growth breakaway has been correlated with the nucleation of faulted vacancy loops on basal planes (c-loops); at lower doses, the irradiated Zr microstructure is characterized by the co-existence of vacancy and interstitial dislocation loops on prismatic planes (a-loops). The goal of this dissertation is split into two categories: 1) to elucidate the mechanisms governing microstructure evolution and develop a computational database to describe such mechanisms; and 2) to incorporate this database into a mechanistically-based cluster dynamics (CD) model capable of describing interstitial and vacancy a-loop co-existence in addition to c-loop nucleation and growth.
This goal requires a computational multi-scale approach that bridges several orders of magnitude in length and time scales. Lower-length scale techniques such as density functional theory (DFT) and molecular dynamics (MD) have been used to simulate interactions at the atomic scale and provide essential parameters, including: 1) Interaction energies of solute and impurities with stacking faults and c-loop precursors; 2) defect production rates from displacement cascades; 3) preferred defect cluster configurations and mobilities; 4) defect binding energies; 5) dislocation loop stress states; and 6) defect capture radii. With these key physics incorporated into a CD model, it was found that an inherent bias exists between vacancy and interstitial a-loops for the capture of same-type defects. The resultant interaction rates drive the simultaneous growth of these loops in the microstructure. The growth of c-loops, on the other hand, is driven by the anisotropy difference of defect cluster diffusion, rather than that of point defects. The work presented in this dissertation marks the first time that these phenomenon have been successfully modeled in a purely mechanistic fashion, and highlights the importance of scale-bridging computational approaches to solve current and future issues of materials performance in extreme nuclear environments.
March-Rico, Jose Francisco, "Scale-Bridging Computational Modeling of Irradiation Effects in Alpha-Zirconium and its Alloys. " PhD diss., University of Tennessee, 2022.