Microstructure evolution of zirconium carbide irradiated by ions
Open Access
- Author:
- Ulmer, Christopher
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 23, 2014
- Committee Members:
- Arthur Thompson Motta, Dissertation Advisor/Co-Advisor
Arthur Thompson Motta, Committee Chair/Co-Chair
Nasim Alem, Committee Member
Kostadin Nikolov Ivanov, Committee Member
Igor Jovanovic, Committee Member
Izabela Szlufarska, Special Member - Keywords:
- nuclear materials
zirconium carbide
ion irradiation
defects
rate theory - Abstract:
- ZrC is a candidate material for use in Generation IV high-temperature, gas-cooled reactor TRISO coated fuel particles, so it is important to understand its behavior under irradiation. The microstructural evolution of ZrC$_x$ under irradiation was studied in situ using the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory. Experiments were performed in which the sample stoichiometry and irradiation temperature were systematically varied. In situ experiments made it possible to continuously follow the microstructure during irradiation using diffraction contrast imaging. Images and diffraction patterns were methodically recorded at chosen dose points. Experiments centered on the irradiation of ZrC$_{0.8}$ and ZrC$_{0.9}$ with 1 MeV Kr ions at temperatures ranging from 20 - 1073 K up to 10 dpa. Initial damage developed as 2 - 4 nm diameter black-dot defects after a threshold dose of approximately 0.1 - 0.5 dpa. As the irradiation temperature increased, the threshold dose for visible defect formation decreased. The density and size of defects increased with additional dose and the density of defects ranged on the order of $10^{22}$ - $10^{23}$ m$^{-3}$ for all experiments. The defect diameter also increased with irradiation temperature, with average defect diameters at 3 dpa ranging from approximately 4 nm at 673 K to 8 nm at 1073 K. No long-range migration of the visible defects or dynamic defect creation and elimination were observed during irradiation, but agglomeration of small defects into loops occurred at 1073 K and resulted in an overall coarsening of the microstructure. The irradiated microstructure was found to not be strongly dependent on the stoichiometry as results for the two stoichiometries studied were nearly identical. No irradiation induced amorphization was observed, even after 5 dpa at 20 K and 10 dpa at 50 K. At the higher temperature (873 K and above), the irradiated microstructure varied with sample thickness and showed a defect-denuded zone in the thin area near the edge. A one-dimensional cluster dynamics rate theory model that only considered the creation and mobility of point defects and their agglomeration into defect clusters was solved and compared with the experimental results. General trends from the simulation results matched the experimental observations: a threshold dose was predicted by the calculation, loop diameter was predicted to increases with dose and temperature, and loop density increased with dose and decreased with temperature, as observed. The spatial distribution showed lower loop size and density near the surface. Additional work is needed to match the experimental results quantitatively for both loop size and density, and the results were found to be sensitive to the chosen temperature.