In the recent years, replacing the current Zr-alloy-based cladding with silicon carbide composites has gained traction in the Department of Energy (DOE) as well as in the industry. The most recent generation of SiC/SiC composites (III), defined by near-stoichiometric chemical composition and crystallinity with reduced oxygen content, has been extensively tested for nuclear applications. They are comprised of a SiC matrix (m) reinforced with long SiC fibers (f) that are woven or braided onto a tubular cladding configuration. The properties of SiC/SiC composites are greatly influenced by the matrix processing methods, fiber architecture and orientation, and interface materials. Oxidation reaction kinetics under steam attack is of critical importance under accident conditions. When exposed to steam, SiC exothermically forms silica, hydrogen and carbon monoxide releasing an energy of 223 kJ/mol, approximately. In general, a competition between a parabolic oxidation process and a linear volatilization process results in paralinear kinetics.
In this work, we capture the oxidation process in a model composite material using a combination of x-ray tomography and mesoscale phase-field simulations. First, the microstructure of the composites is faithfully extracted by x-ray tomography and a full three-dimensional reconstruction is performed. This nondestructive technique allows us to extract all the pertinent microstructural details. Starting with the digitized microstructure, we then simulate the evolution of the pores/pathways accompanying the transport of steam through phase-field modeling. Non-equilibrium chemical thermodynamic models are used to make accurate predictions on the pertinent chemical reactions. The evolution equations (Allen–Cahn and Cahn-Hilliard), which are fully coupled to heat conduction and deformation models, allow us to determine the extent of oxidation processes in the composites under anticipated accident conditions.