Mechanical Properties and Characterization of Coated SiC Fuel Cladding

SiC fiber-reinforced SiC matrix (SiC_f-SiC) composites continue to undergo process modifications to address remaining concerns for Light Water Reactor (LWR) fuel cladding. The critical concerns include the radiolysis-assisted hydrothermal corrosion and the loss of hermeticity during normal operation. Coating technologies are being developed to provide a solution to both problems with no or minimum modification to the SiC_f-SiC clad structures. Several barrier coating options have been considered and examined for these purposes. A comprehensive screening program is being developed to determine their feasibility in LWR environments before the coated fuel claddings are further advanced toward the ideal 'drop-in' replacements for the aging LWR fleet. There have been several coating technologies proposed. All are based on chromium and zirconium interacting with the coolant because they have a proven performance history. However, it is difficult to deposit these elements in reduced form (Zr⁰, Cr⁰) due to the high enthalpy associated with the stable oxide. The Cr chemical deposition techniques use electrochemical 'hard chrome' reduction of Cr^3+/6+ protected by chelating ligands in solution. The collaborative partner for this endeavor is NEO Knoxville, LLC, and two types of hard chrome systems (trivalent and hard chrome) are being investigated. There are also two feasible vapor techniques previously demonstrated: commercial physical vapor deposition (PVD) methods and an experimental, proprietary vacuum plasma spray (VPS) by Plasma Processes, LLC. Both of these use Zr⁰ and Cr⁰ in high vacuum systems. In total, eight coatings and four bond coats are being evaluated. In mid-2016, these were inserted into a reactor and a characterization program is underway. (authors)