July/August_AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 6 1 iTSSe TSS RECENT ADVANCES OF COLD SPRAY TECHNOLOGY IN NUCLEAR ENERGY APPLICATIONS Cold spray technology is a potentially attractive technology for corrosion-resistant coatings, near-net shape manufacturing, and damage mitigation and repair in a variety of applications in the nuclear energy sector. Hwasung Yeom and Kumar Sridharan, FASM* University of Wisconsin, Madison INTRODUCTION Cold spray technology (CST) is a solid-state material deposition process, where feedstock powder particles are accelerated at high velocities onto the surface of a substrate. Above a critical particle velocity, dense adherent coatings form on the substrate due to severe plastic deformation of particles and an adiabatic shear process. The process has been explored for a wide range of industrial applications, for example, for in-situ repair of damaged components and surface coatings to enhance corrosion, fatigue, andwear resistance. The advances of CST in recent years have spurred research and development efforts for its implementation in nuclear energy systems. ACCIDENT TOLERANT FUEL CLADDING DEVELOPMENT Zirconium-alloys (Zr-alloy) have been used for light water reactors (LWRs) as a fuel cladding (tubes containing the urani- um-based fuel) for over five decades due to their high neutron transparency, corrosion resistance in the water coolant, and good mechanical properties under neutron irradiation. How- ever, the Zr-alloys experience unacceptable levels of oxidation and mechanical degradation in high temperature steam envi- ronments as evidenced in the Fukushima-Daiichi nuclear acci- dent in Japan in March 2011. The U.S. Department of Energy (DOE) in conjunction with nuclear industries, universities, and national laboratories embarked on the Accident Tolerant Fuel (ATF) program in 2012 [1] . Coated cladding designs to improve oxidation resistance at high temperatures are regarded as a near-term approach because this does not involve significant engineering changes to the current core design. CST has been explored as one of coating technologies for depositing oxida- tion-resistant coatings on Zr-alloy fuel claddings. Bothmetallic and ceramic coating materials including Cr, FeCrAl alloys with Mo interlayer, and Ti 2 AlC have been deposited on Zr-alloy as ATF coated cladding concepts [2] . The deposition of Cr coatings has been extensively investigated owing to its high melting point (1875°C), its excellent oxidation resistance in high tem- perature water/steam environments, and the high stability of Cr-oxide layer under hydrothermal operating condition. The University of Wisconsin, Madison (UW-Madison), un- der the auspices of the U.S. DOE’s national ATF program, has been developing CST fuel cladding design in collaboration with Westinghouse Electric Company (WEC) since 2014. Opti- mizedmanufacturing recipes were developed by investigating Cr feedstock powder manufactured by various processes (e.g., gas-atomized, electrolytic, and heat-treated powders) and cold spray parameters including the type of propellant gas, its temperature and pressure [3] . An example of Cr cold sprayed tubes manufactured in UW-Madison is shown in Fig. 1a. The surface polished Cr coating exhibits a smooth and bright me- tallic appearance similar to the uncoated Zr-alloy cladding tube. A section of the Zr-alloy tube with uniform Cr coating (shown in Fig. 1b) was exposed to ambient air at 1200 °C for 20 minutes to demonstrate the high-temperature oxidation re- sistance. The Cr coating formed a thin and protective Cr-oxide layer (< 5 µm) while a 100-µm-thick Zr-oxide layer formed on the inner, uncoated surface of the Zr-alloy cladding tube, as shown in Fig. 1c. *Member of ASM International Fig. 1 — (a) Photograph of cold spray Cr coated Zr-alloy cladding tube. Cross-sectional micrograph for (b) as-fabricated coated cladding and (c) oxidized coated cladding at high temperatures (produced by the authors’ group and adopted by Westinghouse for its Accident Tolerant Fuel coated cladding concept) [2] . FEATURE 11