Accident Tolerant Fuels

Cold spray coatings for accident-tolerant fuel cladding

Zirconium-alloy (Zr-alloy) fuel claddings have historically been used in Light Water Reactors (LWRs) for decades. These alloys perform extremely well in normal reactor operating conditions and survive complete in-reactor fuel cycles with minimal oxidation or degradation. However, if a reactor experiences a beyond-design-basis accident, such as loss of coolant to the core, Zr-alloys undergo a highly exothermic reaction with steam resulting in severe oxidation and associated hydrogen generation in the core. Therefore, this group has been developing several accident-tolerant fuel (ATF) coatings for Zr-alloy fuel cladding that are designed to withstand these severe accident scenarios.

Coatings are produced using cold spray, a solid-state deposition method that produces highly dense and adhered coatings with no oxidation of the powder during deposition. This group has produced several single-layer (MAX-phase Ti2AlC, pure Cr) and dual-layer (FeCrAl/Mo) ceramic and metallic coatings, with more advanced coatings under development. We are able to produce very thin (<30 µm) and continuous coatings that are oxidation resistant, cost-effective, and replicate the surface finish of conventional Zr-alloy cladding after post-processing.

SEM images of (a) spherical, gas-atomized Cr powder, (b) cross-section of a polished cold spray Cr coating on Zr, and (c) cross-section of a polished cold spray Cr coating subjected to 1300 °C oxidation in steam for 30 minutes, showing continuous Cr-oxide layer on the surface and Cr-Zr interdiffusion at the coating/substrate interface.

Research and development includes characterizing feedstock powders and cold sprayed coatings using advanced characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Coatings are subjected to a variety of testing in intense environments, such as high pressure water and steam autoclaves (1500-3000 psi), high temperature air and steam oxidation (1000-1500 °C), and neutron irradiation. This work is featured in several publications and three international patents have been filed. Additionally, recognition has been given from our collaborator, Westinghouse, for substantial contribution to their EnCore ® Fuel program, which has successfully produced full-length Cr-coated Zr-alloy lead test rods (LTR) scheduled for insertion in a commercial reactor in early 2019.

Photographs of uncoated cladding tube sections (left images in photo) and as-sprayed Cr coating section (right images in photo): (a) before testing, (b) after 3 minute exposure, (c) after 10 minute exposure, (d) after 20 minute exposure. Sections were exposed to a 1200 °C air environment. While the uncoated Zr-alloy cladding section experienced significant degradation, the cold sprayed Cr coating retained its integrity after the aggressive thermal cycling and exposure.

Future Work

Research and development of advanced dual-layer coatings
TEM of feedstock powders and coatings to further understand coating formation
In-depth observation of coating/substrate interface after high temperature testing
Quantification of coating consumption after high temperature testing
Finite element modeling of cold spray deposition process