Chapter 4 - Accident-tolerant fuels cladding concept: coatings for zirconium alloys

Abstract

The use of coatings to enhance the performance of zirconium alloys in nuclear reactors is considered the most logical evolutionary and near-term solution in the realm of accident-tolerant fuels (ATFs). Because the coating will provide protection by adding a layer in the order of 10-µm thickness to the current zirconium alloy wall, it is deemed that the coating solution may have the fewest licensing barriers for reactor use. It has been demonstrated that chromium coating by a physical vapor deposition process may be the most likely alternative. Chromium will not be a new element in the reactor core because traditional legacy alloys such as austenitic stainless steel type 304 and nickel-based alloy 600 rely on chromium for their corrosion resistance in the reactor environment. Chromium coating will not negatively impact the mechanical behavior of the zirconium alloy cladding and the neutronic balance impact will be minimal. As for the other ATF concepts, the coating of zirconium alloy tubes needs to perform well under normal operation condition and under accident conditions. Many types of coatings were rated for performance in the normal operation and accident conditions and the highest ranking was for chromium-based coatings. The use of chromium coating is meant to provide short-term additional protection to the zirconium alloy cladding in the temperature range up to 1100°C. The top temperature for stability of Cr2O3 in superheated steam is approximately 1200°C. The presence of Cr2O3 on the surface of the cladding coating prevents not only oxygen access but also hydrogen diffusion into the cladding zirconium substrate. Full-length zirconium alloy cladding tubing can be coated in an efficient manner in an industrial production setting. Chromium-coated zirconium alloy fuel rods are currently being evaluated in BWR and PWR commercial nuclear power plant environments. Supporting comprehensive reports for the chromium-coated concepts have been delivered to the Nuclear Regulatory Commission to increase their readiness to receive licensing topical reports from fuel vendors. It is expected that chromium-coated zirconium alloy cladding will be the first ATF concept to receive regulatory approval.