Performance and evolution of cold spray Cr-coated optimized ZIRLO™ claddings under simulated loss-of-coolant accident conditions

Abstract

The performance of Cr-coated Optimized ZIRLO™ as accident tolerant fuel cladding material for pressurized water reactors (PWRs) is assessed. The coating oxidation mechanisms, oxide stability, and the transformation of the Cr-coating/Optimized ZIRLO™ interface are among the studied phenomena. For this purpose, samples were exposed at 1200°C in steam for 3 min, 20 min and 40 min. As-fabricated coated claddings, plus specimens tested in autoclave at 415°C for 90 days in simulated PWR water chemistry were employed for comparison. Characterization techniques such as scanning electron microscopy, energy dispersive x-ray spectroscopy, electron backscattered diffraction, and transmission electron microscopy were used to determine the chemistry and crystalline structure of the various phases formed during the different exposures. When exposed to loss-of-coolant accident (LOCA) conditions for 40 min, a layer of Cr2O3 up to 8 µm thick was measured on the outer surface of the Cr-coating. No significant oxidation of the underlaying Optimized ZIRLO™ alloy occurred, and the applied coating appears to be very effective at delaying the cladding degradation under accident conditions. At the coating-substrate interface, a 1–2 µm thick layer of (Cr,Fe)2Zr Laves phase was found. The presence of this phase appears to have no detrimental effects on the coating performance, and it might play a role in slowing down the dissolution of the coating into the substrate. ZrO2 particles were frequently found at grain boundaries in the coating after exposure to LOCA conditions. For longer exposure time, these particles are expected to grow into a ZrO2-network, creating a fast diffusion path for O, and compromising the oxidation protection offered by the coating.