DLI-MOCVD CrxCy coating to prevent Zr-based cladding from inner oxidation and secondary hydriding upon LOCA conditions

Abstract

Zirconium-based claddings with an outer chromium coating resistant to corrosion are studied and developed as an evolutionary Enhanced Accident Tolerant Fuel (E-ATF) concept for light water reactors. However, in hypothetical LOss-of-Coolant-Accident (LOCA) conditions, following clad ballooning and burst, the outer coating does not allow to protect the inner surface of the cladding from High Temperature (HT) steam oxidation and associated secondary hydriding due to steam starvation occurring within the gap between the clad inner surface and the nuclear fuel pellets.To address this issue, DLI-MOCVD (Direct Liquid Injection of Metal-Organic precursors - Chemical Vapor Deposition) CrxCy coatings have been developed and successfully deposited onto the inner surface of Zr-based cladding tube prototypes. Then, preliminary two-sided oxidation tests have shown that such inner coating is able to increase the resistance to oxidation at HT of the inner clad surface.The present study aimed at performing new steam oxidation tests at 1200 °C on Zircaloy-4 clad prototypes with a 5–20µm-thick CrxCy inner coating, in conditions more representative of LOCA, after a first internal pressure-induced burst step. Additionally, complementary two-sided steam oxidation tests have been carried out up to 1 h at 1200 °C, on short inner and/or outer-coated clad segments. Finally, Post-Quench (PQ) Ring Compression Tests (RCTs), fractographic analysis and deep metallurgical investigations including neutron-tomography have been performed to get more insights into the PQ behavior of the inner-coated clad.Among other results, it is shown that the inner CrxCy coating makes it possible to reduce significantly the oxidation and the associated secondary hydriding of the clad inner surface, after ballooning and burst. After at least 600 s under steam at 1200 °C, the reference uncoated clad fails upon final water quenching while the inner-coated prototype keeps its integrity. PQ RCTs showed a higher strength of the inner-coated material, related to lower oxygen and hydrogen uptakes of the substrate.