[en] Cr-coated M5(Framatome) cladding materials are studied and developed within the CEA-Framatome-EDF French nuclear fuel joint program as Enhanced Accident Tolerant Fuel claddings for Light Water Reactors. The objective of this paper is to bring some insights into the relationship between Equivalent Cladding Reacted (ECR) parameters, oxygen diffusion/partitioning and Post-Quench (PQ) ductility of Cr-coated M5(Framatome) fuel claddings oxidized in steam at 1200 degrees C. The physical meaning of the ECR parameter, evaluated experimentally from the measured Weight Gain (WG) or calculated using time and temperature correlations such as the Baker-Just (BJ) or Cathcart-Pawel (CP) kinetics correlations, is discussed in the light of the benefit brought by Cr coating to oxidation resistance of cladding. As shown in this article, when applied to the Cr-coated M5(Framatome) materials, the 'experimental' ECR derived from WG does not have the same physical meaning than for the uncoated cladding materials. As discussed in the paper, this is fundamentally due to the use of the ECR as a surrogate for retained ductility for uncoated claddings, and to the differences between uncoated and Cr-coated cladding in the high temperature (HT) steam oxidation processes and partitioning of the oxygen between the different layers of the oxidized cladding. It is shown in this article that Cr-coated M5(Framatome) cladding brings significant additional time-at-temperature before full embrittlement of the cladding after one-sided oxidation at 1200 degrees C and quenching, compared to uncoated materials. The oxidation times and associated Baker-Just ECR (BJ-ECR) values, above which the cladding becomes brittle after low temperature quenching, are respectively ten times and three times higher than the ones for the uncoated reference cladding. When analyzing the PQ ductility of the Cr-coated M5(Framatome) cladding using a similar methodology as the one used to derive the ECR criterion for uncoated cladding, the 1-2% ductility limit corresponds to a BJ-ECR of about 50% or higher, for a 12-15 mm-thick Cr-coated cladding tested herein. (authors)

[en] Highlights: • 5–20µm-thick Cr_xC_y coating deposited by DLI-MOCVD onto inner surface of Zircaloy-4 cladding. • Protection against steam oxidation at 1200 °C comparable to outer PVD-deposited 10–15µm-thick Cr coating. • Significant reduction of secondary hydriding after clad ballooning and burst. • Higher mechanical strength of the cladding, upon and after water quenching. 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) Cr_xC_y 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 Cr_xC_y 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 Cr_xC_y 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.