Evaluation of Equivalent Cladding Reacted parameters of Cr-coated claddings oxidized in steam at 1200 °C in relation with oxygen diffusion/partitioning and post-quench ductility

Abstract Cr-coated M5Framatome1 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 M5Framatome fuel claddings oxidized in steam at 1200 °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 M5Framatome 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 M5Framatome cladding brings significant additional time-at-temperature before full embrittlement of the cladding after one-sided oxidation at 1200 °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 M5Framatome 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 μm-thick Cr-coated cladding tested herein.