Effect of Chromium Content on the On-Cooling Phase Transformations and Induced Prior-βZr Mechanical Hardening and Failure Mode (in Relation to Enhanced Accident-Tolerant Fuel Chromium-Coated Zirconium-Based Cladding Behavior upon and after High-Temperature Transients)

Chromium-coated zirconium-based nuclear fuel claddings are studied within the CEA-Framatome-EDF French nuclear fuel joint program as a short-term “enhanced accident-tolerant fuel” concept. It has already been demonstrated that, in hypothetical accident conditions such as in a loss-of-coolant accident (LOCA), 10–20-µm-thick chromium coating slows down the high-temperature (HT) steam oxidation overall kinetics and improves induced postquenching cladding strength and ductility. However, upon HT steam oxidation of chromium-coated zirconium-based nuclear fuel claddings, chromium diffusion occurs within the βZr metallic substrate, thus contributing to the overall chromium coating consumption kinetics. In the present study, it is shown that, depending on the cooling scenario from the high oxidation temperature applied, the mechanical response of the chromium-enriched prior-βZr layer of chromium-coated zirconium-based alloy is quite different. Among the different results obtained and thanks to preliminary thermodynamic calculations and the study of chromium-doped Zr1Nb(O) model alloys, it is shown that after direct water quenching from a high oxidation temperature (i.e., βZr temperature range), the observed hardening and potential embrittlement at room temperature of the chromium-enriched prior-βZr metallic substrate should be related to a martensitic chromium-supersaturated prior-βZr structure formation, with a linear chromium solid-solution strengthening effect up to 1.5 wt.% chromium. Beyond 2.5 wt.% chromium, a smooth decrease of prior-βZr hardness is observed. Improved chromium-enriched prior-βZr layer ductility has been observed following a more LOCA-prototypical “two-step” cooling scenario (with a final water quenching from 700°C) and has been related to the early precipitation of most of the available chromium as coarse ZrCr2 secondary precipitate phases upon cooling from the prior-βZr temperature range.