LOCA; Secondary hydriding; PWR nuclear fuel cladding; Multiscale quantification of oxygen and hydrogen concentrations; Neutron radiography and tomography; EPMA; mu-ERDA;SIMULATED LOCA CONDITIONS; LOW-TIN ZIRCALOY-4; OXYGEN DIFFUSION; STEAM OXIDATION; PHASE-TRANSFORMATION; HYDROGEN; BEHAVIOR; ALLOYS; SPECTROMETRY; QUENCH-L0

This paper gives an overview of a multi-scale experimental study of the secondary hydriding phenomena that can occur in nuclear fuel cladding materials exposed to steam at high temperature (HT) after having burst (loss-of-coolant accident conditions). By coupling information from several facilities, including neutron radiography/tomography, electron probe micro analysis, micro elastic recoil detection analysis and micro laser induced breakdown spectroscopy, it was possible to map quantitatively, at different scales, the distribution of oxygen and hydrogen within M5 (TM)(1) clad segments having experienced ballooning and burst at HT followed by steam oxidation at 1100 and 1200 degrees C and final direct water quenching down to room temperature. The results were very reproducible and it was confirmed that internal oxidation and secondary hydriding at HT of a cladding after burst can lead to strong axial and azimuthal gradients of hydrogen and oxygen concentrations, reaching 3000-4000 wt ppm and 1.0-1.2 wt% respectively within the beta phase layer for the investigated conditions. Consistent with thermodynamic and kinetics considerations, oxygen diffusion into the prior-beta layer was enhanced in the regions highly enriched in hydrogen, where the alpha(O) phase layer is thinner and the prior-beta layer thicker. Finally the induced post-quenching hardening of the prior-beta layer was mainly related to the local oxygen enrichment. Hardening directly induced by hydrogen was much less significant. (C) 2017 Elsevier B.V. All rights reserved.