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This paper presents observations of in situ heating and cooling cycles using synchrotron X-ray diffraction carried out on Zircaloy-4 plates charged to different levels of hydrogen content. The diffraction peak intensities were used to derive the weight fraction of hydrogen in solid solution and the solvi curves upon dissolution and precipitation. The heating and cooling rate did not seem to have a major influence on the solvi. Fast-cooling/dwell experiments showed that the precipitation kinetics are very fast. Finally the evolutions of the lattice strains from individual grain families were studied during dissolution and precipitation. The aim was to understand the role of interphase misfit stresses on the mechanisms of hydride formation and hysteresis between dissolution and precipitation. The results emphasise that changes in dissolved H concentration are responsible for a non-negligible part of the lattice distortion in the matrix, and that the diffraction strains must therefore be analysed with care.

The thermo-mechanical behavior of Zircaloy-4 claddings under simulated post-DNB RIA conditions was investigated. Around twenty experiments were performed in simulated post-DNB conditions, i.e. creep ballooning tests with heating rates greater than 1000 • C/s. Two different levels of pressure of 7 and 11 bar were tested for temperatures of interest ranging from 840 • C to 1020 • C. A complex creep behavior was highlighted in this range of temperature. It appears very well correlated to the phase content present within the material during fast thermal transients. Tests with low thermal transients were also performed and evidence a strong impact of the heating rate on the thermo-mechanical properties of the claddings.

An assessment of the mechanical properties of the highly irradiated fuel claddings under high strain rate has been carried out in the framework of the PROME-TRA program undertaken by the French Institut de Ra-dioprotection et de Sûreté Nucléaire in collaboration with Electricité de France and Commissariat à l'Energie Atom-ique (CEA). Three types of tests, including burst tests, hoop and axial tensile tests, have been performed at CEA-Saclay hot laboratories to determine the cladding tensile properties to use in the SCANAIR code. The prototypicality of each test with regard to the reactivity-initiated accident loading conditions can be addressed and ana-lyzed in terms of strain or stress ratio. The high-strain-rate ductile mechanical properties of irradiated ZIRLO and M5 alloys derived from the PROMETRA program and their comparison to the stress-relieved irradiated Zircaloy-4 are reported. Then, the clad brittle behavior, in particular for highly corroded or spalled Zircaloy-4 cladding, is investigated.