The present paper is focused on the microstructural stability of the Framatome-ANP M5™ (Zr-1%NbO) alloy. Thin foils were cut from M5 cladding tubes irradiated from 1 to 6 annual cycles in EDF PWRs, and examined by analytical transmission electron microscopy (TEM). The results show that within the wide range of irradiation conditions investigated, the initial β-Nb particles undergo no amorphisation and only minor evolutions in size distribution, number density, and composition. Finally, very few ⟨c⟩ component basal loops were detected, so that no sign of accelerated growth regime has to be feared for the fuel burn-ups currently considered (∼70 GWd/t) for PWRs.

In the second part of this paper, other industrial or experimental alloys (Zy-4, M4, Zr-2.5%Nb, ZrNbSnFe) are considered to improve our knowledge about the irradiation-induced evolution of β-Nb and other minor SPP. Thin foils were cut from specimens irradiated in PWRs or MTRs (OSIRIS, SILOE) and also examined by transmission electron microscopy. In addition, Small Angle Neutron Scattering (SANS) was used to provide quantitative and statistical information on the radiation-enhanced precipitation of β-Nb needles and possible other defects or solute clusters. All these results allow a general discussion of evolution of SPP with regard to their initial structure and composition, and of the noteworthy irradiation stability of the M5™ up to high neutron fluence with respect to other alloys.