A. Gentils

Concentration-triggered fission product release from zirconia: consequences for nuclear safety

Crystalline oxide ceramics, more particularly zirconia and spinel, are promising matrices for plutonium and minor actinide transmutation. An important issue concerning these materials is the investigation of their ability to confine radiotoxic elements resulting from the fission of actinides. This letter reports the study of the release, upon annealing or irradiation at high temperature, of one of the most toxic fission product (Cs) in zirconia. The foreign species are introduced by ion implantation and the release is studied by Rutherford backscattering experiments. The results emphasize the decisive influence of the fission product concentration on the release properties. The Cs mobility in zirconia is strongly increased when the impurity concentration exceeds a threshold of the order of a few atomic per cent. Irradiation with medium-energy heavy ions is shown to enhance Cs outdiffusion with respect to annealing at the same temperature.

Lattice location of Cs atoms in cubic ZrO2 single crystals

The lattice location of a specific fission product (Cs) implanted into cubic zirconia single crystals was investigated as a function of the atomic concentration by using Rutherford backscattering and channeling (RBS/C) experiments. At low concentration (<0.5 at.%), a significant Cs substitutional fraction (0.5–0.6) is measured in channeling spectra recorded with the analyzing He beam aligned along the three main axes of the cubic crystal. Angular scans performed across the 〈100〉 axis indicate the formation of Cs-vacancy complexes. The ZrO2 single crystals are strongly damaged when the Cs concentration exceeds a few at.% and the Cs atoms are randomly located in the crystalline lattice. This latter result could be due to the precipitation of implanted Cs atoms.

The feasibility of Al-based oxide precipitation in Fe–10%Cr alloy by ion implantation

This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al+ and O+ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30 nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM.

Nature of Defects Induced by Au Implantation in Hexagonal Silicon Carbide Single Crystals

Pulsed‐slow‐positron‐beam‐based positron lifetime spectroscopy was used to investigate the nature of vacancy defects induced by 20 MeV Au implantation in single crystals 6H‐SiC. Preliminary analysis of the data shows that at lower fluence, below 1014 cm−2, a positron lifetime of 220 ps has been obtained: it could be associated with the divacancy VSi−VC in comparison with the literature. At higher fluence, above 1015 cm−2, a positron lifetime of 260–270 ps, increasing with the incident positron energy, has been observed after decomposition of the lifetime spectra. By comparison with lifetime calculations, open‐volumes such as quadrivacancy (VSi−VC)2 clusters could be associated with this value.