Elastic electron scattering by silver atom

The experimental and theoretical studies of elastic electron scattering by silver atom have been carried out. The experimental investigation was based on crossed beam technique with effusive atomic beam being perpendicularly crossed by electron beam. The measurements were performed at electron-impact energies (E₀) of 10, 20, 40, 60, 80 and 100 eV and for a range of scattering angles (θ) from 10° up to 150°. The absolute differential cross sections (DCSs) have been obtained from the elastic-to-inelastic (the unresolved silver resonant lines 4d¹⁰5p²P_{1/2, 3/2}) intensity ratio at θ = 10° at each E₀. Calculations have been performed using the parameter-free complex optical potential (OP) with the inclusion of spin-orbit interaction for the same E₀. Comparison between present experiment and theory has been made.     

Recovery and restructuring induced by fission energy ions in high burnup nuclear fuel

In light water commercial reactors, extensive change of grain structure was found at high burnup ceramic fuels. The mechanism is driven by bombardment of fission energy fragments and studies were conducted by combining accelerator based experiments and computer-science. Specimen of CeO₂ was used as simulation material of fuel ceramics. With swift heavy ion (Xe) irradiation on CeO₂, with 210 MeV, change of valence charge and lattice deviation of cations were observed by XPS and XRD. Combined irradiations of Xe implantation and swift heavy ion irradiation successfully produced sub-micrometer sized sub-grains, similar as that observed in commercial fuels. Studying components of mechanism scenarios, with first principle calculations using the VASP code, we found stable hyper-stoichiometric defect structures of UO_2+x. Molecular dynamics studies revealed stability of Xe planar defects and also found rapid transport mode of oxygen-vacancy clusters.     

Increase in pyrolytic carbon optical anisotropy and density during processing of coated particle fuel due to heat treatment

Measurements of pyrolytic carbon optical anisotropy and density have been made on a series of tri-isotropic (TRISO) coated particles prepared for the United States Department of Energy’s Advanced Gas Reactor Fuel Development and Qualification (AGR) program. These measurements show the effect of varying the deposition conditions, especially the deposition temperature, on the density and optical anisotropy of the carbon layers. Additional heat treatment studies of the coated particles at various stages illustrate the strong effect of post-deposition thermal processing on these two pyrolytic carbon properties. Such post-deposition heat treatment occurs during SiC deposition and fuel compact firing, resulting in increased anisotropy and density of the pyrolytic carbon layers.     

A method for the quantification of total xenon concentration in irradiated nuclear fuel with SIMS and EPMA

A new method for the quantitative determination of the total xenon concentration in irradiated nuclear fuel is presented. The SIMS measurement of xenon enables the detection of the gas filling bubbles which are not detected with EPMA. The quantification is achieved using the EPMA data as reference at position where no or nearly no bubbles are detected. A new approach using the complementary information given by EPMA, SEM and SIMS is proposed, it opens new horizons for the characterisation of fission gases in irradiated nuclear fuel.     

Microstructural changes induced by low energy heavy ion irradiation in titanium silicon carbide

Low energy ion irradiation was used to investigate the microstructural modifications induced in Ti₃SiC₂ by nuclear collisions. Characterization of the microstructure of the pristine sample by electron back-scatter diffraction (EBSD) shows a strong texturing of TiSi₂, which is a common secondary phase present in Ti₃SiC₂. A methodology based on atomic force microscopy (AFM) was developed to measure the volume swelling induced by ion irradiation, and it was validated on irradiated silicon carbide. The swelling of Ti₃SiC₂ was estimated to 2.2 ± 0.8% for an irradiation dose of 4.3 dpa at room temperature. Results obtained by both EBSD and AFM analyses showed that nuclear collisions induce an anisotropic swelling in Ti₃SiC₂.     

Angular spectra of rainbow scattering at glancing keV He bombardment of NiAl(1 0 0) surface with transverse energies in the range 1-10 eV

Previous simulations of glancing incidence ion-surface interaction have demonstrated that classical dynamics using the row-model have successfully reproduced multimodal azimuthal and polar spectra. These studies have also shown considerable sensitivity to the form of the interatomic potential thus making it a strong test of the validity of such potentials and even allow deduction of the ion-surface potentials. In these simulations the individual pairwise interactions between the projectile and the target atoms have been replaced by cylindrical potentials.Comparison to numerous experimental studies have confirmed the existence of rainbow scattering phenomena and successfully tested the validity of the cylindrical potential used in these simulations. The use of cylindrical potentials avoids stochastic effects due to thermal displacements and allows faster computer simulations leading to reliable angular distributions.In the present work we extend the row-model to consider scattering from binary alloys. Using He⁺ scattered at glancing incidence from NiAl surfaces, Al or Ni terminated, a faster method has been developed to easily and accurately quantize not only the maximum deflection azimuthal angle but all the singular points in the angular distribution. It has been shown that the influence of the surface termination on the rainbow angle and the inelastic losses is small.     

Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation

The influence of the oxide layer morphology on the hydrogen uptake during steam oxidation of (Zr,Sn) and Zr-Nb nuclear fuel rod cladding alloys was investigated in isothermal separate-effect tests and large-scale fuel rod bundle simulation experiments. From both it can be concluded that the concentration of hydrogen in the remaining metal strongly depends on the existence of tangential cracks in the oxide layers formed by the tetragonal - monoclinic phase transition in the oxide, known as breakaway effect. In these cracks hydrogen is strongly enriched. It results in very local high hydrogen partial pressure at the oxide/metal interface and in an increase of the hydrogen concentration in the metal at local regions where such cracks in the oxide layer exist. Due to this effect the hydrogen uptake of the remaining zirconium alloy does not depend monotonically on temperature. Differences between (Zr,Sn) and Zr-Nb alloys are caused by differences in the hydrogen production due to different oxidation kinetics and in the crack forming phase transformation in the oxides as well as in the mechanical stability of the oxides.     

Ion beam analysis of materials in the PBMR reactor

South Africa is developing a new type of high temperature nuclear reactor, the so-called pebble bed modular reactor (PBMR). The planned reactor outlet temperature of this gas-cooled reactor is approximately 900 °C. This high temperature places some severe restrictions on materials, which can be used. The name of the reactor is derived from the form of the fuel elements, which are in the form of pebbles, each with a diameter of 60 mm. Each pebble is composed of several thousands of coated fuel particles. The coated particle consists of a nucleus of UO₂ surrounded by several layers of different carbons and SiC. The diameter of the fuel particles is 0.92 mm. A brief review will be given of the advantages of this nuclear reactor, of the materials in the fuel elements and their analysis using ion beam techniques.