Abstract: This paper presents a discussion of neutron interactions with materials that lead to activation, transmutation, and atomic displacements. The emphasis will be on current applications including neutron irradiation facilities, neutron dosimetry techniques, and computer codes for spectral adjustment and radiation damage calculations.
Abstract: A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...
Abstract: Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small ...
Abstract: Fusion materials research started in the early 1970s following the observation of the degradation of irradiated materials used in the first commercial fission reactors. The technological challenges of fusion energy are intimately linked with the availability of suitable materials capable of reliably withstanding the extremely severe operational conditions of fusion reactors. Although fission and fusion materials exhibit common features, fusion materials research is broader. The harder mono-energetic spectrum associated with the deuterium–tritium fusion neutrons (14.1 MeV compared to <2 MeV on average for fission neutrons) releases significant amounts of hydrogen and helium as transmutation products that might lead to a (at present undetermined) degradation of structural materials after a few years of operation. Overcoming the historical lack of a fusion-relevant neutron source for materials testing is an essential pending step in fusion roadmaps. Structural materials development, together with research on functional materials capable of sustaining unprecedented power densities during plasma operation in a fusion reactor, have been the subject of decades of worldwide research efforts underpinning the present maturity of the fusion materials research programme. For achieving proper safety and efficiency of future fusion power plants, low-activation materials able to withstand the extreme fusion conditions are needed. Here, the irradiation physics at play and fusion materials research is reviewed.
Abstract: The basic physical processes underlying the production of displacement damage in irradiated solids are briefly discussed, including topics from nuclear, atomic, and solid-state physics. Following a general introduction, the concepts of elementary cascade theory are presented as a basis for intuitive descriptions of the damage process. Then the production of primary recoils, mainly by nuclear processes, is discussed in enough detail to prepare a basis for calculating the primary-recoil energy spectra in typical irradiation facilities. The slowing down of fast atomic particles in solids is next discussed as a basis for developing atomistic models of damage production. Finally, several aspects of damage production, as revealed by atomistic simulation models, are outlined.
Abstract: As the most promising plasma-facing material for actual and future nuclear fusion devices, tungsten has to face and withstand a broad variety of severe operational conditions. These comprise high transient thermal loads, neutron irradiation-induced material degradation and transmutation, hydrogen and helium attack at the plasma-facing surface, and thermal fatigue under steady state heat fluxes as part of a plasma-facing component. The characterization and understanding of the material’s behavior under these conditions is essential for finding a grade of tungsten or tungsten alloy compatible with such an extreme environment.
Abstract: Neutron displacement damage-energy cross sections have been calculated for 41 isotopes in the energy range from 10/sup -10/ to 20 MeV Calculations were performed on a 100-point energy grid using nuclear cross sections from ENDF/B-V and the DISCS computer code Elastic scattering is treated exactly including angular distributions from ENDF/B-V Inelastic scattering calculations consider both discrete and continuous nuclear level distributions Multiple (n,xn) reactions use a Monte Carlo technique to derive the recoil distributions The (n,d) and (n,t) reactions are treated as (n,p) and (n,/sup 3/He) as (n,/sup 4/He) The (n,gamma) reaction and subsequent beta-decay are also included, using a new treatment of gamma-gamma coincidences, angular correlations, beta-neutrino correlations, and the incident neutron energy The Lindhard model was used to compute the energy available for nuclear displacement at each recoil energy The SPECTER computer code has been developed to simplify damage calculations The user need only specify a neutron energy spectrum SPECTER will then calculate spectral-averaged displacements, recoil spectra, gas production, and total damage energy (Kerma) The SPECTER computer code package is readily accessible to the fusion community via the National Magnetic Fusion Energy Computer Center (NMFECC) at Lawrence Livermore National laboratory
Abstract: It has long been recognized that thermal neutron irradiations of nickel-bearing materials generate high levels of helium from the sequential 58Ni(n,γ)59Ni(n, α)56Fe nuclear reactions. This process is used to simulate fusion reactor helium-to-displacement damage rates in stainless steel during fission reactor irradiations. However, it has not previously been recognized that the 56Fe recoils will also cause significant displacement damage. At thermal neutron energies the 340 keV 56Fe recoil event will displace 1762 atoms in nickel. The helium (appm)-to-displacement ratio from this process will be 567, a value that can be used to correct previous displacement calculations in nickel-bearing materials. This effect can nearly double the displacement damage in nickel in HFIR at high neutron fluences.
Abstract: Effects of helium on mechanical properties of irradiated structural materials are reviewed. In particular, variations in response to the ratio of helium to displacement damage serve as the focus. Ductility in creep and tensile tests is emphasized. A variety of early work has led to the current concentration on helium effects for fusion reactor materials applications. A battery of techniques has been developed by which the helium to displacement ratio can be varied. Our main discussion is devoted to the techniques of spectral tailoring and isotopic alloying currently of interest for mixed-spectrum reactors. Theoretical models of physical mechanisms by which helium interacts with displacement damage have been developed in terms of hardening to dislocation motion and grain boundary cavitation. Austenitic stainless steels, ferritic/martensitic steels and vanadium alloys are considered. In each case, work at low strain rates, where the main problems may lie, at the helium to displacement ratios appropriate to fusion reactor materials is lacking. Recent experimental evidence suggests that both in-reactor and high helium results may differ substantially from post-irradiation or low helium results. It is suggested that work in these areas is especially needed.
Abstract: This work investigated the sensitivity of microstructural evolution, particularly precipitate development, to increased helium content during thermal aging and during neutron irradiation. Helium (110 at. ppM) was cold preinjected into solution annealed (SA) DO-heat type 316 stainess steel (316) via cyclotron irradiation. These specimens were then exposed side by side with uninjected samples. Continuous helium generation was increased considerably relative to EBR-II irradiation by irradiation in HFIR. Data were obtained from quantitative analytical electron microscopy (AEM) in thin foils and on extraction replicas. 480 refs., 86 figs., 19 tabs.
Abstract: Neutron cross sections for displacements and post-short-term cascade annealing defects are derived from nuclear kinematics calculations of primary atomic recoil energy distributions and the...