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Development of new materials for current and advanced reactor concepts is hampered by long lead times and high cost of reactor irradiations coupled with the paucity of test reactors. Ion irradiation offers many advantages for emulating the microstructures and properties of materials irradiated in reactors but also poses many challenges. Nevertheless, there is a growing body of evidence, primarily for light ion (proton) irradiation showing that many, if not all of the features of the irradiated microstructure and properties, can be successfully emulated by careful selection of irradiation parameters based on differences in the damage processes between ion and neutron irradiation. While much less has been done to benchmark heavy- or self-ion irradiation, recent work shows that under certain conditions, the complete suite of features of the irradiated microstructure can be emulated. This study summarizes the contributions of ion irradiation to our understanding of irradiation effects, the options for emulating radiation effects in reactors, and experience with both proton irradiation and heavy ion irradiation.
JANNUS (Joint Accelerators for Nanosciences and Nuclear Simulation), the unique triple beam facility in Europe, offers the possibility to produce three ion beams simultaneously for nuclear recoil damage and implantation of a large array of ions for well-controlled modeling-oriented experiments. The first triple beam irradiation was performed in March 2010. Along with irradiation developments, continuous efforts have been made to implement ex situ and in situ characterization tools. In this study, we set out the present status of the JANNUS facility of the Saclay site. We focus on the instrumentation used for conducting multi-ion beam irradiations and implantations as well as for characterizing bombarded samples. On-line control of irradiation parameters, in situ modification monitoring using Raman spectroscopy or ion beam induced luminescence, and ex situ characterization by ion beam surface analysis [Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), and elastic recoil detection analysis (ERDA)] of implanted samples are detailed. Some examples of single, dual, and triple beam irradiation configurations are presented. Access to the facility is provided by the French network EMIR for national and international users (http://emir.in2p3.fr/).
We describe aspects of transmission electron microscopy (TEM) technique to image and quantify the defect state following neutron or ion irradiation with an emphasis on experimental considerations. After outlining various neutron and ion irradiation scenarios, including some sample preparation suggestions, we discuss methods to measure defect densities, size distributions, structures, and interstitial or vacancy nature. The importance of the image simulations of Zhou is suggested for guidance to the most accurate quantification of the defect state. It is hoped that the usefulness of the present paper will be greatest for those experiments that compare defect states in materials after different irradiation conditions, or especially those studies designed to benchmark advanced computer model simulations of defect production and evolution. The successful simulation of the defect state in bulk samples neutron irradiated to high dose at high temperature is a goal to which the suggestions in this paper can contribute.
Progress toward combining time-resolved experiments with periodic three-dimensional analysis of the evolved microstructural state has been made recently. In situ electron microscopy is used to observe in real time the development of irradiation defects and the influence of these defects on dislocation behavior. Three-dimensional characterization provides information on the true spatial distribution of defects and clarifies effects of the free surfaces in thin films. This quasi-four dimensional analysis approach has been applied to understand the formation of channels in irradiated alloys, the depth distribution of ion damage in an electron transparent foil, and the dislocation channel interactions with grain boundaries. The new insight obtained from these experiments is highlighted and contrasted with findings from simulations.
The macroscopic properties of materials exposed to irradiation are determined by radiation damage effects which occur on the nanoscale. These phenomena are complex dynamic processes in which many competing mechanisms contribute to the evolution of the microstructure and thus to its end-state. To explore and understand the behavior of existing materials and to develop new technologies, it is highly advantageous to be able to observe the microstructural effects of irradiation as they occur. Transmission electron microscopy with in situ ion irradiation is ideally suited to this kind of study. This review focuses on some of the important factors in designing this type of experiment including sample preparation and ion beam selection. Also presented are a brief history of the development of this technique and an overview of the instruments in operation today including the latest additions.
With its particular ability to image solute clusters in three dimensions and impurity segregation to selected interfaces and grain boundaries, atom probe tomography has provided unique insight into the effects of irradiation on materials microstructures. This article reviews the contribution of atom probe tomography to our understanding of behaviors and responses of structural materials under irradiation. Possible atom probe tomography based approaches and common data analysis methods to analyze the microstructural features often observed in irradiated materials are described. In particular, the analysis of solute clustering, solute segregation, and void imaging are discussed in the context of radiation-induced hardening of austenitic steels and reactor pressure vessel steels, and the development of oxide dispersion strengthened steels, radiation-induced solute segregation to grain boundaries for stress corrosion cracking or corrosion issues, and to understand the swelling response of irradiated materials. While highlighting the unique information that atom probe tomography can offer, common limitations, current challenges, and outstanding technical questions regarding data analysis and interpretation are also presented.
Small specimen testing techniques have a long history in nuclear material research due to the limitations posed by nuclear facilities. The limited space in reactors and the fact that the samples are oftentimes radioactive in addition to the increasing need to obtain mechanical properties from ion beam irradiated samples require small specimen mechanical testing. With the application of modern focused ion beam sample preparation techniques and the enhancement of nanoindentation instruments, the size scale has been moved to even smaller scales and new geometries. Micrometer and even nanometer size samples are feasible, but raise the question of comparability to large scale properties for engineering applications. In this review, we summarize available small scale materials testing techniques and potential shortcomings based on examples from the literature, as well as introduce novel experimental approaches conducted using microcompression testing, microbend bar testing, and nanoindentation at ambient and nonambient conditions.
Designing materials for performance in high-radiation fields can be accelerated through a carefully chosen combination of advanced multiscale modeling paired with appropriate experimental validation. The studies reported in this work, the combined efforts of six universities working together as the Consortium on Cladding and Structural Materials, use that approach to focus on improving the scientific basis for the response of ferritic–martensitic steels to irradiation. A combination of modern modeling techniques with controlled experimentation has specifically focused on improving the understanding of radiation-induced segregation, precipitate formation and growth under radiation, the stability of oxide nanoclusters, and the development of dislocation networks under radiation. Experimental studies use both model and commercial alloys, irradiated with both ion beams and neutrons. Transmission electron microscopy and atom probe are combined with both first-principles and rate theory approaches to advance the understanding of ferritic–martensitic steels.
This paper provides an overview of advanced scanning transmission electron microscopy (STEM) techniques used for characterization of irradiated BCC Fe-based alloys. Advanced STEM methods provide the high-resolution imaging and chemical analysis necessary to understand the irradiation response of BCC Fe-based alloys. The use of STEM with energy dispersive x-ray spectroscopy (EDX) for measurement of radiation-induced segregation (RIS) is described, with an illustrated example of RIS in proton- and self-ion irradiated T91. Aberration-corrected STEM-EDX for nanocluster/nanoparticle imaging and chemical analysis is also discussed, and examples are provided from ion-irradiated oxide dispersion strengthened (ODS) alloys. Finally, STEM techniques for void, cavity, and dislocation loop imaging are described, with examples from various BCC Fe-based alloys.
Ni-based fcc alloys are frequently used as critical structural materials in nuclear energy applications. Despite extensive studies, fundamental questions remain regarding point defect migration and solute segregation as a function of grain boundary character after irradiation. In this study, a coupled experimental and modeling approach is used to understand the response of grain boundary character in a model Ni–5Cr alloy after high temperature heavy-ion irradiation. Radiation-induced segregation and void denuded zones were experimentally examined as a function of grain boundary character, while a kinetic rate theory model with grain boundary character boundary conditions was used to theoretically model Cr depletion in the alloy system. The results highlight major variations in the radiation response between the coherent and incoherent twin grain boundaries, but show limited disparity in defect sink strength between random low- and high-angle grain boundary regimes.
Recent studies have shown that chemical immiscibility is important to achieve enhanced radiation tolerance in metallic multilayers as immiscible layer interfaces are more stable against radiation induced mixing than miscible interfaces. However, as most of these immiscible systems have incoherent interfaces, the influence of coherency on radiation resistance of immiscible systems remains poorly understood. Here, we report on radiation response of immiscible Cu/Fe multilayers, with individual layer thickness h varying from 0.75 to 100 nm, subjected to He ion irradiation. When interface is incoherent, the peak bubble density decreases with decreasing h and reaches a minimum when h is 5 nm. At even smaller h when interface is increasingly coherent, the peak bubble density increases again. However, void swelling in coherent multilayers with smaller h remains less than those in incoherent multilayers. Our study suggests that the coherent immiscible interface is also effective to alleviate radiation induced damage.
We report here the microstructural changes occurring in the zirconium alloy Excel (Zr–3.5 wt% Sn–0.8Nb–0.8Mo–0.2Fe) during heavy ion irradiation. In situ irradiation experiments were conducted at reactor operating temperatures on two Zr Excel alloy microstructures with different states of alloying elements, with the states achieved by different solution heat treatments. In the first case, the alloying elements were mostly concentrated in the beta (β) phase, whereas, in the second case, large Zr3(Mo,Nb,Fe)4 secondary phase precipitates (SPPs) were grown in the alpha (α) phase by long term aging. The heavy ion induced damage and resultant compositional changes were examined using transmission electron microscopy (TEM) in combination with scanning transmission electron microscope (STEM)-energy dispersive x-ray spectroscopy (EDS) mapping. Significant differences were seen in microstructural evolution between the two different microstructures that were irradiated under similar conditions. Nucleation and growth of <c>-component loops and their dependence on the alloying elements are a major focus of the current investigation. It was observed that the <c>-component loops nucleate readily at 100, 300, and 400 °C after a threshold incubation dose (TID), which varies with irradiation temperature and the state of alloying elements. It was found that the TID for the formation of <c>-component loops increases with decrease in irradiation temperature. Alloying elements that are present in the form of SPPs increase the TID compared to when they are in the β phase solid solution. Dose and temperature dependence of loop size and density are presented. Radiation induced redistribution and clustering of alloying elements (Sn, Mo, and Fe) have been observed and related to the formation of <c>-component loops. It has been shown that at the higher temperature tests, irradiation induced dissolution of precipitates occurs whereas irradiation induced amorphization occurs at 100 °C. Furthermore, dose and temperature seem to be the main factors governing the dissolution of SPPs and redistribution of alloying elements, which in turn controls the nucleation and growth of <c>-component loops. The correlation between the microstructural evolution and microchemistry has been found by EDS and is discussed in detail.
The kinetics and morphology of oxides formed during in situ proton irradiation–corrosion experiment were analyzed. Experiments were conducted in 320 °C water with 3 wt ppm H2, while irradiated by a 3.2-MeV proton beam at a current density of 2 µA/cm2 producing a damage rate at 4.4 × 10−7 dpa/s. The resulting oxide was compared with reference samples corroded in an autoclave, and literature data found on in-reactor formed oxide. The corrosion rate of the sample irradiated in situ was 10 times faster than the in-pile corrosion rate. The cracked and porous irradiated oxide consisted of monoclinic equiaxed grains of zirconia with a preferential orientation of the oxide grains. Second phase particles (SPPs) consumed by the oxidation front were rapidly oxidized, but no SPPs were amorphized or dissolved in the metal matrix of the irradiated sample.
Transmission electron microscopy (TEM) studies provide mechanistic understanding of nanoscale processes, whereas advanced synchrotron XRD (SXRD) enables precise measurements on volumes that are more representative of bulk materials. Therefore, the combined strengths of these techniques can provide new insight into irradiation-induced mechanistic processes. In the present study, their application to Zircaloy-2, proton-irradiated to 2.3, 4.7, and 7.0 dpa at 2 MeV and 350 °C and neutron-irradiated to 9.5 and 13.1 × 1025 n m−2 are exemplified. The application of correlative spectral imaging and structural TEM investigations to the phase transformation of Zr(Fe,Nb)2 precipitates in Low-Sn ZIRLO™, neutron-irradiated to 8.9–9 × 1025 n m−2, demonstrates the possibility of a Cr core nucleation site. Anomalous broadening is observed in SXRD profiles, which is believed to be caused by defect clusters and precursors to dislocation loop nucleation. The challenges to quantitative analysis of dislocations by SXRD are highlighted with reference to the segregation of Fe and Ni to basal planes and dislocation cores, observed by spectral imaging in the TEM.
Recent efforts to characterize the nanoscale structural and chemical modifications induced by energetic ion irradiation in nuclear materials have greatly benefited from the application of synchrotron-based x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) techniques. Key to the study of actinide-bearing materials has been the use of small sample volumes, which are particularly advantageous, as the small quantities minimize the level of radiation exposure at the ion-beam and synchrotron user facility. This approach utilizes energetic heavy ions (energy range: 100 MeV–3 GeV) that pass completely through the sample thickness and deposit an almost constant energy per unit length along their trajectory. High energy x-rays (25–65 keV) from intense synchrotron light sources are then used in transmission geometry to analyze ion-induced structural and chemical modifications throughout the ion tracks. We describe in detail the experimental approach for utilizing synchrotron radiation (SR) to study the radiation response of a range of nuclear materials (e.g., ThO2 and Gd2TixZr2−xO7). Also addressed is the use of high-pressure techniques, such as the heatable diamond anvil cell, as a new means to expose irradiated materials to well-controlled high-temperature (up to 1000 °C) and/or high-pressure (up to 50 GPa) conditions. This is particularly useful for characterizing the annealing kinetics of irradiation-induced material modifications.
High performance materials that can withstand radiation, heat, multiaxial stresses, and corrosive environment are necessary for the deployment of advanced nuclear energy systems. Nondestructive in situ experimental techniques utilizing high energy x-rays from synchrotron sources can be an attractive set of tools for engineers and scientists to investigate the structure–processing–property relationship systematically at smaller length scales and help build better material models. In this study, two unique and interconnected experimental techniques, namely, simultaneous small-angle/wide-angle x-ray scattering (SAXS/WAXS) and far-field high-energy diffraction microscopy (FF-HEDM) are presented. The changes in material state as Fe-based alloys are heated to high temperatures or subject to irradiation are examined using these techniques.
Synchrotron-based x-ray techniques are used to bring complementary information to local probes such as atom probe tomography and transmission electron microscopy. Two examples of nuclear materials used for the cladding of fuel assembly are given: oxide dispersion strengthened (ODS) alloys and M5™ Zr-based alloys. In both cases, synchrotron radiation analyses bring original results concerning nanosized secondary phases: for M5™, radiation-enhanced precipitation of β-Nb precipitates has been evidence and the crystallographic structure (lattice parameter and Nb content) is reported for the first time and for irradiated ODS, the dissolution of larger oxides is evidenced while a finer distribution of complex Y–Ti–O oxides still acts as obstacles for dislocations.
The progressive build up of fission products inside different nuclear reactor components can lead to significant damage of the constituent materials. We demonstrate the use of time-domain thermoreflectance (TDTR), a nondestructive thermal measurement technique, to study the effects of radiation damage on material properties. We use TDTR to report on the thermal conductivity of optimized ZIRLO, a material used as fuel cladding in nuclear reactors. We find that the thermal conductivity of optimized ZIRLO is 10.7 ± 1.8 W m−1 K−1 at room temperature. Furthermore, we find that the thermal conductivities of copper–niobium nanostructured multilayers do not change with helium ion irradiation doses of 1015 cm−2 and ion energy of 200 keV, demonstrating the potential of heterogeneous multilayer materials for radiation tolerant coatings. Finally, we compare the effect of ion doses and ion beam energies on the measured thermal conductivity of bulk silicon. Our results demonstrate that TDTR can be used to quantify depth dependent damage.
We describe measurements aimed at tracking the subsurface energy deposition of ionic radiation by encapsulating an irradiated oxide target within multiple, spatially separated metal–oxide–semiconductor (MOS) capacitors. In particular, we look at incident kinetic energy and potential energy effects in the low keV regime for alkali ions (Na+) and multicharged ions (MCIs) of ArQ+ (Q = 1, 4, 8, and 11) incident on the as-grown layers of SiO2 on Si. With the irradiated oxide encapsulated under Al top contacts, we record an electronic signature of the incident ionic radiation through capacitance–voltage (C–V) measurements. Both kinetic and potential energy depositions give rise to shifted C–V signatures that can be directly related to internal electron–hole pair excitations. The MCI data reveal an apparent power law dependence on charge state, which is at odds with some prior thin foil studies obtained at higher incident energies.