Scientific understanding of any kind of radiation effects starts from the primary damage, i.e. th... more Scientific understanding of any kind of radiation effects starts from the primary damage, i.e. the defects that are produced right after an initial atomic displacement event initiated by a high-energy particle. In this Review, we consider the extensive experimental and computer simulation studies that have been performed over the past several decades on what the nature of the primary damage is. We review both the production of crystallographic or topological defects in materials as well as radiation mixing, i.e. the process where atoms in perfect crystallographic positions exchange positions with other ones in nondefective positions. All classes of materials except biological materials are considered. We also consider the recent effort to provide alternatives to the current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model for metals. We present in detail new complementary displacement production estimators ("athermal recombination corrected dpa", arc-dpa) and atomic mixing ("replacements per atom", rpa) functions that extend the NRT-dpa, and discuss their advantages and limitations.
Atomic collision processes are fundamental to numerous advanced materials technologies such as el... more Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-...
Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with l... more Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100 nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally. Published by Elsevier B.V.
ABSTRACT The use of a fusion component testing facility to study and establish during the ITER er... more ABSTRACT The use of a fusion component testing facility to study and establish during the ITER era the remaining scientific and technical knowledge needed by fusion Demo is considered and described in this paper. This use aims to test components in an integrated fusion nuclear environment to discover and understand the underpinning physical properties, and to develop improved components for further testing, in a time-efficient manner. It requires a design with extensive modularization and remote handling of activated components, and flexible hot-cell laboratories. It further requires reliable plasma conditions to avoid disruptions and minimize impact, and designs to reduce the divertor heat flux. As the plasma duration is extended through the ITER level (~10 3 s) and beyond, physical properties with increasing time constants would become accessible for testing and R&D. The longest time constants of these are expected to be many days (~10 5 – 10 6 s). Progressive stages of research operation are envisioned in deuterium, deuterium-tritium at the ITER-level, and deuterium-tritium with increasingly longer plasma durations.
Recent experimental observations have made it clear that cavity formation can occur in light-wate... more Recent experimental observations have made it clear that cavity formation can occur in light-water reactor internal components fabricated from austenitic stainless during the course of their service life. In order to assess the potential for cavity swelling in these components at end-of-life doses, it is necessary to develop a validated computational model that incorporates the relevant physical mechanisms and accounts for recent experiment data. Such a modeling activity is underway; the model development and some preliminary results are described. For the relatively low temperatures involved, cavity formation is shown to be sensitive to both the temperature and the rate of helium production by nuclear transmutation reactions. This report includes a brief review of the relevant microstructural data, discussion of the current model's status and planned further development, and a description of the microstructural modeling that is planned to fully define the potential for cavity evolution under light water reactor operating conditions.
Effects of Radiation on Materials: 20th International Symposium, 2000
An extensive database of atomic displacement cascades in iron has been developed using the method... more An extensive database of atomic displacement cascades in iron has been developed using the method of molecular dynamics (MD). More than 300 simulations have been completed at 100K with energies between 0.1 and 100 keV. This encompasses nearly all energies relevant to fission reactor irradiation environments since a 100 keV MD cascade corresponds to the average iron cascade following a collision with a 5.1 MeV neutron. Extensive statistical analysis of the database has determined representative average values for several primary damage parameters: the total number of surviving point defects, the fraction of the surviving point defects contained in clusters formed during cascade cooling, and a measure of the size distribution of the in-cascade point defect clusters. The cascade energy dependence of the MD-based primary damage parameters has been used to obtain spectrum-averaged defect production cross sections for typical fission reactor neutron energy spectra as a function of depth through the reactor pressure vessel. The attenuation of the spectrum-averaged cross sections for total point defect survival and the fraction of either interstitials or vacancies in clusters are quite similar to that for the NRT dpa. However, the cross sections derived to account for the energy dependence of the point defect cluster size distributions exhibit a potentially significant variation through the vessel. The production rate of large interstitial clusters decreases more rapidly than dpa whereas the production of large vacancy clusters is slower than dpa.
Http Dx Doi Org 10 1080 14786435 2010 521528, Oct 23, 2010
Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at f... more Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.
The influence of temperature variations on microstructural evolution in austenitic stainless stee... more The influence of temperature variations on microstructural evolution in austenitic stainless steels is discussed in order to help interpret the response of materials in the HFIR-RB-13J temperature variation experiment. A kinetic microstructural evolution model developed for irradiated austenitic stainless steels was modified to provide a fully dynamic calculation of point defect and point defect cluster concentrations. Using the modified model,
ABSTRACT We have initiated fundamental investigations of 15 MeV Ni-ion induced defect clusters in... more ABSTRACT We have initiated fundamental investigations of 15 MeV Ni-ion induced defect clusters in single crystal Fe and Fe-Cr using diffuse scattering measurements near Bragg reflections combined with molecular dynamics (MD) and self-evolving atomistic kinetic Monte Carlo (SEAK-MC) simulations. Synchrotron x-ray diffuse scattering measurements performed near the (002) reflection of 001oriented Fe and Fe(30%)Cr single crystals are analyzed within the so-called asymptotic regime using scattering cross-sections based on MD simulated local lattice distortions and SEAK-MC generated interstitial and vacancy cluster configurations. Measurements for Ni-ion irradiations of Fe and Fe-Ni with doses corresponding to 0.2 and 1 displacements per atom (DPA) at ambient temperature will be presented and discussed in connection with the local Bragg scattering interpretation of defect cluster diffuse scattering in ion-irradiated Cu. Methods for calculating diffuse scattering cross sections directly from MD simulations of atomic displacements around vacancy and interstitial loops within the single defect approximation will be considered and the importance of such approaches for complex defect clusters will be addressed.
Attempts to correlate radiation-induced microstructural changes with changes in mechanical proper... more Attempts to correlate radiation-induced microstructural changes with changes in mechanical properties rely on a well-established theory to compute the resolved shear stress required to move dislocations through a field of obstacles. However, this microstructure-based shear ...
ABSTRACT We investigated dislocation nucleation and defect formation underneath a spherical inden... more ABSTRACT We investigated dislocation nucleation and defect formation underneath a spherical indenter which possesses atomic steps on its surface. Atomic-scale simulations of Cu (111) nanoindentation were performed. Our simulation results reveal that dislocations nucleate from surface ledges formed by atomic steps on indenter surfaces. We found that stepped indenters promote concurrent activation of three inclined {111} planes, which leads to an increased probability of forming threefold symmetric defects and punching prismatic loops along threefold symmetric directions. A new junction structure was observed and found to unzip during the formation of prismatic loops. The formation and destruction of defect structures can be explained using a conventional theory of dislocation reactions.
ABSTRACT This book encompasses a rich seam of current information on the vast and multidisciplina... more ABSTRACT This book encompasses a rich seam of current information on the vast and multidisciplinary field of nuclear materials employed in fission and prototype fusion systems. Discussion includes both historical and contemporary international research in nuclear materials, from Actinides to Zirconium alloys, from the worlds leading scientists and engineers. Synthesizes pertinent current science to support the selection, assessment, validation and engineering of materials in extreme nuclear environments. The work discusses the major classes of materials suitable for usage in nuclear fission, fusion reactors and high power accelerators, and for diverse functions in fuels, cladding, moderator and control materials, structural, functional, and waste materials.
Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displac... more Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascades in iron with energies up to 50 keV (corresponding to a primary knock-on atom (PKA) energy of 79 keV) at 100 K, up to 20 keV at 600 K, and up to 10 keV at 900 K. The cascade damage production has been characterized in
Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in... more Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in thin films have a number of remarkable features. A particular example is the interaction between a screw dislocation and a stacking fault tetrahedron (SFT) in Cu, which can be directly compared with in situ observations of quenched or irradiated fcc metals. If the specimen is thin, the dislocation velocity is slow, and the temperature is high enough, a segment of the original SFT can be transported towards the surface via a double cross-slip mechanism and fast glide of an edge dislocation segment formed during the interaction. The mechanisms observed in the simulations provide an explanation for the results of in situ straining experiments and the differences between bulk and thin film experiments.
Scientific understanding of any kind of radiation effects starts from the primary damage, i.e. th... more Scientific understanding of any kind of radiation effects starts from the primary damage, i.e. the defects that are produced right after an initial atomic displacement event initiated by a high-energy particle. In this Review, we consider the extensive experimental and computer simulation studies that have been performed over the past several decades on what the nature of the primary damage is. We review both the production of crystallographic or topological defects in materials as well as radiation mixing, i.e. the process where atoms in perfect crystallographic positions exchange positions with other ones in nondefective positions. All classes of materials except biological materials are considered. We also consider the recent effort to provide alternatives to the current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model for metals. We present in detail new complementary displacement production estimators ("athermal recombination corrected dpa", arc-dpa) and atomic mixing ("replacements per atom", rpa) functions that extend the NRT-dpa, and discuss their advantages and limitations.
Atomic collision processes are fundamental to numerous advanced materials technologies such as el... more Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-...
Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with l... more Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100 nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally. Published by Elsevier B.V.
ABSTRACT The use of a fusion component testing facility to study and establish during the ITER er... more ABSTRACT The use of a fusion component testing facility to study and establish during the ITER era the remaining scientific and technical knowledge needed by fusion Demo is considered and described in this paper. This use aims to test components in an integrated fusion nuclear environment to discover and understand the underpinning physical properties, and to develop improved components for further testing, in a time-efficient manner. It requires a design with extensive modularization and remote handling of activated components, and flexible hot-cell laboratories. It further requires reliable plasma conditions to avoid disruptions and minimize impact, and designs to reduce the divertor heat flux. As the plasma duration is extended through the ITER level (~10 3 s) and beyond, physical properties with increasing time constants would become accessible for testing and R&D. The longest time constants of these are expected to be many days (~10 5 – 10 6 s). Progressive stages of research operation are envisioned in deuterium, deuterium-tritium at the ITER-level, and deuterium-tritium with increasingly longer plasma durations.
Recent experimental observations have made it clear that cavity formation can occur in light-wate... more Recent experimental observations have made it clear that cavity formation can occur in light-water reactor internal components fabricated from austenitic stainless during the course of their service life. In order to assess the potential for cavity swelling in these components at end-of-life doses, it is necessary to develop a validated computational model that incorporates the relevant physical mechanisms and accounts for recent experiment data. Such a modeling activity is underway; the model development and some preliminary results are described. For the relatively low temperatures involved, cavity formation is shown to be sensitive to both the temperature and the rate of helium production by nuclear transmutation reactions. This report includes a brief review of the relevant microstructural data, discussion of the current model's status and planned further development, and a description of the microstructural modeling that is planned to fully define the potential for cavity evolution under light water reactor operating conditions.
Effects of Radiation on Materials: 20th International Symposium, 2000
An extensive database of atomic displacement cascades in iron has been developed using the method... more An extensive database of atomic displacement cascades in iron has been developed using the method of molecular dynamics (MD). More than 300 simulations have been completed at 100K with energies between 0.1 and 100 keV. This encompasses nearly all energies relevant to fission reactor irradiation environments since a 100 keV MD cascade corresponds to the average iron cascade following a collision with a 5.1 MeV neutron. Extensive statistical analysis of the database has determined representative average values for several primary damage parameters: the total number of surviving point defects, the fraction of the surviving point defects contained in clusters formed during cascade cooling, and a measure of the size distribution of the in-cascade point defect clusters. The cascade energy dependence of the MD-based primary damage parameters has been used to obtain spectrum-averaged defect production cross sections for typical fission reactor neutron energy spectra as a function of depth through the reactor pressure vessel. The attenuation of the spectrum-averaged cross sections for total point defect survival and the fraction of either interstitials or vacancies in clusters are quite similar to that for the NRT dpa. However, the cross sections derived to account for the energy dependence of the point defect cluster size distributions exhibit a potentially significant variation through the vessel. The production rate of large interstitial clusters decreases more rapidly than dpa whereas the production of large vacancy clusters is slower than dpa.
Http Dx Doi Org 10 1080 14786435 2010 521528, Oct 23, 2010
Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at f... more Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.
The influence of temperature variations on microstructural evolution in austenitic stainless stee... more The influence of temperature variations on microstructural evolution in austenitic stainless steels is discussed in order to help interpret the response of materials in the HFIR-RB-13J temperature variation experiment. A kinetic microstructural evolution model developed for irradiated austenitic stainless steels was modified to provide a fully dynamic calculation of point defect and point defect cluster concentrations. Using the modified model,
ABSTRACT We have initiated fundamental investigations of 15 MeV Ni-ion induced defect clusters in... more ABSTRACT We have initiated fundamental investigations of 15 MeV Ni-ion induced defect clusters in single crystal Fe and Fe-Cr using diffuse scattering measurements near Bragg reflections combined with molecular dynamics (MD) and self-evolving atomistic kinetic Monte Carlo (SEAK-MC) simulations. Synchrotron x-ray diffuse scattering measurements performed near the (002) reflection of 001oriented Fe and Fe(30%)Cr single crystals are analyzed within the so-called asymptotic regime using scattering cross-sections based on MD simulated local lattice distortions and SEAK-MC generated interstitial and vacancy cluster configurations. Measurements for Ni-ion irradiations of Fe and Fe-Ni with doses corresponding to 0.2 and 1 displacements per atom (DPA) at ambient temperature will be presented and discussed in connection with the local Bragg scattering interpretation of defect cluster diffuse scattering in ion-irradiated Cu. Methods for calculating diffuse scattering cross sections directly from MD simulations of atomic displacements around vacancy and interstitial loops within the single defect approximation will be considered and the importance of such approaches for complex defect clusters will be addressed.
Attempts to correlate radiation-induced microstructural changes with changes in mechanical proper... more Attempts to correlate radiation-induced microstructural changes with changes in mechanical properties rely on a well-established theory to compute the resolved shear stress required to move dislocations through a field of obstacles. However, this microstructure-based shear ...
ABSTRACT We investigated dislocation nucleation and defect formation underneath a spherical inden... more ABSTRACT We investigated dislocation nucleation and defect formation underneath a spherical indenter which possesses atomic steps on its surface. Atomic-scale simulations of Cu (111) nanoindentation were performed. Our simulation results reveal that dislocations nucleate from surface ledges formed by atomic steps on indenter surfaces. We found that stepped indenters promote concurrent activation of three inclined {111} planes, which leads to an increased probability of forming threefold symmetric defects and punching prismatic loops along threefold symmetric directions. A new junction structure was observed and found to unzip during the formation of prismatic loops. The formation and destruction of defect structures can be explained using a conventional theory of dislocation reactions.
ABSTRACT This book encompasses a rich seam of current information on the vast and multidisciplina... more ABSTRACT This book encompasses a rich seam of current information on the vast and multidisciplinary field of nuclear materials employed in fission and prototype fusion systems. Discussion includes both historical and contemporary international research in nuclear materials, from Actinides to Zirconium alloys, from the worlds leading scientists and engineers. Synthesizes pertinent current science to support the selection, assessment, validation and engineering of materials in extreme nuclear environments. The work discusses the major classes of materials suitable for usage in nuclear fission, fusion reactors and high power accelerators, and for diverse functions in fuels, cladding, moderator and control materials, structural, functional, and waste materials.
Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displac... more Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascades in iron with energies up to 50 keV (corresponding to a primary knock-on atom (PKA) energy of 79 keV) at 100 K, up to 20 keV at 600 K, and up to 10 keV at 900 K. The cascade damage production has been characterized in
Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in... more Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in thin films have a number of remarkable features. A particular example is the interaction between a screw dislocation and a stacking fault tetrahedron (SFT) in Cu, which can be directly compared with in situ observations of quenched or irradiated fcc metals. If the specimen is thin, the dislocation velocity is slow, and the temperature is high enough, a segment of the original SFT can be transported towards the surface via a double cross-slip mechanism and fast glide of an edge dislocation segment formed during the interaction. The mechanisms observed in the simulations provide an explanation for the results of in situ straining experiments and the differences between bulk and thin film experiments.
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Papers by Roger Stoller