With respect to fuel the issues, we illustrate insights from atomistic simulations of defect ener... more With respect to fuel the issues, we illustrate insights from atomistic simulations of defect energetics, evolution and thermal transport properties. Specifically, we have used atomic-level simulation methods to determine the interaction of metallic fission products with the core of edge dislocations in UO2. Detailed analysis shows that the trends in segregation energy can be understood in terms of bulk behavior and continuum elasticity, opening up an efficient method for rapidly generating databases of segregation properties for input to mesoscale codes. We also analyze the results of molecular dynamics simulation of void formation in UO2 in the bulk and close to a grain boundary. The microstructural evolution during burnup, results in a progressive degradation in the thermal transport properties. We present the results of thermal conductivity calculations within the lattice dynamics framework in UO2 using electronic-structure calculations for the interatomic force constants. Compar...
This article was originally published in the Comprehensive Nuclear Materials published by Elsevie... more This article was originally published in the Comprehensive Nuclear Materials published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution's administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution's website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Golubov S.I.
Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena... more Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena at the mesoscale have typically been simulated using models based on reaction rate theory, such as mean field rate theory (MFRT) or Monte Carlo. These mesoscale models are appropriate for application to problems that involve intermediate-length scales, and timescales from those characteristic of diffusion to long-term microstructural evolution ($ls to years). Although the MFRT and Monte Carlo models can be used simulate the same phenomena, some of the details are handled quite differently in the two approaches. Models employing the rate theory have been extensively used to describe radiation-induced phenomena such as void swelling and irradiation creep. The primary approximations in such models are time and spatial averaging of the radiation damage source term, and spatial averaging of the microstructure into an effective medium. Kinetic Monte Carlo models can account for these spatial and temporal correlations; their primary limitation is the computational burden, which is related to the size of the simulation cell. Even with modern computers, the maximum simulation cell size and the maximum dose (typically much less than 1 dpa) that can be simulated are limited. In contrast, even very detailed MFRT models can simulate microstructural evolution for doses up 100 dpa or greater in clock times that are relatively short. Within the context of the effective medium, essentially any defect density can be simulated. A direct comparison of MFRT and object kinetic MC simulations has been made in the domain of point defect cluster dynamics modeling, which is relevant to the evolution (both nucleation and growth) of radiation-induced defect structures. Overall, the agreement between the two methods is best for irradiation conditions that produce a high density of defects (lower temperature and higher displacement rate) and for materials that have a relatively high density of fixed sinks such as dislocations.
This report presents results of investigations of damage accumulation during neutron irradiation ... more This report presents results of investigations of damage accumulation during neutron irradiation of pure iron and EUROFER 97 steel with or without prior helium implantation. The defect microstructure, in particular the cavities, was characterized using Positron Annihilation Spectroscopy (PAS) and Transmission Electron Microscopy (TEM). The PAS investigations revealed a clear difference between the He implantation effects in Fe and EUROFER 97 at 623 K. For both materials the mean positron lifetimes increased with He dose in the range 1 – 100 appm, although the increase was stronger for Fe than for EUROFER 97 and for both materials smaller for implantation at 623 K than at 323 K. This lifetime increase is due primarily to the formation of He bubbles. For He doses of 10 – 100 appm cavity sizes and densities in Fe were estimated to be 1.7 – 2.8 nm and 4 14×10 m, respectively. Neutron irradiation after He implantation in general leads to an increase of both cavity sizes and densities. Es...
ABSTRACTModels of radiation growth proposed to date are all based on the assumption that the prim... more ABSTRACTModels of radiation growth proposed to date are all based on the assumption that the primary damage is produced by neutron irradiation in the form of single defects. These models do not account for the features of the cascade damage: intra-cascade clustering of self‑interstitial atoms (SIAs) and their one‑dimensional diffusion. During the last twenty years, a ‘Production Bias Model’ has been developed, which shows that the damage accumulation in cubic metals depends crucially on the cascade properties. The cascades in hcp zirconium are similar to those in cubic crystals; hence the model can provide a realistic framework for the hcp metals as well. In this work we present such a model in application to low-temperature (below 300°C) radiation growth.
In order to help understand the evolution of stacking fault tetrahedra (SFTs) in a cascade produc... more In order to help understand the evolution of stacking fault tetrahedra (SFTs) in a cascade producing irradiation environment, the available information on the behaviour of SFTs observed under different experimental conditions has been briefly reviewed. Effects of thermal annealing and irradiations on the stability of pre-existing SFTs produced by quenching and aging are also included in the review. Some results on the effects of thermal annealing of irradiationinduced SFTs are presented and discussed. The analysis of these observations leads to three significant conclusions: (a) during irradiation SFTs produced in the cascades are likely to interact with vacancies, self-interstitial atoms (SIAs) and SIA clusters, (b) interaction with SIAs and their clusters may cause both shrinkage and transformation of SFTs into Frank loops and (c) both during irradiation and annealing the lifetime of SFTs is determined not only by their thermal stability but also by their stability against transformation to loops. These facts must be taken into account in the theoretical treatments of damage accumulation. Published by Elsevier B.V. q Presented as an invited talk in the Kiritani Memorial Session at the ICFRM-11 held at Kyoto, Dec. 7-12, 2003.
Molecular dynamics is a useful tool for simulating cascade damage in metals and alloys, but the t... more Molecular dynamics is a useful tool for simulating cascade damage in metals and alloys, but the time scale accessible to molecular dynamics is only about 10-10 s. Kinetic Monte Carlo can be used to simulate annealing of cascade damage to permit analysis of the longer time evolution of cascade damage. We conducted a series of such annealing simulations in α-Fe. The number of surviving displacements before annealing is ~0.3 of the Norgett-Robinson-Torrens (NRT) value in the case of primary knock-on atoms with energy more than ~10 keV, and it decreased by ~30% during the annealing at 300 K because of recombination of vacancies and self-interstitial atoms. The recombination ratio increased as the annealing temperature increased. These results can be meaningfully applied in models such as mean field reaction rate theory used to simulate long-term radiation damage accumulation. We also demonstrated that 1D motion of small SIA clusters can substantially influence the long-term accumulation of cascade damage.
A mean-field, cluster dynamics model of the microstructure evolution in austenitic steels of ligh... more A mean-field, cluster dynamics model of the microstructure evolution in austenitic steels of light water reactors which reproduces the incubation period of swelling has been developed for the first time. In agreement with observations, it predicts that, although the void nucleation starts from the very beginning of irradiation, their growth beyond certain, relatively small size is delayed until the onset of the transient period of swelling. Such a delayed growth of voids is explained by the frequent interaction of voids randomly distributed over the volume with one-dimensionally migrating clusters of self-interstitial atoms, which are produced in cascades of atomic displacements. The incubation period of swelling is followed by the transient stage, when voids start to grow with increasing rate due to development of the experimentally-observed spatial correlations between voids and extended defects, such as second-phase precipitates and dislocations, which screen voids from the mobil...
In the climb-enabled glide model of irradiation creep, the plastic deformation is defined by the ... more In the climb-enabled glide model of irradiation creep, the plastic deformation is defined by the elastic deflections of pinned dislocations, which is an inconsistency. We argue that this relation is incorrect; instead, as in other pinning-unpinning-type models, the dislocations move from one set of obstacles to another, so that the inter-obstacle spacing determines creep rate, whereas the dependence on the applied stress is only implicit in the unpinning time.
The transport properties of ''interface-engineered'' edge-type YBa 2 Cu 3 O 7 Josephson junctions... more The transport properties of ''interface-engineered'' edge-type YBa 2 Cu 3 O 7 Josephson junctions are investigated in detail. We have investigated the dependence of the current-voltage characteristics on external magnetic field, temperature, and microwave irradiation and compare them to the resistively shunted junction model. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport of quasiparticles and Cooper pairs in the investigated ''interface-engineered'' junctions. We have studied the properties of junctions for which La doped YBa 2 Cu 3 O 7 is used for the superconducting electrodes. We will propose a model for the undoped and the La doped case which takes into account a barrier which consists of a series connection of a normal conducting layer and an insulator, containing superconducting microconstrictions.
ABSTRACT There are over 100 nuclear power plants operating in the U.S. that range in age from 15 ... more ABSTRACT There are over 100 nuclear power plants operating in the U.S. that range in age from 15 to 40 years old. May plants have obtained license extension for operation to 60 years, and operation beyond that is currently being assessed. As plants age, it is anticipated that some internal components may require repair by welding. Welding repair of irradiated nuclear reactor materials (such as austenitic stainless steels) is especially challenging because of the existence of large amounts of helium in the steel matrix after intense neutron exposure. Under the influence of high temperatures and high tensile stresses during welding, rapid formation of large helium bubbles can occur at grain boundaries, resulting in intergranular cracking in the heat-affected zone (HAZ). In this study, a refined helium bubble nucleation and growth kinetic model is developed based on the literature empirical model in the form of closed-form formulas. Especially, the material kinetic parameters needed by the empirical model are refined using a mesoscale master equation model for helium-vacancy cluster evolution in austenite matrix. Experimental results of high-temperature tensile tests of irradiated steel are used to validate the refined model, where the dependence of bubble size on the angle between grain boundary orientation and primary tensile stress direction are discussed. Finally, a three-dimensional thermal-stress model incorporating the refined helium model is used to calculate the transient temperature and stress conditions and the resulting helium bubble growth kinetics under those transients. The predicted bubble size distribution is compared with the experimental data available in the open literature. The effect of welding parameters on the cracking tendency is evaluated using the numerical model. The quantitative knowledge of helium bubble growth serves as much-needed prerequisite for understanding and mitigation of helium induced intergranular cracking during welding repair of irradiated nuclear reactor materials.
... (41) This nucleation rate corresponds to a specific steady-state SDF g(x) which satis-fies th... more ... (41) This nucleation rate corresponds to a specific steady-state SDF g(x) which satis-fies the equation ... The main prediction of the analytical analysis (Katz and Wiedersich 1971) is that theSDFf(x, t) of voids, starting from the delta function (equation (8)), approaches g(x). At any ...
Microstructural features evolving in crystalline solids from diffusion-reaction kinetics of mobil... more Microstructural features evolving in crystalline solids from diffusion-reaction kinetics of mobile components depend crucially on the dimension of the underlying diffusion process which is commonly assumed to be three-dimensional ͑3D͒. In metals, irradiation-induced displacement cascades produce clusters of selfinterstitials performing 1D diffusion. Changes between equivalent 1D diffusion paths and transversal diffusion result in diffusion-reaction kinetics between one and three dimensions. An analytical approach suggests a single-variable function ͑master curve͒ interpolating between the 1D and 3D limiting cases. The analytical result is fully confirmed by kinetic Monte Carlo simulations.
Static and molecular dynamics simulations have been used with different types of interatomic pote... more Static and molecular dynamics simulations have been used with different types of interatomic potentials to investigate the structure, properties and stability of self-interstitial atom (SIA) clusters produced during irradiation. In α-iron (Fe), faulted clusters of <110> dumbbells are unstable for all the potentials. The most stable SIA clusters are sets of parallel <111> crowdions. Large clusters of this type form
With respect to fuel the issues, we illustrate insights from atomistic simulations of defect ener... more With respect to fuel the issues, we illustrate insights from atomistic simulations of defect energetics, evolution and thermal transport properties. Specifically, we have used atomic-level simulation methods to determine the interaction of metallic fission products with the core of edge dislocations in UO2. Detailed analysis shows that the trends in segregation energy can be understood in terms of bulk behavior and continuum elasticity, opening up an efficient method for rapidly generating databases of segregation properties for input to mesoscale codes. We also analyze the results of molecular dynamics simulation of void formation in UO2 in the bulk and close to a grain boundary. The microstructural evolution during burnup, results in a progressive degradation in the thermal transport properties. We present the results of thermal conductivity calculations within the lattice dynamics framework in UO2 using electronic-structure calculations for the interatomic force constants. Compar...
This article was originally published in the Comprehensive Nuclear Materials published by Elsevie... more This article was originally published in the Comprehensive Nuclear Materials published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution's administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution's website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Golubov S.I.
Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena... more Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena at the mesoscale have typically been simulated using models based on reaction rate theory, such as mean field rate theory (MFRT) or Monte Carlo. These mesoscale models are appropriate for application to problems that involve intermediate-length scales, and timescales from those characteristic of diffusion to long-term microstructural evolution ($ls to years). Although the MFRT and Monte Carlo models can be used simulate the same phenomena, some of the details are handled quite differently in the two approaches. Models employing the rate theory have been extensively used to describe radiation-induced phenomena such as void swelling and irradiation creep. The primary approximations in such models are time and spatial averaging of the radiation damage source term, and spatial averaging of the microstructure into an effective medium. Kinetic Monte Carlo models can account for these spatial and temporal correlations; their primary limitation is the computational burden, which is related to the size of the simulation cell. Even with modern computers, the maximum simulation cell size and the maximum dose (typically much less than 1 dpa) that can be simulated are limited. In contrast, even very detailed MFRT models can simulate microstructural evolution for doses up 100 dpa or greater in clock times that are relatively short. Within the context of the effective medium, essentially any defect density can be simulated. A direct comparison of MFRT and object kinetic MC simulations has been made in the domain of point defect cluster dynamics modeling, which is relevant to the evolution (both nucleation and growth) of radiation-induced defect structures. Overall, the agreement between the two methods is best for irradiation conditions that produce a high density of defects (lower temperature and higher displacement rate) and for materials that have a relatively high density of fixed sinks such as dislocations.
This report presents results of investigations of damage accumulation during neutron irradiation ... more This report presents results of investigations of damage accumulation during neutron irradiation of pure iron and EUROFER 97 steel with or without prior helium implantation. The defect microstructure, in particular the cavities, was characterized using Positron Annihilation Spectroscopy (PAS) and Transmission Electron Microscopy (TEM). The PAS investigations revealed a clear difference between the He implantation effects in Fe and EUROFER 97 at 623 K. For both materials the mean positron lifetimes increased with He dose in the range 1 – 100 appm, although the increase was stronger for Fe than for EUROFER 97 and for both materials smaller for implantation at 623 K than at 323 K. This lifetime increase is due primarily to the formation of He bubbles. For He doses of 10 – 100 appm cavity sizes and densities in Fe were estimated to be 1.7 – 2.8 nm and 4 14×10 m, respectively. Neutron irradiation after He implantation in general leads to an increase of both cavity sizes and densities. Es...
ABSTRACTModels of radiation growth proposed to date are all based on the assumption that the prim... more ABSTRACTModels of radiation growth proposed to date are all based on the assumption that the primary damage is produced by neutron irradiation in the form of single defects. These models do not account for the features of the cascade damage: intra-cascade clustering of self‑interstitial atoms (SIAs) and their one‑dimensional diffusion. During the last twenty years, a ‘Production Bias Model’ has been developed, which shows that the damage accumulation in cubic metals depends crucially on the cascade properties. The cascades in hcp zirconium are similar to those in cubic crystals; hence the model can provide a realistic framework for the hcp metals as well. In this work we present such a model in application to low-temperature (below 300°C) radiation growth.
In order to help understand the evolution of stacking fault tetrahedra (SFTs) in a cascade produc... more In order to help understand the evolution of stacking fault tetrahedra (SFTs) in a cascade producing irradiation environment, the available information on the behaviour of SFTs observed under different experimental conditions has been briefly reviewed. Effects of thermal annealing and irradiations on the stability of pre-existing SFTs produced by quenching and aging are also included in the review. Some results on the effects of thermal annealing of irradiationinduced SFTs are presented and discussed. The analysis of these observations leads to three significant conclusions: (a) during irradiation SFTs produced in the cascades are likely to interact with vacancies, self-interstitial atoms (SIAs) and SIA clusters, (b) interaction with SIAs and their clusters may cause both shrinkage and transformation of SFTs into Frank loops and (c) both during irradiation and annealing the lifetime of SFTs is determined not only by their thermal stability but also by their stability against transformation to loops. These facts must be taken into account in the theoretical treatments of damage accumulation. Published by Elsevier B.V. q Presented as an invited talk in the Kiritani Memorial Session at the ICFRM-11 held at Kyoto, Dec. 7-12, 2003.
Molecular dynamics is a useful tool for simulating cascade damage in metals and alloys, but the t... more Molecular dynamics is a useful tool for simulating cascade damage in metals and alloys, but the time scale accessible to molecular dynamics is only about 10-10 s. Kinetic Monte Carlo can be used to simulate annealing of cascade damage to permit analysis of the longer time evolution of cascade damage. We conducted a series of such annealing simulations in α-Fe. The number of surviving displacements before annealing is ~0.3 of the Norgett-Robinson-Torrens (NRT) value in the case of primary knock-on atoms with energy more than ~10 keV, and it decreased by ~30% during the annealing at 300 K because of recombination of vacancies and self-interstitial atoms. The recombination ratio increased as the annealing temperature increased. These results can be meaningfully applied in models such as mean field reaction rate theory used to simulate long-term radiation damage accumulation. We also demonstrated that 1D motion of small SIA clusters can substantially influence the long-term accumulation of cascade damage.
A mean-field, cluster dynamics model of the microstructure evolution in austenitic steels of ligh... more A mean-field, cluster dynamics model of the microstructure evolution in austenitic steels of light water reactors which reproduces the incubation period of swelling has been developed for the first time. In agreement with observations, it predicts that, although the void nucleation starts from the very beginning of irradiation, their growth beyond certain, relatively small size is delayed until the onset of the transient period of swelling. Such a delayed growth of voids is explained by the frequent interaction of voids randomly distributed over the volume with one-dimensionally migrating clusters of self-interstitial atoms, which are produced in cascades of atomic displacements. The incubation period of swelling is followed by the transient stage, when voids start to grow with increasing rate due to development of the experimentally-observed spatial correlations between voids and extended defects, such as second-phase precipitates and dislocations, which screen voids from the mobil...
In the climb-enabled glide model of irradiation creep, the plastic deformation is defined by the ... more In the climb-enabled glide model of irradiation creep, the plastic deformation is defined by the elastic deflections of pinned dislocations, which is an inconsistency. We argue that this relation is incorrect; instead, as in other pinning-unpinning-type models, the dislocations move from one set of obstacles to another, so that the inter-obstacle spacing determines creep rate, whereas the dependence on the applied stress is only implicit in the unpinning time.
The transport properties of ''interface-engineered'' edge-type YBa 2 Cu 3 O 7 Josephson junctions... more The transport properties of ''interface-engineered'' edge-type YBa 2 Cu 3 O 7 Josephson junctions are investigated in detail. We have investigated the dependence of the current-voltage characteristics on external magnetic field, temperature, and microwave irradiation and compare them to the resistively shunted junction model. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport of quasiparticles and Cooper pairs in the investigated ''interface-engineered'' junctions. We have studied the properties of junctions for which La doped YBa 2 Cu 3 O 7 is used for the superconducting electrodes. We will propose a model for the undoped and the La doped case which takes into account a barrier which consists of a series connection of a normal conducting layer and an insulator, containing superconducting microconstrictions.
ABSTRACT There are over 100 nuclear power plants operating in the U.S. that range in age from 15 ... more ABSTRACT There are over 100 nuclear power plants operating in the U.S. that range in age from 15 to 40 years old. May plants have obtained license extension for operation to 60 years, and operation beyond that is currently being assessed. As plants age, it is anticipated that some internal components may require repair by welding. Welding repair of irradiated nuclear reactor materials (such as austenitic stainless steels) is especially challenging because of the existence of large amounts of helium in the steel matrix after intense neutron exposure. Under the influence of high temperatures and high tensile stresses during welding, rapid formation of large helium bubbles can occur at grain boundaries, resulting in intergranular cracking in the heat-affected zone (HAZ). In this study, a refined helium bubble nucleation and growth kinetic model is developed based on the literature empirical model in the form of closed-form formulas. Especially, the material kinetic parameters needed by the empirical model are refined using a mesoscale master equation model for helium-vacancy cluster evolution in austenite matrix. Experimental results of high-temperature tensile tests of irradiated steel are used to validate the refined model, where the dependence of bubble size on the angle between grain boundary orientation and primary tensile stress direction are discussed. Finally, a three-dimensional thermal-stress model incorporating the refined helium model is used to calculate the transient temperature and stress conditions and the resulting helium bubble growth kinetics under those transients. The predicted bubble size distribution is compared with the experimental data available in the open literature. The effect of welding parameters on the cracking tendency is evaluated using the numerical model. The quantitative knowledge of helium bubble growth serves as much-needed prerequisite for understanding and mitigation of helium induced intergranular cracking during welding repair of irradiated nuclear reactor materials.
... (41) This nucleation rate corresponds to a specific steady-state SDF g(x) which satis-fies th... more ... (41) This nucleation rate corresponds to a specific steady-state SDF g(x) which satis-fies the equation ... The main prediction of the analytical analysis (Katz and Wiedersich 1971) is that theSDFf(x, t) of voids, starting from the delta function (equation (8)), approaches g(x). At any ...
Microstructural features evolving in crystalline solids from diffusion-reaction kinetics of mobil... more Microstructural features evolving in crystalline solids from diffusion-reaction kinetics of mobile components depend crucially on the dimension of the underlying diffusion process which is commonly assumed to be three-dimensional ͑3D͒. In metals, irradiation-induced displacement cascades produce clusters of selfinterstitials performing 1D diffusion. Changes between equivalent 1D diffusion paths and transversal diffusion result in diffusion-reaction kinetics between one and three dimensions. An analytical approach suggests a single-variable function ͑master curve͒ interpolating between the 1D and 3D limiting cases. The analytical result is fully confirmed by kinetic Monte Carlo simulations.
Static and molecular dynamics simulations have been used with different types of interatomic pote... more Static and molecular dynamics simulations have been used with different types of interatomic potentials to investigate the structure, properties and stability of self-interstitial atom (SIA) clusters produced during irradiation. In α-iron (Fe), faulted clusters of <110> dumbbells are unstable for all the potentials. The most stable SIA clusters are sets of parallel <111> crowdions. Large clusters of this type form
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