Primary Knock-on Atom Energy Research Articles

An insight into radiation resistance of D019 Ti3Al intermetallics

A molecular dynamics study of radiation damage created in collision cascades in D019 Ti3Al intermetallic compound and a Ti-Al binary disordered solid solution with the same chemical composition is carried out. Collision cascades are initiated by either Al or Ti primary knock-on atoms (PKA) with PKA energy 5 keV ≤EPKA≤ 20 keV in the two materials at temperature T ranging from 100 K to 900 K. A series of at least 32 different cascades for each (EPKA, T) set was simulated in order to imitate an isotropic spatial and random temporal distribution of PKAs, generate representative sampling and obtain statistically reliable quantitative results. The numbers of Frenkel pairs, 〈NFPα2〉 and 〈NFPα〉, formed in D019 Ti3Al intermetallics and Ti-25at.%Al disordered solid solution, respectively, were averaged over collision cascades with the same (EPKA, T) and used to quantify the radiation resistance against primary damage formation. It is shown that the relationship 〈NFPα2〉≤〈NFPα〉 is satisfied under all simulation conditions, i.e. D019 ordered crystal structure of Ti3Al intermetallics is an important feature affecting its radiation tolerance. Nevertheless, a relatively high resistance against the formation of primary radiation defects has been retained even after complete disordering of D019 Ti3Al, which points out to the existence of additional mechanisms governing its radiation tolerance.

Multi-model quantification of defects in irradiated lithium titanate

Lithium based ceramics (Li2TiO3, Li4SiO4 etc.) in the packed pebble form, used in breeder blankets to breed tritium, are promising candidates for future fusion reactors. Neutron induced trap sites in the ceramic breeder affect the diffusion and inventory of tritium and thereby influence the quantity of tritium recovered. In the present work, a multi model framework has been employed to quantify the number of defects, which can be used to evaluate trap-sites, due to neutron irradiation in porous Li2TiO3 pebbles. This framework is based on a three stage calculation viz. (i) radiation transport, (ii) obtaining the primary knock-on atom (PKA) energy spectrum and (iii) quantifying the ensuing damage by collision cascade simulations. The output of one stage is coupled to the next stage and finally the defect distribution is obtained. We use radiation transport calculations to obtain the neutron spectrum in ceramic pebbles. This is used as an input to SPECTER, a code that uses evaluated cross-sections, to obtain the Primary Knock-on Atom (PKA) energy spectrum. The PKA energy spectrum is then used to find the number and distribution of vacancies and displacements in porous Li2TiO3 pebbles using binary collision approximation (BCA) based Monte Carlo simulations. The vacancy distribution and the number of vacancies in Li2TiO3 have been evaluated for the ITER neutron source using the above frame work. The PKA energy spectrum of Li, Ti and O PKAs, have been obtained using SPECTER, and their average energies are 680, 120 and 320 keV respectively. The collision cascade simulations shows the expected Bragg peak for the number of defects created and has a range of 6–8 microns for O and Li PKA respectively and a fraction of a micron for Ti PKA. The total number of displaced atoms which is equal to the number of vacancies, at the end of the collision cascades, obtained using the above framework has also been used to estimate the displacements per atom (dpa) in Li2TiO3 and this shows a good agreement with the dpa calculated using the NRT model.

Atomistic simulation of the effect of carbon content and carbon-rich region on irradiation response of α-Fe on picosecond timescale

α-Fe with low carbon content is a base material which is commonly used in manufacturing of Reactor Pressure Vessel (RPV) of commercial nuclear power plants. Carbon is generally diffused to α-Fe matrix to improve some of its mechanical properties. The presence of carbon may alter the irradiation response of the steel. In the current study, using molecular dynamics simulations, we have investigated the influence of carbon (∼in either dispersed form or carbon-rich region as chain) in the primary damage states of α-Fe low carbon steels. It is found that carbons in dispersed form have no significant effect on the self-interstitial atoms (SIAs) in α-Fe. While, carbon-rich (C-rich as pseudo-precipitation) region clearly increased the number of SIAs in bulk. A simple power formula was proposed to estimate the number of defects as a function of primary knock-on atom (PKA) energy and the number of atoms in the C-rich region.Cluster analyses of the defects formed in α-Fe containing 0–0.1 at.% dispersed carbon and with a C-rich region (with carbon chain of 0–100 atoms positioned in octahedral positions) showed that the presence of carbon in dispersed form had no effect on the average number of defect clusters (both the SIAs and the vacancies) in α-Fe containing 0–0.1 at.%. On the other hand, the average number of defect clusters for the α-Fe containing C-rich region apparently increases with increasing the number of carbon atoms in carbon chain. In addition, the average number of the interstitial clusters was significantly higher than that of the vacancy clusters.

Radiation Defects in Aluminum: MD Simulations of Collision Cascades in the Bulk of Material

The molecular dynamics method has been used to simulate displacement cascades created by primary knock-on atoms (PKAs) with energies EPKA= 5, 10, 15, and 20 keV in aluminum at temperatures T = 100, 300, and 600 K. A series of 24 cascades was simulated for each pair of the parameters (EPKA, T) to ensure a representative sampling. The number of Frenkel pairs, the fraction of vacancies (evac) and self-interstitial atoms (SIA) (eSIA) in point defect clusters, the average yield of vacancy (Yvac) and SIA (YSIA) clusters per cascade, and the average relaxation time τc have been determined as a function of (EPKA, T). It has been shown that displacement cascades of in aluminum decompose into several sub-cascades along PKA trajectory. Such a spatial structure of cascades is responsible for the absence of the dependence of the values of 〈evac〉, 〈eSIA〉, 〈Nvac〉, 〈NSIA〉, and τc on the energy of PKAs.

Characterization of radiation damage in TiO2 using molecular dynamics simulations

Molecular dynamics simulations are carried out to characterize irradiation effects in TiO2 rutile, for wide ranges of temperatures (300–900 K) and primary knock-on atom (PKA) energies (1–10 keV). The number of residual defects decreases with increased temperature and decreased PKA energy, but is independent of PKA type. In the ballistic phase, more oxygen than titanium defects are produced, however, the primary residual defects are titanium vacancies and interstitials. Defect clustering depends on the PKA energy, temperature, and defect production. For some 10 keV PKAs, the largest cluster of vacancies at the peak of the ballistic phase and after annealing has up to ≈1200 and 100 vacancies, respectively. For the 10 keV PKAs at 300 K, the energy storage, primarily in residual Ti vacancies and interstitials, is estimated at 140–310 eV. It decreases with increased temperature to as little as 5–180 eV at 900 K. Selected area electron diffraction patterns and radial distribution functions confirm that although localized amorphous regions form during the ballistic phase, TiO2 regains full crystallinity after annealing.

Molecular dynamics simulations of the primary irradiation damage in Zirconium

Atom-scale numerical calculations were performed to investigate the damage behavior in close-packed hexagonal zirconium (HCP-Zr) by primary irradiation through a molecular dynamics (MD) study. The influences of Primary Knock-on Atom (PKA) energy (EPKA), the PKA incident direction and the ambient temperature (T) on the cascade collision were studied comprehensively. The results show that the athermal recombination corrected displacements per atom (arc-dpa) model is more accurate in comparison with the Norgett-Robinson-Torrens (NRT) model when predicting the steady vacancy (Ns). It was found that the vacancy peak (Npeak), the peak time (tpeak), and the steady time (ts) increase as EPKA and T increase. The steady vacancy also increases with the increase of EPKA. It was also found that Ns increases and subsequently decreases by increasing T, suggesting that there is a suitable temperature to maximize Ns when EPKA is constant. In addition, it was discovered that the PKA incident direction has little effect on vacancy dynamic history. It was proved that EPKA can worsen the irradiation damage in crystal Zr, but T can relieve such damage. In addition, the incident direction was found to have an insignificant effect on the damage. This study highlights that the steady vacancy concentration (C) can characterize the material irradiation damage.

Effects of stacking fault energies on formation of irradiation-induced defects at various temperatures in face-centred cubic metals

ABSTRACTBy using the six sets of interatomic potentials for face-centred cubic metals that differ in the stacking fault energy (SFE) while most of the other material parameters are kept almost identical, we conducted molecular dynamics simulations to evaluate the effects of SFE on the defect formation process through collision cascades. The simulations were performed at 100, 300 and 600 K, with a primary knock-on atom energy of 50 keV. The number of residual defects is not dependent on the SFE at all the temperatures. For clusters of self-interstitial atoms (SIAs), their clustering behaviour does not depend on the SFE, either. However, the ratio of glissile SIA clusters tends to decrease with increasing SFE. This is because perfect loops, the edges of which split into two partial dislocations with stacking fault structures between them in most cases, prefer to form at lower SFEs. The enhanced formation of glissile SIA clusters at lower SFEs can also be observed even at increased temperature. Because most large vacancy clusters have stacking fault structures, they preferentially form at lower SFE; however, it is observed only at the lowest temperature, where the mean size increases with decreasing SFE. At higher temperatures, because of their extremely low number density, the vacancy clustering behaviour does not depend on the SFEs.

A combined BCA-MD method with adaptive volume to simulate high-energy atomic-collision cascades in solids under irradiation

We present a method combining the Binary Collision Approximation (BCA) and Molecular Dynamics (MD) with adaptive volume to simulate high-energy collision cascades in solids under irradiation. The results obtained with the BCA-MD method with adaptive volume are in very good agreement with those obtained with full MD calculations. The BCA-MD predicts very well in a broad range of Primary Knock-on Atom (PKA) energies the different phases of evolution of the cascade, the number of Frenkel pairs, the fraction of point defects in clusters and their size distribution. The BCA-MD method proposed here achieves a considerable acceleration of MD simulations with a speedup factor that increases with the PKA energy. For a PKA energy of 80 keV in Fe, the speedup measured is higher than 30. For a PKA energy of 0.5 MeV in Fe, by comparison with MD simulations performed by other authors, we estimate that the BCA-MD accelerates MD simulations by a factor 312.

Propagation of uncertainties in basic nuclear reaction data to uncertainties in the parameters of primary radiation damage by neutrons

The primary radiation damage formation in materials subjected to neutron radiation is very sensitive to the energy spectra of primary knock-on atoms (PKAs) and their collision cascade in the crystal lattice. The estimated values of the primary radiation damage metrics, such as displacements per atom (dpa) cross sections, integrated dpa, PKA spectrum and spectrum averaged PKA energy are uncertain because of uncertainties present in both the basic nuclear reaction data and the displacement damage model parameters. The Total Monte Carlo (TMC) methodology, which uses the random evaluated nuclear data files for nuclides, is well recognized to propagate the uncertainties in basic nuclear data to the derived quantities in nuclear reactor physics. The uncertainties in the above mentioned primary radiation damage parameters are obtained in this study by propagating the uncertainties in basic evaluated nuclear data, following the TMC methodology. The knowledge of uncertainties in the spectrum averaged PKA energies also enables us to ascertain the uncertainties in the production of vacancies through self-ion simulations using SRIM-2013 code. The present study is an outcome of our systematic approach towards establishing the procedure. This includes pre-processing a large number of TENDL nuclear data random files using NJOY-2016, developing an indigenous code CRaD for handling these point and multigroup data libraries and computing primary radiation damage metrics and their associated uncertainties including energy-energy and nuclear reaction channel-channel correlation matrices. The uncertainties in dpa cross sections and PKA spectra due to uncertainties in basic nuclear data are found to be comparatively larger than the uncertainties in respective integrated parameters; viz. dpa and average PKA energy. This study also confirms the non-Gaussian distributions of derived primary radiation damage parameters.

Distribution of defect clusters in the primary damage of ion irradiated 3C-SiC

We report a statistical analysis of sizes and compositions of clusters produced in cascades during irradiation of SiC. The results are obtained using molecular dynamics (MD) simulations of cascades caused by primary knock-on atoms (PKAs) with energies between 10 eV and 50 keV. The results are averaged over six crystallographic directions of the PKA and integrated over PKA energy spectrum derived from the Stopping and Range of Ions in Matter (SRIM) code. Specific results are presented for 1 MeV Kr ion as an example of an impinging particle. We find that distributions of cluster size n for both vacancies and interstitials obey a power law f=A/nS and these clusters are dominated by carbons defects. The fitted values of A and S are different than values reported for metals, which can be explained through different defect energetics and cascade morphology between the two classes of materials. In SiC, the average carbon ratio for interstitial clusters is 91.5%, which is higher than the ratio of C in vacancy clusters, which is 85.3%. Size and composition distribution of in-cascade clusters provide a critical input for long-term defect evolution models.

Molecular dynamics simulations of displacement cascades in nanotwinned Cu

With large-scale molecular dynamics simulations, we investigate systematically displacement cascades in nanotwinned Cu with different twin spacings. Coherent twin boundaries (CTBs) are proven to be effective defect sinks, and they exhibit preferential absorption of interstitials. The existence of twin boundary affected zone is confirmed, and its width depends on the energy of primary knock-on atom. When stacking fault tetrahedra nucleate near CTBs, their growth are suppressed, resulting in truncation. CTB migration is observed, which is induced by transient internal stress arising from collision cascades.

Molecular Dynamics Simulations of Radiation Damage in Ti-Al based Structural Intermetallics

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Atomistic simulations to characterize the influence of applied strain and PKA energy on radiation damage evolution in pure aluminum

Knowledge of defects generation, their mobility, growth rate, and spatial distribution is the cornerstone for understanding the surface and structural evolution of a material used under irradiation conditions. In this study, molecular dynamics simulations were used to investigate the coupled effect of primary knock-on atom (PKA) energy and applied strain (uniaxial and hydrostatic) fields on primary radiation damage evolution in pure aluminum. Cascade damage simulations were carried out for PKA energy ranging between 1 and 20 keV and for applied strain values ranging between −2% and 2% at the fixed temperature of 300 K. Simulation results showed that as the atomic displacement cascade proceeds under uniaxial and hydrostatic strains, the peak and surviving number of Frenkel point defects increases with increasing tension; however, these increments were more prominent under larger volume changing deformations (hydrostatic strain). The percentage fraction of point defects that aggregate into clusters increases under tension conditions; compared to the reference conditions with no strain, these increases are around 13% and 7% for interstitials and vacancies, respectively (under 2% uniaxial strain), and 19% and 11% for interstitials and vacancies, respectively (under 2% hydrostatic strain). Clusters formed of vacancies and interstitials were both larger under tensile strain conditions, with increases in both the average and maximum cluster sizes. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their size distributions under expansion/compression strain conditions were higher for higher PKA energies. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.

Molecular dynamics simulations of the coupled effects of strain and temperature on displacement cascades in α-zirconium

Abstract In this article, we conducted molecular dynamics simulations to investigate the effect of applied strain and temperature on irradiation-induced damage in alpha-zirconium. Cascade simulations were performed with primary knock-on atom energies ranging between 1 and 20 KeV, hydrostatic and uniaxial strain values ranging from −2% (compression) to 2% (tensile), and temperatures ranging from 100 to 1000 K. Results demonstrated that the number of defects increased when the displacement cascade proceeded under tensile uniaxial hydrostatic strain. In contrast, compressive strain states tended to decrease the defect production rate as compared with the reference no-strain condition. The proportions of vacancy and interstitial clustering increased by approximately 45% and 55% and 25% and 32% for 2% hydrostatic and uniaxial strain systems, respectively, as compared with the unstrained system, whereas both strain fields resulted in a 15–30% decrease in vacancy and interstitial clustering under compressive conditions. Tensile strains, specifically hydrostatic strain, tended to produce larger sized vacancy and interstitial clusters, whereas compressive strain systems did not significantly affect the size of defect clusters as compared with the reference no-strain condition. The influence of the strain system on radiation damage became more significant at lower temperatures because of less annealing than in higher temperature systems.

Primary radiation damage characterization of α-iron under irradiation temperature for various PKA energies

The understanding of radiation-induced microstructural defects in body-centered cubic (BCC) iron is of major interest to those using advanced steel under extreme conditions in nuclear reactors. In this study, molecular dynamics (MD) simulations were implemented to examine the primary radiation damage in BCC iron with displacement cascades of energy 1, 5, 10, 20, and 30 keV at temperatures ranging from 100 to 1000 K. Statistical analysis of eight MD simulations of collision cascades were carried out along each [110], [112], [111] and a high index [135] direction and the temperature dependence of the surviving number of point defects and the in-cascade clustering of vacancies and interstitials were studied. The peak time and the corresponding number of defects increase with increasing irradiation temperature and primary knock-on atom (PKA) energy. However, the final number of surviving point defects decreases with increasing lattice temperature. This is associated with the increase of thermal spike at high PKA energy and its long timespan at higher temperatures. Defect production efficiency (i.e., surviving MD defects, per Norgett-Robinson-Torrens displacements) also showed a continuous decrease with the increasing irradiation temperature and PKA energy. The number of interstitial clusters increases with both irradiation temperature and PKA energy. However, the increase in the number of vacancy clusters with PKA energy is minimal-to-constant and decreases as the irradiation temperature increases. Similarly, the probability and cluster size distribution for larger interstitials increase with temperature, whereas only smaller size vacancy clusters were observed at higher temperatures.

A study to compute integrated dpa for neutron and ion irradiation environments using SRIM-2013

Displacements per atom (dpa), estimated based on the standard Norgett–Robinson–Torrens (NRT) model, is used for assessing radiation damage effects in fast reactor materials. A computer code CRaD has been indigenously developed towards establishing the infrastructure to perform improved radiation damage studies in Indian fast reactors. We propose a method for computing multigroup neutron NRT dpa cross sections based on SRIM-2013 simulations. In this method, for each neutron group, the recoil or primary knock-on atom (PKA) spectrum and its average energy are first estimated with CRaD code from ENDF/B-VII.1. This average PKA energy forms the input for SRIM simulation, wherein the recoil atom is taken as the incoming ion on the target. The NRT-dpa cross section of iron computed with “Quick” Kinchin-Pease (K-P) option of SRIM-2013 is found to agree within 10% with the standard NRT-dpa values, if damage energy from SRIM simulation is used. SRIM-2013 NRT-dpa cross sections applied to estimate the integrated dpa for Fe, Cr and Ni are in good agreement with established computer codes and data. A similar study carried out for polyatomic material, SiC, shows encouraging results. In this case, it is observed that the NRT approach with average lattice displacement energy of 25 eV coupled with the damage energies from the K-P option of SRIM-2013 gives reliable displacement cross sections and integrated dpa for various reactor spectra. The source term of neutron damage can be equivalently determined in the units of dpa by simulating self-ion bombardment. This shows that the information of primary recoils obtained from CRaD can be reliably applied to estimate the integrated dpa and damage assessment studies in accelerator–based self-ion irradiation experiments of structural materials. This study would help to advance the investigation of possible correlations between the damages induced by ions and reactor neutrons.

Cascade Collision near the Grain Boundary of Fe-Cr Alloy by MD Simulation

Using molecular dynamics method to study the cascade collision for the case of PKA(Primary Knock-on Atom) atoms at different distance from the grain boundary(GB) of iron chromium alloy. It is found that the PKA atoms at the GB will produce a large size cluster (size from 11 to 409 ) consisting of interstitial and vacancies, and many small clusters (number from 5 to 50). The size and number of the cluster depend heavily on PKA energy, while depend weakly on temperature. The PKA atom at distance of 1nm from the GB, sometimes produces large size defect clusters both inside and outside the GB region. When the PKA atom is at 1nm, 2nm and even 3nm, 4nm from the GB, the GB will effectively absorb the interstitial atoms. It is found that the atomic ratio of Cr-interstitial to total interstitial produced at the GB region is much less than one at outside of GB region.

Effects of grain boundaries on irradiation-induced defects in tungsten by molecular dynamics simulations

The effects of two different symmetric tilt grain boundaries (GBs), Σ13[001](230) GB and Σ17[001](140) GB, on displacement cascade processes in tungsten were investigated using molecular dynamics simulations. By quantifying the number of interstitials and vacancies surviving after irradiation with the kinetic energy of primary knock-on atom energies of 1, 3 and 5 keV, respectively, in these simulations, it is found that the GBs have dual nature for radiation-induced defects: They absorb interstitials while leaving more vacancies to survive in the grains. The net effect is that the number of total surviving defects in the GB system is not always less than that in the single crystal. These defect behaviors are understood by quantitatively analyzing the recovery fraction of irradiation-induced defects, the time to reach steady state and the mobility of vacancies and interstitials. It is also found that the Σ17 GB is a more effective sink of radiation-induced point defects than the Σ13 GB. One of the main reasons is that the Σ17 GB has a higher GB energy.

Theoretical and experimental study of the dark signal in CMOS image sensors affected by neutron radiation from a nuclear reactor

The CMOS image sensors (CISs) are irradiated with neutron from a nuclear reactor. The dark signal in CISs affected by neutron radiation is studied theoretically and experimentally. The Primary knock-on atoms (PKA) energy spectra for 1 MeV incident neutrons are simulated by Geant4. And the theoretical models for the mean dark signal, dark signal non-uniformity (DSNU) and dark signal distribution versus neutron fluence are established. The results are found to be in good agreement with the experimental outputs. Finally, the dark signal in the CISs under the different neutron fluence conditions is estimated. This study provides the theoretical and experimental evidence for the displacement damage effects on the dark signal CISs.

Opportunities and limitations for ion beams in radiation effects studies: Bridging critical gaps between charged particle and neutron irradiations

Ion beams are indispensible for understanding radiation effects in materials, enabling access to conditions challenging to perform with neutrons. However, ion irradiations can produce potential artifacts from beam rastering, high displacement damage rate effective “temperature shifts”, subthreshold collisions, high ionization rates, and non-monoenergetic primary knock-on atom energies. The influence of near-surface denuded zones and implanted ion effects is analyzed, including diffusional broadening effects. At high ion irradiation energies, “swift heavy ion” effects can lead to enhanced defect production or recovery. Ion energies of 10–50MeV typically offer a good compromise for minimizing near-surface, implanted ion, and swift heavy ion effects.