Radiation-enhanced Diffusion Coefficient Research Articles

Computational determination of a primary diffusion mode in [formula omitted]U-10Mo under irradiation

Low enriched uranium (< 20 %235U)-molybdenum (U-Mo) monolithic fuel is the primary candidate for high-performance research and test reactors and is in the process of being qualified to replace highly-enriched uranium (≥ 20 %235U) fuel. As part of the qualification process, it is critical to understand and predict the behavior of fission gas bubbles under irradiation, which affects fuel swelling and fuel failure. Mechanistic fuel models are being developed that can both reproduce the existing experimental data for fuel swelling, and be further applied to irradiation conditions beyond the experimental scope. Diffusion of species under irradiation conditions is an important parameter in the mechanistic fuel models; however, no temperature-relevant experimental diffusion data exists. In the present work, radiation-enhanced diffusion coefficients of U, Mo, and Xe in γU-10wt.%Mo were calculated in the temperature range between 300 K and 1400 K via rate-theory models and molecular dynamics simulations with an embedded-atom method interatomic potential for the U-Mo-Xe system. Accordingly, total diffusion coefficients under relevant irradiation conditions are determined using previously obtained intrinsic thermal diffusion and radiation-driven diffusion coefficients, as well as the newly calculated radiation-enhanced diffusion coefficients presented herein. Radiation-enhanced diffusion of U and Mo was dominant in the intermediate temperature range, whereas radiation-enhanced diffusion of Xe did not significantly contribute to total diffusion of Xe at the relevant fission rate densities. Radiation-enhanced diffusion of Xe became faster than both intrinsic thermal diffusion and radiation-driven diffusion at a fission rate density of 5×1022fiss/m3/s, which is higher than the typical fission rate density range in research reactors. The temperature regime where radiation-enhanced diffusion of each element dominated was dependent on the fission rate density. The total diffusion coefficients of U, Mo, and Xe, updated in this work, will be utilized as parameters in the mechanistic fuel models to help predict the behavior of fission gas bubbles under irradiation more accurately.

Enhanced diffusion of Cr in 20Cr-25Ni type alloys under proton irradiation at 670 °C

Alloy 709 (Fe-20Cr-25Ni-1.5Mo-0.26Nb in weight percentages) and its model alloy (Fe-20Cr-25Ni) were subjected to 2 MeV proton irradiation up to 1.5 displacements per atom at ∼670 °C for 90 h. Significant compositional changes were observed in the irradiated zone of both alloys, which was correlated with the radiation-enhanced diffusion. The radiation-enhanced diffusion coefficient of Cr was experimentally determined by fitting the Cr depletion profiles and theoretically analyzed considering vacancy-assisted lattice diffusion, dislocation pipeline diffusion, etc. The estimation of the radiation-enhanced diffusion in the 20Cr-25Ni type alloy in this study is useful in predicting its performance in radiation damage environment. The methodology of investigating radiation-enhanced diffusion adds to the nuclear-material screening capability of ion-irradiation technologies.

Stability of β″ nano-phases in Al-Mg-Si(-Cu) alloy under high dose ion irradiation

The microstructure of a 6061-T6 Al alloy subjected to ion irradiation at 95 and 165 displacements per atom (dpa) has been evaluated by transmission electron microscopy and atom-probe tomography. The initial microstructure of the alloy is dominated by needle-shaped β″ precipitates. After 95 dpa irradiation, the β″ precipitates display a slight lattice distortion and are partially dissolved, together with the formation of a new phase. After 165 dpa irradiation, the β″ precipitates are completely dissolved, the new phase has grown and a high density of clusters rich in Mg, Si, Cu and Cr is observed. The determination of ballistic versus radiation enhanced diffusion coefficients shows that enhanced diffusion is predominant for β” dissolution. The formation and growth of the new particles may be caused by radiation induced segregation. Solute drag by vacancies or mixed dumbbell interstitials migration could explain the diffusion of some elements as Si or Cr towards the new particles.

Time dependence of silica optical properties during the implantation of fast hydrogen ions: Experiment

The luminescence technique has been applied to study long-time hydrogen ion implantation of silica. It was found that the changes of the spectrum shape for the luminescence radiation are caused by diffusion of hydrogen from the implanted layer. Analysis of the luminescence radiation spectra showed that hydrogen passivated the non-bridging oxygen hole centers (HBOHCs) and modified the oxygen deficiency centers (ODC).As the absorption dose grows a different behavior of luminescence in the blue (maximum at 456nm) and the red (maximum at 645nm) bands is demonstrated. Luminescence radiation intensity associated with the ODCs increased during the implantation due to the growth of modificated ODC’s contribution. At the same time luminescence radiation intensity associated with NBOHCs had a non-monotonic dependence on the absorption dose.On the basis of our theoretical model and the experimental data, we calculated the average radiation-enhanced diffusion coefficient of hydrogen, the product of average value of the cross-section of the radiation defect creation and the initial portion of defects and the reaction rate constant for non-bridging oxygen passivation by the hydrogen.

Enhanced diffusion under alpha self-irradiation in spent nuclear fuel: Theoretical approaches

Various theoretical approaches have been developed in order to estimate the enhanced diffusion coefficient of fission products under alpha self-irradiation in spent nuclear fuel. These simplified models calculate the effects of alpha particles and recoil atoms on mobility of uranium atoms in UO 2. They lead to a diffusion coefficient which is proportional to the volume alpha activity with a proportionality factor of about 10 −44 (m 5). However, the same models applied for fission lead to a radiation-enhanced diffusion coefficient which is approximately two orders of magnitude lower than values reported in literature for U and Pu. Other models are based on an extrapolation of radiation-enhanced diffusion measured either in reactors or under heavy ion bombardment. These models lead to a proportionality factor between the alpha self-irradiation enhanced diffusion coefficient and the volume alpha activity of 2 × 10 −41 (m 5).

Mass transfer of metal ion implantation into metal targets at elevated temperatures

A mass transfer model for metal ion implantation into a metal target at elevated temperatures has been built up based on transport of ions in matter and radiation enhanced diffusion. It is used to calculate concentration-depth profiles and compositional changes of the implanted species. The ion implantation at elevated temperatures was simulated by a dynamic Monte Carlo (MC) method, which takes into account a local saturation in the crystalline target by using a maximum atomic fraction allowed in the matrix. For the diffusion process, the transport of the implanted species was obtained from the diffusion equations for the implanted species and nonequilibrium vacancies. The radiation enhanced diffusion coefficient was obtained by taking into account linear annealing of the defects. A nonequilibrium vacancy source function and surface sputtering were introduced into the diffusion equations. Concentration-depth profiles of Cr, Fe and Ni ions implanted into Al at a temperature range from 200 to 510 °C were calculated. The calculated results principally were consistent with measured concentration-depth profiles obtained by Rutherford backscattering spectroscopy (RBS). In some cases deviations occur, which are discussed.

Radiation-enhanced diffusion and fission gas release from recrystallised grains in high burn-up \\hbox{UO}_{2} nuclear fuel

The effective diffusion coefficient that gives a steady-state xenon concentration of 0.2-0.3wt% in the recrystallised grains of high burn-up UO 2 fuel is calculated to lie in the range 10 m 24 to 10 m 22 m 2 s m 1 . These values are one to three orders of magnitude lower than the value currently accepted for the radiation-enhanced diffusion coefficient. The time required to reach the steady-state concentration depends on the local fission rate, the grain size distribution and the precise magnitude of the radiation-enhanced diffusion coefficient, and can take from 2 to 10 years. Additional calculations reveal that substantially less than 10% of the fission gas inventory is released from the original UO 2 grains in the outer region of the fuel prior to recrystallisation. In contrast, with a diffusion coefficient of 10 m 22 m 2 s m 1 more than 80% of the fission gas is released from the recrystallised grains of the high burn-up structure in one year.

Radiation enhanced diffusion in MgO

Radiation enhanced diffusion and ion beam mixing of O18, Ca, and Zn buried tracer layers in MgO, grown by molecular beam epitaxy, have been measured following irradiation with 2.0 MeV Kr+ and 1.0 MeV Ne+, He+, and H+ from 30 to 1500 °C. The ion beam mixing parameter varied between 1.0 and 5.0 Å5 eV−1 for the different tracers at 30 °C and increased slowly with increasing temperature. These results are consistent with ballistic mixing. In the highest temperature range investigated, 1350–1500 °C, the radiation enhanced diffusion coefficient for O18 was proportional to the square root of the irradiation flux and displayed an apparent activation enthalpy of 1.2 eV. These dependencies on flux and temperature are indicative of recombination-limited kinetics, with the measured activation enthalpy representing one-half the migration enthalpy of anion vacancies. From 1150 to 1350 °C an activation enthalpy of 4.1 eV was obtained. The unexpectedly high value is attributed to the dissociation energy of small vacancy clusters. Measurements on the cation sublattice were limited to temperatures below 900 °C owing to the excessive thermal diffusion associated with extrinsic vacancies, which are present for trivalent impurity charge compensation.

Sink Strength Evolution of Heavy Ion Irradiated Nickel

AbstractAtom transport under irradiation is determined by the concentration of freely migrating defects, which depends on the dynamical equilibrium between production and annihilation rates. In order to determine effective values of both of these quantities for the case of ion irradiation, spatially resolved self-diffusion measurements were performed on single crystals of nickel which contained several thin tracer layers at different depths.For fixed depth the radiation-enhanced diffusion coefficient (DK) was determined as function of displacement rate (K0) and fluence (Φ). The DK essentially representing the ratio of the rates of production and annihilation was found to be proportional to K0 for 800 K irradiation temperature and to K00.4for Ni and K00.4for Kr irradiation at 950 K. It is independent of Φ for 800 K and decreases with increasing Φ for 950 K.

Atom transport under ion irradiation

Diffusion coefficients in nickel single crystals under heavy ion irradiation were measured near the irradiated surface by using multi-layer specimens. Analysis of the depth dependence of these diffusion coefficients yields information on the effective fraction η of the displacement rate, with which transporting, i.e., ‘freely migrating’ defects are produced, and on the radiation-induced sink strength. Upper limits of η = 7% are estimated for a self-ion irradiation at 950 K. By increasing the irradiation temperature from 800 to 950 K the effective sink strength was observed to decrease while η increases. The relation between swelling rates and the magnitude of the radiation-enhanced diffusion coefficient is briefly discussed.

Composition profiles at initial stage of altered layer formation on Cu-Pt alloy surfaces under Ar+ ion bombardment

The dynamic process of subsurface altered layer formation in a Cu-Pt alloy under Ar+ ion bombardment has been studied by sequential Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS) measurements. For this study, we recently developed a compact coevaporator, which enables Cu and Pt to be deposited at a constant composition ratio (within ±3 at. %) onto a Si substrate cooled to liquid-nitrogen temperature. The average surface composition of the alloy film was monitored by AES throughout the deposition procedure, and the composition of the topmost atomic layer was measured by ISS right after the end of the film deposition. The data show that the topmost atomic layer was considerably Cu-rich while subsurface atomic layers were of constant composition. During Ar+ ion bombardment, however, the surface composition of Cu, measured by sequential AES and ISS, gradually decreased with time toward a steady-state value. The temporal evolution of the composition profile is described. The surface-segregation constant and radiation-enhanced diffusion coefficient for the Cu-Pt alloy under Ar+ ion bombardment are inferred from the measured composition profile.

Enhanced Diffusion of High-Temperature Implanted Aluminum in Silicon Carbide

AbstractThe diffusion of aluminum in silicon carbide during high-temperature A1+ ion implantation was studied using secondary ion mass spectrometry (SIMS). Transmission electron microscopy (TEM) has been used to determine the microstructure of the implanted sample. A 6H-SiC wafer was implanted at a temperature of 1800 °C with 40 keV Al ions to a dose of 2 x 1016 cm-2. It was established that an Al step-like profile starts at the interface between the crystal region and the damaged layer. The radiation enhanced diffusion coefficient of Al at the interface was determined to be Di = 2.8 x 10-12 cm2/s, about two orders of magnitude higher than the thermally activated diffusion coefficient. The Si vacancy-rich near-surface layer formed by this implantation condition is believed to play a significant role in enhanced Al diffusion.

Sputtering and radiation-enhanced segregation in amorphous CuTi alloys

Binary amorphous copper-titanium alloys of compositions 44, 53, 65 and 69 at% copper were sputtered with 2 keV Ar + ions. The surface concentrations were then determined by Auger electron spectroscopy and ion scattering spectrometry. It was found that Cu is sputtered preferentially as it segregates to the surface. A recently developed model for radiation-enhanced surface segregation was used to fit the experimental data. The segregation energy decreases with increasing Cu concentration from 2500 Jmol −1 to zero for the highest concentration, in qualitative agreement with several predictions on the segregating species in the alloy. The radiation-enhanced diffusion coefficients have values in the range (2–3) × 10 −20 m 2 s . From the concentration profiles the depth of the altered layer was estimated to about 4 nm which coincides well with the range of the defect distribution caused by the argon ions according to TRIM calculations. A Cu Ti sputtering ratio of 6 was needed for the best possible fit of the results.

Preferential sputtering and radiation enhanced segregation in palladium-platinum alloys

Five palladium-platinum alloys of different compositions ranging from 10 to 90 at% Pd were sputtered with 1 keV Ar + ions. The surface concentrations were then determined with Auger electron spectroscopy (AES) and low energy ion scattering spectroscopy (ISS). The observed concentrations differed largely from bulk values. This difference is explained in terms of the difference in sputtering yields for platinum and palladium and the radiation enhanced segregation of palladium. The surface was then mechanically cleaned in situ. The surface concentrations measured subsequently corresponded excellently with the bulk values confirming the calibration procedure for the calculation of the surface concentrations. A recently developed model for surface segregation was used to fit the experimental data. Values for the surface segregation energy Δ G and the radiation enhanced diffusion coefficient D were obtained in the range 2000–8000 J mol and (1.2–8.0)×10 −20 m 2 s , respectively.

Preferential sputtering and radiation enhanced segregation in palladium-platinum alloys

Five palladium-platinum alloys of different compositions ranging from 10 to 90 at% Pd were sputtered with 1 keV Ar+ ions. The surface concentrations were then determined with Auger electron spectroscopy (AES) and low energy ion scattering spectroscopy (ISS). The observed concentrations differed largely from bulk values. This difference is explained in terms of the difference in sputtering yields for platinum and palladium and the radiation enhanced segregation of palladium. The surface was then mechanically cleaned in situ. The surface concentrations measured subsequently corresponded excellently with the bulk values confirming the calibration procedure for the calculation of the surface concentrations. A recently developed model for surface segregation was used to fit the experimental data. Values for the surface segregation energy ΔG and the radiation enhanced diffusion coefficient D were obtained in the range 2000–8000 Jmol and (1.2–8.0)×10−20m2s, respectively.

Point defect sinks in self-ion-irradiated nickel: A self-diffusion investigation

The diffusion coefficient of 63Ni in pure nickel after irradiation with 300 keV Ni ions has been measured directly using a secondary ion mass spectrometer. The calculated displacement rate for this irradiation ranged from 1.2×10−5 to 3.1×10−2 dpa/s, the dose from 0.2 to 102 dpa, and the temperatures from 293 to 950 K. Between 293 and 650 K the irradiation-induced diffusion coefficient is temperature independent for a displacement rate of 1.2×10−2 dpa/s. In this temperature range mass transport by atomic mixing prevails over diffusion via point defects (radiation-enhanced diffusion). Normalized to the displacement rate K′ the diffusion coefficient describing atomic mixing attains a value of Dmix /K′=1.3×10−18 m2 /dpa. The temperature and displacement rate dependence of the radiation-enhanced diffusion coefficient is discussed in the framework of a rate equation model. The analysis yields a production rate of freely migrating defects of about 1.5% of the calculated displacement rate. The effective concentration of point defect sinks was derived for nickel under heavy ion irradiation. This effective sink concentration Cs showed no dependence on displacement rate but a considerable temperature dependence, e.g., Cs =2×10−6 at 850 K and Cs =2.5×10−7 at 950 K. The conditions for a stationary effective sink concentration in self-ion irradiated nickel are discussed.

Radiation-enhanced diffusion in amorphous Ni-Zr alloys.

Radiation-enhanced diffusion of Cu and Au tracer impurity atoms during ${\mathrm{Kr}}^{+}$ irradiation has been measured in amorphous ${\mathrm{Ni}}_{50}$${\mathrm{Zr}}_{50}$ and ${\mathrm{Ni}}_{35}$${\mathrm{Zr}}_{65}$ alloys in the range 80-573 K. At low temperatures, the spreading of the thin impurity layers was due to ion beam mixing and was independent of temperature. Above \ensuremath{\cong}400 K, the diffusion coefficient of Cu in ${\mathrm{Ni}}_{50}$${\mathrm{Zr}}_{50}$ shows Arrhenius behavior with an apparent activation enthalpy of \ensuremath{\cong}0.55 eV, and it depends on the square root of dose rate. The radiation-enhanced diffusion coefficient is greater for Cu than Au and is also larger in the Zr-rich alloy.

Sputter-induced compositional modifications in a Ni-Au alloy

Surface and subsurface composition changes induced by heating and by ion sputtering of a Ni-6 at% Au alloy at temperatures between 25 and 600°C were measured in situ, using the ion-scattering-spectroscopy technique. Upon heating, Au atoms segregated to the alloy surface; the enthalpy and entropy of Gibbsian adsorption were found to be −0.45 eV and −1.09K. During sputtering, the steady-state surface composition was noticeably temperature-dependent above ~400°C, which could be interpreted in terms of significant contributions of the second atom layer to the sputtered-atom flux. Steady-state concentration profiles measured after rapid cooling of the target to room temperature were analyzed to gain information about the temperature dependence of the effective altered-layer thickness, from which radiation-enhanced diffusion coefficients in the bombarded alloy were determined.

Measurements of radiation-induced segregation and diffusion in an ion irradiated alloy using implanted manganese

The radiation-induced segregation and diffusion of 30 keV managenese implants in nickel under steady 75 keV Ni + bombardment a. 500°C has been measured for doses up to 7.2 × 10 19 m −2. Under irradiation the Mn implant profile broadens by radiation-enhanced diffusion and its peak moves inwards. This movement is due to the drift of Mn atoms in the point-defect flux set up by ion bombardment. The drift velocity has been measured as a function of depth. The radiation-enhanced diffusion coefficient of managenese solute can also be calculated directly from the experimental data over a limited range of depths. The behaviour of the manganese is interpreted in terms of underlying point-defect concentration profiles, calculated using rate theory or a cascade diffusion theory. The ratio of the measured rates of solute drift and diffusion is shown to agree, within the errors, with the theory of drift induced by a vacancy flux, in the limit of weak solute/vacancy binding. Comparison of the measured diffusion coefficient with the value expected from the calculated vacancy concentration suggests that only (9 ±5%) of atomic displacements give rise to freely diffusing vacancies, under the conditions of the experiment.

Study of Ion Bombardment-Induced SubSurface Compositional Modifications in Ni-Cu Alloys at Elevated Temperatures by Ion Scattering Spectroscopy

ABSTRACTChanges in the subsurface composition of Ni-40 at.% Cu alloys during 3-keV Ne+ bombardment at temperatures between 25 and 700°C were studied by means of ion scattering spectroscopy.Both the time evolution of the composition in the surface atom layer during ion bombardment and subsurface concentration profiles after rapid specimen cooling to room temperature were measured as a function of temperature. Radiation-enhanced diffusion coefficients were derived from the effective altered-layer thicknesses obtained. A comparison of the experimental measurements with theoretical calculations based on a phenomenological model enabled the identification of processes and kinetics responsible for subsurface compositional modifications.