Helium Retention Research Articles

The influence of high grain boundary density on helium retention in tungsten

In order to study the influence of a high grain boundary density on the amount, size and distribution of defects produced by pulsed helium (625keV) irradiation in tungsten, we have carried out Object Kinetic Monte Carlo (OKMC) simulations in both monocrystalline and nanocrystalline tungsten. The parameterization of the OKMC code (MMonCa) includes binding energies obtained with our in-house Density Functional Theory (DFT) calculations. In the interior of a grain in nanocrystalline tungsten the mixed HenVm clusters are larger and have a lower He/V ratio. Thus, they are less pressurized clusters. The total elastic strain energy remains almost constant with the increasing number of pulses, contrary to its increase in monocrystalline tungsten. A better response to helium irradiation is therefore expected in nanocrystalline tungsten, opening a new path to investigate these nanostructured materials for fusion purposes.

Crystal orientation effects on helium ion depth distributions and adatom formation processes in plasma-facing tungsten

We present atomistic simulations that show the effect of surface orientation on helium depth distributions and surface feature formation as a result of low-energy helium plasma exposure. We find a pronounced effect of surface orientation on the initial depth of implanted helium ions, as well as a difference in reflection and helium retention across different surface orientations. Our results indicate that single helium interstitials are sufficient to induce the formation of adatom/substitutional helium pairs under certain highly corrugated tungsten surfaces, such as {1 1 1}-orientations, leading to the formation of a relatively concentrated layer of immobile helium immediately below the surface. The energies involved for helium-induced adatom formation on {1 1 1} and {2 1 1} surfaces are exoergic for even a single adatom very close to the surface, while {0 0 1} and {0 1 1} surfaces require two or even three helium atoms in a cluster before a substitutional helium cluster and adatom will form with reasonable probability. This phenomenon results in much higher initial helium retention during helium plasma exposure to {1 1 1} and {2 1 1} tungsten surfaces than is observed for {0 0 1} or {0 1 1} surfaces and is much higher than can be attributed to differences in the initial depth distributions alone. The layer thus formed may serve as nucleation sites for further bubble formation and growth or as a source of material embrittlement or fatigue, which may have implications for the formation of tungsten “fuzz” in plasma-facing divertors for magnetic-confinement nuclear fusion reactors and/or the lifetime of such divertors.

Retention and thermal release behavior of deuterium and helium implanted in SiC

Retention and release of ion implanted deuterium (D) and helium (He) in silicon carbide (SiC) were studied with respect to damage accumulation and annealing behavior using ion beam analysis techniques. α-SiC single crystals were irradiated by 10keV D2+ and He+ at 300 and 770K, and depth profiles of retained atoms and lattice disorder were measured during the implantation and successive heat treatments. For the sample pre-irradiated with He at a fluence of 1.0×1021ions/m2, the thermal release of D atoms was completed at an annealing temperature of 1370K, while the retained D atoms still remained in the sample without pre-irradiation. In contrast to the facilitated thermal release of D by He pre-irradiation, the He release temperature increased in the sample followed by D ion implantation. No significant difference of He retention and damage accumulation behavior was observed between the samples implanted at 300 and 770K. The D retention and D-ion-induced disorder for 770K implantation, was reduced to approximately 1/2 comparing to that of the sample implanted at 300K. The D retention at 770K was not affected by the He pre-implantation induced damage.

Effect of the displacement damage from argon ion irradiation on the synergistic effect of helium–hydrogen in tungsten

The effect of argon ion pre-irradiation on helium and hydrogen ion irradiation was investigated in tungsten. At the same time, comparative experiments were carried out on the irradiation of helium and hydrogen ions in tungsten. Without the argon ion irradiation, the energy of 35keV hydrogen ions mainly accelerated the coalescence of defects created by the 60keV helium ions, the irradiation damage degree increased with hydrogen ion fluence increasing. With the argon ion irradiation, lots of voids were created by argon ion irradiation, which increased the helium and hydrogen retention and the synergistic effect of helium–hydrogen in tungsten. In the same hydrogen fluence, the surface damage degree with argon ion pre-irradiation was higher than that without argon ion pre-irradiation.

Characterization of helium implanted Fe–Cr alloys by means of positron annihilation methods

Doppler broadening spectroscopy and positron lifetime measurements have been used for the characterization of radiation damage and helium effects on the microstructure of the ferritic Fe–Cr alloy with 12% (wt.) chromium content. Severe displacement damage (up to 100dpa) was introduced by the implantation of helium ions with fluence 1.25×1018cm−2. The temperature during the implantation process was below 80°C. Positron lifetime spectra obtained from slow positron beam experiments confirmed the presence of large voids in the region of the displacement peak. Further investigation by Doppler broadening spectroscopy in the conventional and slow positron beam setups revealed a strong retention of helium in this region, which contributes to the broadening of the momentum peak and at the same time it reduces the positron lifetime in radiation-induced vacancy clusters. The measured Doppler broadened profiles of the implanted materials reasonably correspond with the calculated data showing the helium effect range within 5–12×10−3m0c. Both experimental techniques show empty vacancy clusters to be a dominant type of defect in the first 400nm region, while helium filling of these defects occurs further in depth. This is in a good agreement with the results of SRIM simulation.

The role of radiation damage on retention and temperature intervals of helium and hydrogen detrapping in structural materials

An experimental study of hydrogen/deuterium behavior in ferritic–martensitic stainless steels EP-450 (Cr13Mo2NbVB), EP-852 (Cr13Mo2VS), and RUSFER-EK-181 (Fe12Cr2WVTaB) is presented. The effect of displacement damage (dpa) resulting from irradiation with helium, hydrogen, and argon ions on features of deuterium detrapping and retention in steels was studied using ion implantation, nuclear reaction depth profiling, and thermal desorption spectrometry techniques. Numerical simulation on the basis of the continuum rate theory was applied for obtaining thermodynamic parameters of deuterium trapping and desorption in steels.

Deuterium retention in the carbon co-deposition layers deposited by magnetron sputtering in D2/He atmosphere

Carbon was deposited on Si and W substrates using a D2/He plasma in a radio frequency magnetron sputtering system. The deposited layers were examined with ion beam analysis (IBA), Raman spectra analysis (RS) and scanning electron microscopy (SEM). The growth rate of the layers deposited at 2.5Pa total pressure and 300K decreased with increasing He fraction in the D2/He gas mixture. The deuterium concentration in the layers deposited on the Si substrate increased from 14% to 28% when the flow rate of the He gas relative to the D2 gas was varied from 0.125 to 0.5, but the deuterium concentration in the layers on a W substrate decreased from 24% to 14%. Deuterium or helium retention and the layer thickness all significantly decreased when the substrate temperature was increased from 423K to 773K. Raman analysis showed that the deposited layers were amorphous deuterated-carbon layers (named a-C: D layer) and the extent of bond disorder increased dramatically with the increasing helium content in the film. Blisters and bubbles occurred in the films for high helium content in the films, and surface cracking and exfoliation were also observed.

A rate theory study of helium bubble formation and retention in Cu–Nb nanocomposites

A spatially dependent rate theory model for helium migration, clustering, and trapping on interfaces between Cu and Nb layers is introduced to predict the evolution of the concentrations of He clusters of various sizes during implantation and early annealing. Migration and binding energies of point defects and small clusters in bulk Cu and Nb are found using conjugate gradient minimization and the nudged elastic band method. This model is implemented in a three-dimensional framework and used to predict the relationship between helium bubble formation and the nano-composite microstructure, including interfacial free volume, grain size, and layer thickness. Interstitial and vacancy-like migration of helium is considered. The effects of changing layer thickness and interfacial misfit dislocation density on the threshold for helium bubble nucleation are found to match experiments. Accelerated helium release due to interfaces and grain boundaries is shown to occur only when diffusion rates on interfaces and grain boundaries are greatly increased relative to the bulk material.

Effect of ion flux on helium retention in helium-irradiated tungsten

Helium retention in irradiated tungsten leads to swelling, pore formation, sample exfoliation and embrittlement with deleterious consequences in many applications. In particular, the use of tungsten in future nuclear fusion plants is proposed due to its good refractory properties. However, serious concerns about tungsten survivability stems from the fact that it must withstand severe irradiation conditions. In magnetic fusion as well as in inertial fusion (particularly with direct drive targets), tungsten components will be exposed to low and high energy ion irradiation (helium), respectively. A common feature is that the most detrimental situations will take place in pulsed mode, i.e., high flux irradiation. There is increasing evidence of a correlation between a high helium flux and an enhancement of detrimental effects on tungsten. Nevertheless, the nature of these effects is not well understood due to the subtleties imposed by the exact temperature profile evolution, ion energy, pulse duration, existence of impurities and simultaneous irradiation with other species. Object Kinetic Monte Carlo is the technique of choice to simulate the evolution of radiation-induced damage inside solids in large temporal and space scales. We have used the recently developed code MMonCa (Modular Monte Carlo simulator), presented at COSIRES 2012 for the first time, to study He retention (and in general defect evolution) in tungsten samples irradiated with high intensity helium pulses. The code simulates the interactions among a large variety of defects and during the irradiation stage and the subsequent annealing steps. The results show that the pulsed mode leads to significantly higher He retention at temperatures higher than 700K. In this paper we discuss the process of He retention in terms of trap evolution. In addition, we discuss the implications of these findings for inertial fusion.

Helium retention and early stages of helium-vacancy complexes formation in low energy helium-implanted tungsten

Tungsten has been selected as the material of the divertor of the ITER fusion reactor. In operation, tungsten will be submitted to high alpha particles bombardment. The consequence of helium implantation is a major issue for the reliability of tungsten components. The aim of the study was to investigate the behavior of helium implanted in tungsten at low energy and low flux. 320eV Helium ions were introduced by plasma immersion at the flux of 2.5×1018ion/m−2/s−1. The helium behavior was investigated by Nuclear Reaction Analysis and the evolution of the tungsten lattice by Positron Annihilation Spectroscopy (PAS). Helium-implanted tungsten exhibits a low retention rate (13.6% at 9.4×1019Hem−2) which decreases with the implantation fluence. The desorption of helium starts at low temperature (<400K). SEM analysis after annealing over 973K shows sparse pores probably due to bubbles opening at the surface. The creation of helium-filled defects in the near surface layer (0.5 to ∼20nm) was followed by PAS. A low level of damages was introduced by 12MeV proton irradiation, prior to He introduction and allowed to examine the influence of pre-existing defects on the helium trapping. The PAS results suggest that the early stage of the formation of helium-filled vacancy clusters does not require the presence of pre-existing vacancy and thus proceed from the trap mutation phenomenon.

Survivability of First-Wall Materials in Fusion Devices: An Experimental Study of Material Exposure to Pulsed Energetic Ions

The confining walls in future fusion power plants will be subjected to an intense energetic bombardment from X-rays, ions, and neutrons. This is true for both direct-drive inertial fusion energy (IFE) and magnetic fusion energy (MFE) designs. We focus in this paper on the threat spectra presented by energetic ions. X-rays are predicted to present a less significant threat in direct-drive IFE, and neutron effects cannot be readily simulated in current experimental facilities. For the experimental results presented herein, the energetic ions are generated in the Repetitive High-Energy Pulsed Power 1 (RHEPP-1) facility at Sandia National Laboratories. Depending upon whether the ion pulses are of nitrogen (previous database) or helium (this paper), the pulse width varies from 100 ns to as much as 500 ns, respectively. While this is short compared to ˜500-μs transient events anticipated in MFE operation, data from both IFE and MFE experiments for tungsten exposure are shown to exhibit similar fluence thresholds when thermal diffusion is taken into account by use of the heat flux parameter H = Power density × t1/2, where t is the characteristic event time duration.Long-term exposure of tungsten to RHEPP-1 nitrogen pulses indicates that above a level of ˜1 J · cm−2 /pulse, polycrystalline tungsten roughens severely, the cause of which appears to be thermomechanical distress, with loosening of grains near the surface the primary result. This roughening is correlated with unacceptable mass loss. While this occurs below melting temperatures, allowing the surface to melt by raising the per-pulse fluence does not appear to be a viable approach to smoothing the surface. Oriented grain material such as ITER-specified tungsten performs significantly better than polycrystalline tungsten, but under helium exposure it appears to suffer additional surface deterioration that appears to be connected to helium pore and bubble formation at absorbed implantation levels of mid-1015 He/cm2. This level is below previously reported levels of concern for helium retention effects and well short of required survival duration. Experiments with three-dimensional "needle" geometries, designed to increase the effective surface area for heat absorption and reduce helium implantation in depth, show promising results that need further investigation to confirm long-term survival.

Effects of alloying elements on thermal desorption of helium in Ni alloys

It is well known that the minor elements Si and Sn can suppress the formation of voids in Ni alloys. In the present study, to investigate the effects of Si and Sn on the retention of helium in Ni alloys, Ni, Ni–Si, and Ni–Sn alloys were irradiated by 5keV He ions at 723K. Thermal desorption spectroscopy (TDS) was performed at up to 1520K, and microstructural observations were carried out to identify the helium trapping sites during the TDS analysis. Two peaks, at 1350 and 1457K, appeared in the TDS spectrum of Ni. On the basis of the microstructural observations, the former peak was attributed to the release of trapped helium from small cavities and the latter to its release from large cavities. Small-cavity helium trapping sites were also found in the Ni–Si and Ni–Sn alloys, but no large cavities were observed in these alloys. In addition, it was found that the oversized element Sn could trap He atoms in the Ni–Sn alloy.

Nuclear reaction and heavy ion ERD analysis of wall materials from controlled fusion devices: Deuterium and nitrogen-15 studies

Time-of-flight HIERDA (26 MeV 127I 7+) and micro-NRA (2.5 MeV 3He) were used to determine the composition of graphite and tungsten plasma-facing components (PFC) exposed at the TEXTOR and Tore Supra tokamaks. High sensitivity and resolution of HIERDA allowed, for the first time, for studies of nitrogen-15 showing that the gas injected during tokamak discharges is trapped on PFC, 3–7 × 10 15 N cm −2. Also helium retention in tungsten has been identified: up to 8 × 10 15 He cm −2. Deuterium distribution on the main limiters of Tore Supra is not uniform on tiles extracted from the erosion- and deposition-dominated areas. This is measured both on macro- (points 5 mm apart) and micro-scale (30 μm). The mapping with μ-NRA revealed the D content variation by a factor 40–50 in regions 1 × 2 mm 2: 1.2–40 × 10 17 D cm −2 and 4–230 × 10 18 D cm −2 in the erosion and deposition zones, respectively. In summary, the measurements of 15N contributed to material mixing studies and improved understanding of deuterium retention on PFC.

Surface Pore Formation in Helium Implanted Fine-Grain Tungsten and Tungsten Needles as Engineered First Wall and Divertor Plate Materials

Surface morphology changes of sub-micron tipped tungsten needles (W.N.) and an engineered fine-grain tungsten (FGW) were studied after implantation with He ions at reactor relevant conditions. Surface and subsurface pore formation was observed on all of the samples by using scanning electron microscopy (SEM) and focused ion beam (FIB) milling. Additionally, helium retention analysis was performed on the FGW and compared to several previously studied W materials.Three samples of FGW were irradiated with 30 keV 3He ions to 3×1017 He+/cm2 at 700 °C, 9×1017 He+/cm2 at 850 °C, and 1×1019 He+/cm2 at 1050 °C. SEM analysis revealed that the threshold for visible pore formation was below ˜1018 He+/cm2. The sample irradiated to the highest fluence showed “coral-like” morphology on the surface, and FIB analysis showed that the sub-surface semi-porous layer extended almost one micron below the surface. The percentage of implanted helium retained in the samples ranged from 4.5-40%.Two W.N. were implanted with 100 keV 4He ions to conditions of 3×1018 He+/cm2 at 700 °C and 1.3x1019 He+/cm2 at 1000 °C. Extensive pore formation was observed on both specimens. FIB analysis revealed that a sub-surface semi-porous layer developed after ion implantation that extended ˜300 nm in the W.N. implanted to the lower dose, and over 1500 nm in the needle implanted to the higher dose. This second needle also exhibited a drastic morphology change, which appears to be a result of the unraveling of the grains at the tip.

Helium retention and surface morphology of oxidized vanadium alloy

The effects of the surface oxidation on the helium desorption and retention behaviors of vanadium alloy were investigated. V–4Cr–4Ti alloy, which was thermally oxidized at 873K for 15min with 0.05Pa, was irradiated by helium ions with energy of 5keV at room temperature. Then, the helium desorption and retention behaviors were evaluated using a technique of thermal desorption spectroscopy. The changes in surface structures by the irradiation were also evaluated. Helium desorption behavior in low temperature region were significantly changed by the oxidation. Helium desorption around 700K became large for the oxidized sample. The amount of retained helium and the blister size in the oxidized sample were smaller than of those in the non-oxidized one. These results indicate that the surface oxidation significantly affects the helium retention and desorption behaviors of the vanadium alloy.

Helium diffusion behavior and its retention in LaNiAl alloy from molecular dynamic simulations

Molecular dynamic method has been used to investigate the diffusion behavior of helium atom in the LaNiAl alloy system. The results have shown that diffusion coefficient of helium atom increases as the temperature increases from 300 to 1500K, which indicates that helium atom gets more active via thermal desorption. The diffusion coefficient in LaNi5 and LaNi4Al are about 1.5×10−5 and 1.1×10−6cm2/s at 1300K, respectively, and the diffusion barrier of He are 1.45 and 0.52eV, respectively. The helium diffusion is shown to be enhanced with temperature increasing. Compared with metallic La, Ni, Al and Pd, the simulation result implies that helium atom is more stable and difficult to diffuse in LaNi5 and LaNi4Al alloys, which are the most promising materials for helium retention in experiments. Our results indicate the correlation between diffusion behavior of helium atom and capacity of materials for helium retention. Therefore, it can be considered as a feasible method to evaluate the helium retention capacity of materials via determining the diffusion properties of interstitial helium atom.

Lithium concentration dependence of implanted helium retention in lithium silicates

Abstract Helium ions of 500 keV were implanted with a fluence of 1.4 × 1017 ion/cm2 into various lithium silicates to investigate whether a threshold level of helium retention exists in Li-containing silicate ceramics similar to that found in SiOx in previous work. The composition and phases of the as prepared lithium silicates were determined by proton backscattering spectrometry (p-BS) and X-ray diffraction (XRD) methods with an average error of ±10%. Electrostatic charging of the samples was successfully eliminated by wrapping the samples in Al foil. The amounts of the retained helium within the samples were determined by subtracting the non-implanted spectra from the implanted ones. The experimental results show a threshold in helium retention depending on the Li concentration. Under 20 at.% all He is able to escape from the material; at around 30 at.% nearly half of the He, while over 65 at.% all implanted He is retained. With compositions expressed in SiO2 volume percentages, a trend similar to those reported of SiOx previously is found.

Retention and thermal desorption of helium in amorphous and crystalline FeBSi alloys

Helium retention in metals is related to their atomic structure and the type of defects they contain. In order to investigate the dependence of helium retention on structure, amorphous Fe79B16Si5 and crystalline FeBSi alloys were irradiated by helium ions at room temperature. In the crystalline alloy irradiated with 5 keV He+ ions, three types of helium trapping sites were found: surface defects produced by the irradiation, interstitial-type dislocation loops, and voids. Although these defects did not exist in the amorphous FeBSi alloy, we did observe thermal desorption peaks related to all three types. In addition, helium was released during the crystallization of amorphous FeBSi that had been irradiated by He+ ions.

Performances of inert gas glow discharges for reductions of fuel hydrogen retention and helium retention

In order to clarify the effects of inert gas discharges on the reduction of fuel hydrogen retention, the inert gas discharge has to be similarly conducted in the same apparatus with the same conditions. In the present experiment, first, the hydrogen glow discharge was conducted to measure the fuel hydrogen retention. During the discharge, the amount of retained hydrogen was measured by a technique of residual gas analysis. The inert gas discharges, He, Ne and Ar discharges, then, were carried out after the hydrogen discharge to measure the amount of removed hydrogen. The fractions of removed hydrogen after the He, Ne and Ar discharges were 0.8, 0.3 and 0.1, respectively. The projected range of helium ion is comparable with that of the hydrogen ion, so that the helium impact desorption well removed the hydrogen. The sputter-etching rate of argon is significantly high, so that the wall surface is quickly covered by the deposited layer. Thus, the removal ratio of the Ar discharge becomes lower than that of the He or Ne discharge. The H 2, Ne or Ar discharge was carried out after the He discharge to reduce the helium retention. It was found that the Ne and Ar discharges significantly reduced the helium retention, although the H 2 discharge little reduced the helium retention. The reduction of the Ne discharge was larger than that of the Ar discharge. The amounts of the retained He, Ne and Ar were a few times, two orders and three orders of magnitude smaller than that of hydrogen.

Helium desorption in 3He implanted tungsten at low fluence and low energy

The behaviour of helium in polycrystalline 3He implanted tungsten at low energy (60 keV) and low fluence (2 × 10 13 cm −2) has been studied as a function of post-implantation annealing temperature until 1873 K by means of Nuclear Reaction Analysis. Helium desorption has been observed only from ∼1500 K, suggesting a helium trapping at mono-vacancies. Only ∼75% of the implanted helium has been released after the annealing during 1 h at high temperature (1873 K); besides, the desorption rate decreased from 1673 K. The presence of a second type of helium trapping site is likely to explain this strong helium retention.