Helium Trapping Research Articles

The ferrite/perovskite interface and helium partition in nano-structured ferritic alloys from the first-principles

Nano-oxides are largely responsible for the excellent mechanical properties and irradiation tolerance of nano-structured ferritic alloys (NFAs) for nuclear reactor applications. In this work, the roles of perovskite YTiO3 and its interface in trapping helium in NFAs were investigated from the first-principles. Similarly as other two Y-Ti-oxide phases (Y2TiO5 and Y2Ti2O7), bulk YTiO3 can trap insoluble helium at its interstitial sites, but with a lower trapping ability that is only comparable to matrix vacancies. The ferrite/YTiO3 interface phase diagram was constructed based on the experimental orientation relationship and the calculated interface formation energy, and the lowest-energy interface structure was predicted as the ns-Ti or the stoichiometric. Helium always prefers to consume individual interfacial vacancies and interstitial sites to the extent possible, before forming higher-order helium-vacancy clusters at the interface. Similarly as Y2Ti2O7, YTiO3 preferably traps helium at its interface, followed by its bulk interior and the ferritic matrix. However, in view of all the bulk and interface results, perovskite YTiO3 cannot compete with pyrochlore Y2Ti2O7 in trapping helium in NFAs.

Ab initio study of helium behavior near stacking faults in 3C-SiC

First-principles calculations are used to investigate the effects of stacking faults (SFs) on helium trapping and diffusion in cubic silicon carbon (3C-SiC). Both extrinsic and intrinsic SFs in 3C-SiC create a hexagonal stacking sequence. The hexagonal structure is found to be a strong sink of a helium interstitial. Compared to perfect 3C-SiC, the energy barriers for helium migration near the SFs increase significantly, leading to predominant helium diffusion between the SFs in two dimensions. This facilitates the migration of helium towards interface traps, as confirmed by previous experimental reports on the nanocrystalline 3C-SiC containing a high density of SFs. This study also reveals that the formation of helium interstitial clusters near the SFs is not energetically favored. The findings from this study enhance our comprehension of helium behavior in faulted 3C-SiC, offering valuable insights for the design of helium-tolerant SiC materials intended for reactor applications.

Behavior of helium diffusion sinks in apatite: Evidence from continuous ramped heating analysis of borehole and well-characterized samples

Diffusion sinks have been proposed to explain the complex high-temperature 4He release often seen in Continuous Ramped Heating (CRH) analysis of apatites, which often correlates with over-dispersed and anomalously old (U-Th)/He ages. In this work, we first conducted CRH experiments with various ramping rates and cycled CRH experiments that confirm that the high-temperature 4He release from diffusion sinks is systematically temperature sensitive at laboratory time scales. We then analyzed apatite grains from the KTB borehole (German Continental Deep Drilling program) that have well-constrained thermal histories to test whether different thermal histories could alter the trapping of 4He at geologic time scales. The KTB samples show large intra-sample single-grain age dispersion, with most ages being older than predictions from a 4He radiation-damage diffusion model RDAAM. Individual grains from shallower depths show complex 4He outgassing behaviors, including the expected low-temperature volume-diffusion loss, sudden release spikes, and a higher- temperature-dependent component. Grains from the current partial retention zone and greater depths retain considerable helium and exhibit exclusively complex outgassing behavior dominated by high-temperature release in the laboratory. KTB samples show a systematic difference with depth in the proportion of low- and high-temperature gas-release gas components. Deeper samples, having long open-system thermal histories, contain a greater proportion of 4He that appears to have been trapped compared to the simpler volume-diffusion behavior exhibited by shallower samples that experienced a simpler monotonic cooling history. While greatly reduced, the high-temperature component is still present in the highest-temperature samples analyzed, well hotter than the partial retention zone. These observations confirm that in apatite trapping and release of helium from sinks is temperature dependent at geological time scales as well. This supports the suggestion that additional thermal-history information from apatites might be obtained using 4He/3He analysis, with laboratory release of 3He providing information about trap distribution and kinetics and 4He providing information about the geologic thermal history.

Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

Abstract A series of high-entropy alloys (HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method. In order to study the irradiation evolution of helium bubbles in the FeCoNiCr-based HEAs with γ′ precipitates, these samples are irradiated by 100-keV helium ions with a fluence of 5×1020 ions/m2 at 293 K and 673 K, respectively. And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles. Transmission electron microscope (TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles. Experimental results reveal that nanosized, spherical, dispersed, coherent, and ordered L12-type Ni3Ti γ′ precipitations are introduced into FeCoNiCr(Ni3Ti)0.1 HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K. Under the ageing treatment conditions adopted in this work, γ′ nanoparticles are precipitated in FeCoNiCr(Ni3Ti)0.1 HEAs, with average diameters of 15.80 nm, 37.09 nm, and 62.50 nm, respectively. The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm, 1.65 nm, and 1.58 nm, respectively. The improvement in the irradiation resistance of FeCoNiCr(Ni3Ti)0.1 HEAs is evidenced by the diminution in bubbles size. Furthermore, the FeCoNiCr(Ni3Ti)0.1 HEAs containing γ′ precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles, which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries. Subsequently, annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy (E a) for helium bubbles, resulting in larger helium bubble size. This study provides guidance for improving the irradiation resistance of L12-strengthened high-entropy alloy.

Effects of Y2O3 nanoparticles / Ni matrix interface on the mechanical properties and helium swelling resistance in NiMoCr-Y2O3 alloys

The use of oxide dispersion-strengthened (ODS) nickel-based alloys has emerged as a promising choice for structural materials in molten salt reactors. In this study, a novel Y2O3 nanoparticles dispersion-strengthened nickel-base alloy was prepared by powder metallurgy method. The NiMoCr-Y2O3 alloy exhibits superior yield strengths and ultimate tensile strengths, surpassing those of the Hastelloy N (HN) alloy, both at room temperature and high temperatures. This enhancement can be attributed to the dispersion-strengthening effect imparted by Y2O3 nanoparticles. Additionally, the NiMoCr-Y2O3 alloy displays commendable plasticity. Nevertheless, it is worth noting that the stress concentration at the Y2O3 nanoparticles/Ni matrix interfaces makes it susceptible to stress-induced breakage, resulting in lower plasticity compared to the HN alloy. Transmission electron microscopy (TEM) observations reveal that the Y2O3 nanoparticles /Ni matrix interfaces effectively trap helium (He) bubbles, leading to a reduction in helium bubble size. Furthermore, X-ray absorption fine structure (XAFS) analysis indicates a higher vacancy density in NiMoCr-Y2O3 alloy compared to HN alloys during irradiation, contributing to an increased number of helium bubbles in the NiMoCr-Y2O3 alloy. This effect is primarily due to the preferential absorption of a significant quantity of interstitial particles by the Y2O3 nanoparticles/Ni matrix interfaces. Consequently, the volume fraction of helium bubbles in the NiMoCr-Y2O3 alloy (∼0.12 %) is notably lower than that observed in the HN alloy (∼0.52 %), showing that NiMoCr-Y2O3 alloys exhibit excellent He swelling resistance.

Exploration for Helium in the Phanerozoic

Almost all the helium discovered worldwide has been found by chance in the drilling for hydrocarbons. Targets were usually anticlinal structures located by seismic surveys. The association of helium and natural gas in reservoirs is purely coincidental because the source rocks for each are different—natural gas is mainly produced from diagenesis of carbon rich shale whereas Helium- 4 is derived mainly from the decay of uranium/thorium in the crust. A new geochemical exploration system for helium in the Phanerozoic is considered highly desirable by industry. The current exploration system in the Phanerozoic uses a modified petroleum system concept that has been used successfully for decades to high-grade plays and de-risk oil and gas prospects. Like a petroleum system, the helium system is identified by its source rock, reservoir, trap, seal, and migration pathways. However, this approach is expensive taking years to complete and can be a limiting factor for countries/provinces/states to develop their resources. As a precursor to ground helium/hydrogen surveys for exploration in the Phanerozoic, hyperspectral (satellite) surveys assess huge areas for their helium potential as well as any associated hydrocarbons. These areas may be even devoid of any previous exploration for hydrocarbons. This is followed with geochemical soil gas surveys of the more prospective trends to locate drilling locations. Both methods ascertain whether helium anomalies, which represent helium reservoirs at depth, are associated with hydrocarbons or nitrogen. Helium reservoirs associated with nitrogen are higher in helium content but are deeper, close to the basement. Hyperspectral and geochemical soil gas surveys are also applicable for projects that begin with helium analysis of old wells followed by seismic and drilling. Typically, exploration companies lease vast areas surrounding the legacy well, but their initial focus is on seismic in the area around the legacy well to determine the size and configuration of the helium reservoir and trap penetrated by the well and this is followed by drilling. It is not known at this point whether this reservoir is the best prospect because the helium discovered is usually associated with hydrocarbon (HC), not with nitrogen with much higher helium concentration in the lower Phanerozoic. Rarely do legacy wells penetrate deep enough to this level. Any cost-effective exploration program for helium is best accomplished by hyperspectral and follow-up geochemical soil gas surveys.

Superfluid Helium Drops Levitated in High Vacuum.

We demonstrate the trapping of millimeter-scale superfluid helium drops in high vacuum. The drops are sufficiently isolated that they remain trapped indefinitely, cool by evaporation to 330mK, and exhibit mechanical damping that is limited by internal processes. The drops are also shown to host optical whispering gallery modes. The approach described here combines the advantages of multiple techniques, and should offer access to new experimental regimes of cold chemistry, superfluid physics, and optomechanics.

Examination of Early-Stage Helium Retention and Release in Dispersion-Strengthened Tungsten Alloys

Tungsten is the material of choice for plasma-facing components in the divertor region of future nuclear fusion reactors. Exposure to low-energy helium ion irradiation results in microstructural changes as helium is trapped at defects in the tungsten matrix. High-temperature exposure results in the formation of helium bubbles in the subsurface. Dispersion-strengthened tungsten materials are tungsten-based materials with added transition metal carbides to alter the impurity distribution and grain structure. In this work, the thermal release of helium from dispersion-strengthened tungsten is investigated. After irradiation at 1073 K to a 1024 m−2 fluence, thermal desorption spectroscopy was performed to elucidate the helium trapping and desorption behavior. Post-desorption microscopy was performed to correlate the microstructural changes with helium release spectra. The amount of desorbed helium was highest in the 1.1 and 5 wt% alloys, and significantly lower in the 10 wt% alloys. Helium bubbles were observed in the pure tungsten and 1.1 wt% alloys within the tungsten grains. Correlating the composition with helium release spectra revealed the importance of tailoring grain size and oxide vacancy concentrations by varying the dispersoid content on the helium retention and release behavior. These first results of helium desorption from dispersion-strengthened tungsten indicate compositionally dependent retention and reveal the need to examine helium retention in advanced tungsten alloys under reactor-relevant exposure.

Effect of pre-irradiation defects on helium trapping and diffusion in RAFM steel

In order to analyze the effect of pre-irradiation defects on helium trapping and diffusion behaviors in detail by slow positron beam, Si ion with high energies (300 keV and 6 MeV) and 100 eV helium plasma were sequentially introduced into reduced activation ferritic/martensitic steel. Massive microscopic defects were observed in Si ion irradiated specimens. Low energy helium (∼100 eV) atoms could diffuse into much deeper region in metals without displacement damage. In the diffusion process of helium atoms, they would be trapped by the Si-ion pre-irradiation defects. The formation of He-vacancy complexes would affect the annihilation of positrons with the electrons in pre-irradiation defects and decrease the S parameters. The shallow region defects induced by 300 keV irradiation have more obvious advantage of hindering the diffusion of helium over the deep defects than that in 6 MeV irradiation. Thus, the pre-introduced defects can trap effectively helium atoms and hinder their diffusion.

Towards better understanding the question of helium-driven swelling in RAFM-ODS alloys

We irradiated three 9Cr alloys: Fe9Cr model alloy, reduced-activation ferritic/martensitic (RAFM), and advanced oxide dispersion strengthened ferritic (RAFM-ODS) alloys, with 100keV He ions at 723K to the peaked concentration of 6500 appm (low helium level) and 65000 appm (high helium level), respectively. The question of helium-driven swelling in Fe9Cr, RAFM, and RAFM-ODS alloys was investigated by multiple characterizations. Homogeneous helium bubbles, main in the form of VmHen (n < m), nucleated on different sinks, producing swelling behavior and dominating the microstructures of irradiated Fe9Cr, RAFM, and RAFM-ODS steels. The consequent swelling rate values in Fe9Cr, RAFM, and RAFM-ODS were ∼ 0.15% (1.33%), ∼ 0.058% (0.83%), and ∼ 0.048% (0.73%) in the low (high) levels, respectively. The RAFM-ODS steel exhibits better tolerance to helium bubble swelling and superior grain size stability than Fe9Cr and RAFM, which is manifested by the smaller average sizes and the lower number densities of bubbles. It can be attributed that the oxides in RAFM-ODS steel could provide more effective helium trapping sites that sequester the helium into smaller bubbles and away from the precipitate-matrix interfaces and grain boundaries. In addition, the existence of high density of oxide particles could also delay desorption peaks of helium atoms from RAFM-ODS steel compared to RAFM and Fe9Cr.

Helium irradiation-induced swelling resistance of heterogeneous nanoparticles in an iron-based multi-principal element alloy

In this work, we studied the resistance to helium-irradiation-induced swelling of the Fe6Cr1.2Mn0.8Cu1.5Mo0.3V0.2 alloy containing highly thermally stable Cu nanoprecipitates for nuclear applications using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The results show that the Cu nanoprecipitate/matrix interface and the nanoprecipitate itself can trap helium bubbles, creating open volume sinks of helium bubbles; large helium bubbles tend to be dispersed at the interface, while small helium bubbles are distributed in the matrix. The SAXS results indicate that the average size of helium bubbles is 1.3–1.7 nm, and the distribution of helium bubbles in the two samples varies due to the difference in Cu nanoprecipitation/matrix interface density, which was also verified via TEM. In addition, a low swelling ratio (0.05%/dpa) of the alloy was calculated, further confirming the feasibility of heterogeneous nanoprecipitation management of helium damage in irradiation tolerant materials. These nanostructured materials have the potential to reduce the helium effect by providing more nucleation sites for helium bubbles.

Crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy

The crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy have been systematically investigated in this study. After the thermal exposure at 750 °C, the secondary MC carbides can be observed in the matrix in the forms of stacking fault precipitates (SFP) and flake-shaped ones. At the first stage of exposure, the SFP preferentially nucleates around the primary MC carbides, and then spread throughout the matrix. The flake-shaped MC carbides were also formed at the dislocations and pile-ups. After the long-term exposure, the SFP starts to degenerate, while flake-shaped ones still remain and become coarsen. All secondary MC carbides possess the cube-on-cube orientation relationship with the matrix but the different interface structures. For the SFP, the semi-coherent interface is along the {111} planes. In the flake-shaped MC carbides, it is along {001} planes. All secondary MC carbides possess the same chemical constitution, that is approximately (Mo0.66Nb0.24Cr0.1) C. Based on the first-principle calculations, the Mo doping in NbC was prove to decrease the chemical interface energy and favor the nucleation process. The evolution mechanism of the secondary MC carbides was also briefly discussed by exploring the role of crystal defects (stacking faults and dislocations). Our results can help to deepen the understanding of the helium trapping mechanism of MC carbides.

First lithium experiments in HIDRA and evidence of helium retention during quasi-steady-state stellarator plasma operations

Recent experiments in Hybrid Illinois Device for Research and Applications (HIDRA) have had operational discharges between t discharge = 60 and 1000 s using electron cyclotron resonant heating (ECRH) of the plasma. This means that quasi-steady-state plasma discharges reach conditions to study long-pulse plasma material interactions (PMIs). The newly commissioned HIDRA-Material Analysis Test-stand PMI diagnostic is used to place a drop of lithium onto a heated tungsten surface, transfer the sample in-vacuo and expose it in a helium plasma. Helium is of interest as there is an open question to whether lithium will be able to remove helium ash in real fusion devices. The introduction of the W-Li sample in HIDRA resulted in evaporation of lithium into the helium plasma during a 600 s pulse and caused a reduction of over 90% in neutral pressure during the discharge. It was also observed that the plasma density and temperature increased by over 2.5 times. Using spectroscopy and a helium collisional radiative model, the peak temperature and density of the helium plasma can be monitored during the discharge. During lithium evaporation, as significant lithium ionization occurs, there is a 85% drop in the HIDRA vessel neutral pressure, despite a constant flow rate of He gas. This reduction in neutral pressure is supported by spectroscopy data with corresponding reductions in He I line intensities (587 nm, 667 nm, 706 nm, and 728 nm), as well as those of other impurities. At one point in the discharge a lithium plasma is created, as indicated by an increase in Li+ emission and a complete reduction in He+ emission, but the electron density jumps from n e = 3 × 1018 m−3 to over n e = 8 × 1018 m−3 while the core temperature stays relatively constant between T e = 16 eV and 20 eV. Once lithium has completely evaporated from the sample and the majority of the ionized lithium has diffused from the plasma to the vessel walls, pressure and spectroscopy data paired with He collisional radiative model calculations shows a re-establishment of a helium plasma in a low recycling regime. In this regime, the density drops down to n e = 2 × 1018 m−3 and the electron temperature increases from T e = 20 eV to over T e = 50 eV indicating an increase in helium heating efficiency. This is also indicated by the He+ emission re-establishing and having a higher intensity. In this paper we show the results from the first lithium campaign in HIDRA. In the presence of lithium, and in particular when lithium ions are present, the helium disappears from the plasma via an as of yet unknown complex relationship that needs to be further studied. The most likely explanation is that the lithium ions are distributed around the vessel and able to trap helium to the surface turning HIDRA into a large gettering surface. These results have potential implications on future plasma facing component design using liquid lithium for impurity and recycling control using limiters and divertors.

Implications of Microstructure in Helium-Implanted Nanocrystalline Metals.

Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and—moreover—diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to saturation in both samples, bubble density in Fe was over 300% greater than in Ni. To interrogate the kinetics of helium diffusion and trapping, a rate theory model is developed that substantiates that helium is more readily captured within grains in helium-vacancy complexes in NC Fe, whereas helium is more prone to traversing the grain matrices and migrating to GBs in NC Ni. Our results suggest that (1) grain boundaries can affect bubble swelling in grain matrices significantly and can have a dominant effect over crystal structure, and (2) an NC-Ni-based material can yield superior resistance to irradiation-induced bubble growth compared to an NC-Fe-based material and exhibits high potential for use in extreme environments where swelling due to He bubble formation is of significant concern.

The microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions

Oxide dispersion strengthened (ODS) steels with excellent irradiation swelling resistance and high temperature creep resistance are regarded as one of the priority candidate materials for the first wall/blanket materials in fusion reactors. Microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions at R.T. and 400 °C respectively have been studied by transmission electron microscopy and nanoindentation technique. Three main oxides were identified, including high-density dispersed Y-Ti-O nanoclusters, tens of nanometers Y2TiO5 and a few hundred nanometers Y2Ti2O7 particles with the largest size but the least density. Helium bubbles were homogeneously distributed in the matrix in both irradiated specimens and the aggregation state of bubbles near GBs was related to the nature of GB. The average size of bubbles increased and the density decreased in the steel irradiated at elevated temperature. A defect-depleted zone was observed near a GB with a Y2TiO5 precipitate imbedded and a large helium bubble was present at the interface of the Y2TiO5 particle/GB. It suggests that the GBs combined with Y2TiO5 can enhance the aggregation of helium bubbles since GBs can act as defect sinks as well as diffusion channels and Y2TiO5 has a high ability of trapping helium atoms. Significant irradiation hardening was obtained at both irradiation temperatures. The relationship between microstructure and the irradiation hardening was discussed.

Role of Ni on the helium diffusion, stability and energetics of vacancy–type clusters in Fe–6.25Cr–3.13Ni (at.%) ternary alloys: From first-principles

Ab initio methods based on DFT are utilized to study the role of Ni addition on the interstitial helium diffusion, energetics and stability of HenVm (n = 0–6, m = 0–4) clusters in austenitic Fe–6.25Cr–3.13Ni (at.%) ternary alloys. The limit vacancy concentration considered in our simulation will be as close to the real irradiation conditions as possible. The presence of Ni obviously hinders the long-range migration of an individual helium between tetrahedral and octahedral interstitials, or between tetrahedral gaps, but greatly promotes the movement between octahedral gaps, and the travel between octahedral (tetrahedral) gaps cannot cross the tetrahedral (octahedral) interstitials. The addition of Ni enhances the attractions between interstitial helium, drives the short-range migration and tends to form small helium bubbles; on the other hand, it seriously hinders the trapping of helium by multiple-vacancies, and reduces the risk of helium bubble nucleation. In addition, the effects of Ni on the stability and energetics of helium vacancy complexes are evaluated, and it is confirmed that Ni improves the stability of the cluster models and enhances the irradiation expansion resistance of the alloy configurations, although it is not conducive to the formation of HenVm clusters. Our findings provide a theoretical reference for understanding the thermal helium desorption spectra and helium-induced embrittlement or hardening under irradiation conditions.

Studies on the role of ion mass and energy in the defect production in irradiation experiments in tungsten

Experimental investigations on the role of ion mass and the primary knock-on atoms (PKA) spectrum in the defect type, structure and defect production efficiency is presented in ion-irradiation experiments in tungsten using a combination of positron annihilation spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy. Recrystallized tungsten foils were irradiated using low- (helium), medium- (boron) and high-mass (gold) ions of MeV energy for a comparable dpa and implantation range at room temperature. Depending on the ion mass and the PKA spectrum, distinctly different defect structures were observed at the atomistic as well as meso-scales. While no indication of dislocation lines was observed in 3 MeV helium irradiated samples, the boron and gold ions showed extensive dislocation line formation. The cluster shape depends on the PKA energy and the cluster density depends on the irradiation fluence. The depth profile analysis of the defects in the helium-irradiated samples showed extensive helium trapping throughout the implantation range. Significant sub-surface helium trapping is observed within 700 nm from the surface, indicating that they moved towards the surface from their mean implantation depth of 4500 nm. The study also shows a correlation between carbon and helium profiles in the samples.

Helium partitioning to the core-shelled Ta nanoclusters in nanocrystalline Cu-Ta alloy

In this work, a nanocrystalline (NC) Cu-10at.%Ta alloy is irradiated with helium at different temperatures to assess the stability and effectiveness of Ta nanoclusters in trapping helium and suppressing swelling. Advanced microstructural characterization of the room-temperature irradiated specimens indicated the presence of small He-bubbles (∼1-2 nm) at the peak damage depth mainly at the core and along the interface of Ta nanoclusters with Cu matrix. Few bubbles were found along grain boundaries, with much smaller bubbles homogenously distributed within the copper lattice. High-temperature irradiation exhibited bubbles of ∼3-5 nm, which were primarily associated with nanoclusters as compared to other locations, with no observed faceting of the bubbles. Atom probe analysis confirmed helium partitioning to the Ta nanoclusters indicating the effective entrapment of these He atoms.

The influence of Cr on He trapping behavior and the coupling effect of Cr/He on the mechanical behavior of the C14-Laves Fe2W phase: First–principle and quasi-harmonic approximation studies

The influence of chromium (Cr) on the thermal properties of C14–Laves Fe2W phase were studied in detail based on the quasi–harmonic approximation. The effect of Cr on the helium (He) trapping behavior of the C14–Laves Fe2W phase was also investigated based on the defect formation energy with density functional theory (DFT). The results indicated that Cr could not only decrease the values of the Gibbs energy and bulk modulus of the C14–Laves Fe2W phase, but could also increase the thermal expansion coefficient of the C14–Laves Fe2W phase. In addition, Cr could reduce the capacity of the C14–Laves Fe2W phase to trap He. The coupling effect of Cr and He on the mechanical behavior of the C14–Laves Fe2W was studied, wherein both Cr and He could reduce the tensile strength that was aligned to the [0001] of C14–Laves Fe2W phase. The relation between the mechanical behavior of the C14-Laves Fe2W or Cr doped Fe2W phases and their thermal properties were also discussed. The fracture mechanisms of the C14–Laves Fe2W phases, with or without Cr/He–doping, along the (0001) grain plane was analyzed based on the charge density and partial densities of states (pDOS).

Post-mortem analyses of gap facing surfaces of tungsten tiles of T-10 ring limiter

• The entire surface of gaps was covered by Li evaporated and splashed from Li limiter. • Heavy carbide precipitates were found deep in narrow gaps. • Deuterium and helium are accumulated in the tiles due to ion implantation. • Mainly regular but not cleaning discharges were responsible for effects in gaps. • High frequency auto-oscillating discharges were suggested to play a role in processes in gaps. Surfaces facing the gap between W tiles of the ring limiter of tokamak T-10 were analyzed after T-10 decommissioning using LIBS, SEM/EDA, XRD, TDS, and NRA techniques. Gaps with the width of 5 mm and 0.1 mm were nearly completely covered to their full depths of 22 and 15 mm, respectively, by a deposited film. The film was formed mainly by deposition of lithium that came from Li limiter and transformed in air to Li 2 CO 3 and Li 2 O. Carbon was deposited from volatile hydrocarbons sputtered from the tokamak walls. Besides, carbon appeared due to chemical reaction with lithium in air. Chemical interactions of W with C, O, and Li led to formation W 2 C, WC, WO 2 , and Li 2 WO 4 . Carbides formed in W over the entire surface to the full depth of the gaps. Trapping of deuterium and helium in tiles was demonstrated. Possible influence of auto-oscillating discharges on ionization and ion trapping of C,D, and He in gaps is discussed.