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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Understanding the release of helium atoms from nanochannel tungsten: a molecular dynamics simulation

The design of highly radiation-tolerant plasma-facing materials (PFMs) is of great importance for fusion reactors. Our recent experiments have shown that nanochannel tungsten (W) films have clearly superior radiation tolerance properties. In the present work, helium clustering and release from nanochannel tungsten were studied by molecular dynamics simulations. The effects of temperature and vacancy concentration on the helium release from a tungsten cylinder were investigated. Our results show that nanochannel W that consists of thin W cylinders releases He atoms more quickly than bulk W with flat surfaces, thus greatly reducing the He concentration and suppressing the formation and growth of He bubbles, which leads to increased radiation tolerance. Moreover, the micro-structural changes due to increasing He fluence are smaller in nanochannel W than those in bulk W. Although vacancies in nanochannel W will trap He atoms, the nanochannel W also has a stronger tendency to stabilize helium retention than bulk W. The mechanism of helium release from nanochannel W was also examined. The results reported here are beneficial for guiding future work in the design of radiation resistant PFMs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Influence of the ionization-energy losses of high-energy bismuth ions on the development of helium blisters in silicon

Understanding the behavior of helium in solids under conditions of intense ionizing radiation is of particular interest in solving many problems of nuclear, fusion, and space materials science and also in microelectronics. The observed effect of suppressing the formation of helium blisters on the surface of helium ion-doped silicon as a result of irradiation with high-energy bismuth ions is reported in this publication. It is suggested that a possible decrease in the concentration of helium atoms in silicon is due to their radiationinduced desorption from the area of doping in terms of the high-impact ionization of bismuth ions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Helium Energetic Neutral Atoms from the Heliosphere: Perspectives for Future Observations

Observations of energetic neutral atoms (ENAs) allow for remote sensing of plasma properties in distant regions of the heliosphere. So far, most of the observations have concerned only hydrogen atoms. In this paper, we present perspectives for observations of helium energetic neutral atoms (He ENAs). We calculated the expected intensities of He ENAs created by the neutralization of helium ions in the inner heliosheath and through the secondary ENA mechanism in the outer heliosheath. We found that the dominant source region for He ENAs is the inner heliosheath. The obtained magnitudes of intensity spectra suggest that He ENAs can be observed with future ENA detectors, as those planned on Interstellar Mapping and Acceleration Probe . Observing He ENAs is most likely for energies from a few to a few tens of keV/nuc. Estimates of the expected count rates show that the ratio of helium to hydrogen atoms registered in the detectors can be as low as 1:10{sup 4}. Consequently, the detectors need to be equipped with an appropriate mass spectrometer capability, allowing for recognition of chemical elements. Due to the long mean free paths of helium ions in the inner heliosheath, He ENAs are produced also in the distant heliospheric tail. This implies that observations of He ENAs can resolve its structure, which seems challenging from observations of hydrogen ENAs since energetic protons are neutralized before they progress deeper in the heliospheric tail.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Transition in helium bubble strengthening of copper from quasi-static to dynamic deformation

Damage from low-temperature irradiation and the subsequent degradation of materials performance pose significant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (>> 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (~10 µm thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s – 10 8 /s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. Here, this discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

36 MATERIALS SCIENCE↗
DOE OSTI2025

MX precipitate behavior in an irradiated advanced Fe-9Cr steel: Helium effects on phase stability

As part of an ongoing series aimed at optimizing Fe-9Cr reduced activation ferritic/martensitic (RAFM) alloys for fusion energy systems, this study explores MX precipitate behavior under dual-ion irradiations, specifically examining correlations between helium transmutation and irradiation-induced damage. Utilizing single and dual-beam ion irradiation, the research explores the combined effects of helium (10–25 appm He/dpa), temperature (400–600 °C), and damage levels (15–100 dpa) on the microstructural evolution of CNA9 steel, a variant of Castable Nanostructured Alloys (CNAs). The study demonstrates that helium co-implantation hinders radiation-enhanced coarsening of MX-TiC precipitates at 500 and 600 °C, maintaining MX-TiC precipitate stability at moderate damage levels (15 dpa) but failing to prevent complete precipitate dissolution at higher damage levels (≥50 dpa) when irradiated at 500 °C. Here, a generalized precipitate stability model suggests that helium-induced suppression of diffusion alters the balance between recoil resolution and back diffusion for MX-TiC precipitates, enhancing the current understanding of precipitate behavior under damage and transmutation simulated dual-ion irradiation conditions.

Characterization↗
DOE OSTI2021

Helium tribology of Inconel 617 subjected to laser peening for high temperature nuclear reactor applications

Inconel 617 is among the best candidates for utilization in high temperature gas cooled reactor tribo-components. However, the combined effects of sliding contact, along with intermittent idle times and very high temperature material degradation, deteriorates the alloy tribological performance, especially under a helium atmosphere. Laser peening is a surface treatment technique which can enhance the properties at the surface and subsurface by generating deep residual stresses and enhanced microstructure. In this work, we report the tribological behavior of regular laser peened as well as thermally-engineered laser peened Inconel 617 under helium and air atmospheres at 800 °C. In addition to friction and wear studies, the specimens are characterized by different analytical techniques to further understand the mechanisms involved in the peening process and sliding contact. Regardless of the peening process and post-process treatment types, it is observed that laser peening improves the tribological characteristics of Inconel 617. Interestingly, laser peening followed by helium thermal aging shows highly enhanced tribological behavior. This is attributed to the strengthening effect of the laser peening on the surface oxides providing an excellent and lasting protective and lubricating film under helium exposure.

42 ENGINEERING↗
DOE OSTI2013

Ab initio study of helium behavior in titanium tritides

Ab initio calculations based on density functional theory have been performed to investigate the relative stability of titanium tritides and the helium behavior in stable titanium tritides. The results show that the β-phase TiT1.5 without two tritium along the [100] direction (TiT1.5[100]) is more stable than other possible structures. The stability of titanium tritides decrease with the increased generation of helium in TiT1.5[100]. In addition, helium generated by tritium decay prefers locating at a tetrahedral site, and favorably migrates between two neighbor vacant tetrahedral sites through an intermediate octahedral site in titanium tritides, with a migration energy of 0.23 eV. Furthermore, helium is easily accumulated on a (100) plane in β-phase TiT1.5[100].

Ab initio calculation↗
DOE OSTI2025

Operation of helium sub-atmospheric multistage cryogenic centrifugal compressor trains: Part 1 – Steady state modeling and speed ratio selection

Helium cryogenic systems which can provide cooling below the normal boiling point of helium (approximately 4.2 K) are often required by superconducting radio-frequency niobium resonators utilized in modern high-energy particle accelerators. Achieving temperatures below 4.2 K generally involves operating a cryogenic vessel with liquid helium under sub-atmospheric conditions, thereby lowering the saturation pressure and corresponding saturation temperature. Over the last several decades, multi-stage cryogenic centrifugal compressor trains (CC’s) have been operated efficiently and reliably within large-scale cryogenic systems to continuously evacuate helium vapor generated by a device within the vessel, maintaining sub-atmospheric conditions in the vessel while pressurizing the return vapor to above atmospheric conditions. Traditionally, these CC systems have been operated using empirically derived control philosophies and insight gathered from previous operational experience. Recent efforts at the Facility for Rare Isotope Beams (FRIB) have been aimed at the development of a theoretical basis to characterize the operation of multi-stage cryogenic centrifugal compressor train and utilizing predictive model results to generate control parameters. The objective of this research was identifying operational points which adequately balance cryogenic system efficiency, stability, and overall ease of operation. Furthermore, this manuscript provides an overview of the predictive model development, characterization of the FRIB cryogenic centrifugal compressors and implementation of the predicted performance results during steady-state system operation.

Compressor train control↗
DOE OSTI2021

Heat and mass transfer during a sudden loss of vacuum in a liquid helium cooled tube - Part III: Heat deposition in He II

A sudden loss of vacuum can be catastrophic for superconducting particle accelerators. In such an event, air leaks into the liquid-helium-cooled accelerator beamline tube and condenses on its inner surface, causing rapid boiling of the helium and dangerous pressure build-up. Understanding the coupled heat and mass transfer processes is important for the design of the beamline cryogenic system. Our past experimental study on nitrogen gas propagating in a copper tube cooled by normal liquid helium (He I) has revealed a nearly exponential slowing down of the gas front. A theoretical model that accounts for the interplay of the gas dynamics and the condensation was developed, which successfully reproduced various key observations. However, since many accelerator beamlines are actually cooled by superfluid helium (He II) in which the heat transfer is via a non-classical thermal counterflow mode, we need to extend our work to the He II cooled tube. This paper reports our systematic measurements using He II and the numerical simulations based on a modified model that accounts for the He II heat-transfer characteristics. By tuning the He II peak heat-flux parameter in our model, we have reproduced the observed gas dynamics in all experimental runs. The fine-tuned model is then utilized to reliably evaluate the heat deposition in He II. Finallly, this work not only advances our understanding of condensing gas dynamics but also has practical implications to the design codes for beamline safety.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

First-principles calculations of the stability and incorporation of helium, xenon and krypton in uranium

While metallic fuels have a long history of reactor use, their fundamental physical and thermodynamic properties are not well understood. Many metallic nuclear fuels are body-centered cubic alloys of uranium that swell under fission conditions, creating fission product gases such as helium, xenon and krypton. In this paper, helium, xenon, and krypton point defects are investigated in the a and ? phases of metallic uranium using first principles calculations. A density functional theory (DFT) framework is utilized with projector augmented-wave (PAW) pseudopotentials. Formation and incorporation energies of He, Xe, and Kr are calculated at various defect positions for the prediction of fission gas behavior in uranium. In most cases, defect energies follow a size effect, with helium incorporation and formation energies being the smallest. The most likely position for the larger Xe and Kr atoms in uranium is the substitutional site. Helium atoms are likely to be found in a wide variety of defect positions due to the comparable formation energies of all defect configurations analyzed. This is the first detailed study of the stability and incorporation of fission gases in uranium.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

High Flux Helium Irradiation of Dispersion-Strengthened Tungsten Alloys and Effects of Heavy Metal Impurity Layer Deposition

Tungsten has been chosen as the plasma-facing material (PFM) for the divertor region in ITER and also a candidate PFM for future plasma-burning nuclear fusion reactors. During fusion device operation, PFMs will be exposed to low-energy He irradiation at high temperatures, resulting in sub-surface bubbles and surface morphology changes such as pores and fuzz. Carbide dispersion-strengthened W materials may enhance the ductility of W, but their behavior under high flux He irradiation remains unclear. In this work, the response of dispersion-strengthened tungsten materials to high flux, low energy He irradiation at high temperature is examined. Tungsten alloyed with 1, 5, or 10 wt. % tantalum carbide or titanium carbide exposed to these conditions result in surface pores, coral-like feature growth and sub-surface helium bubbles. Reactor-relevant helium irradiation (5 x 10 26 m -2 fluence) combined with high powered laser pulses to simulate off-normal reactor events does not significantly alter the surface morphology, as the surface nanostructures appear stable and cracks are only observed on a localized region of one sample. However, specimens show the development of an impurity layer on the surface, likely impurity deposition from the sample holder during irradiation, resulting in a mixed material layer on the surface. Helium bubbles exist in this impurity layer, and obscure conclusions about helium interactions with the carbide dispersoids. Nonetheless, it is clear that the dispersoid microstructure limits He bubble formation and subsequent surface nanostructuring, attributed to the dispersoid composition.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Suppression of Helium Induced Cracking in Laser Repair Welding of Highly Irradiated Stainless Steels

Welding repair of irradiated nuclear reactor materials such as austenitic stainless steels used for the reactor internals has been challenging due to irradiation generated helium in the material matrix over an extended period of time. In this study, improved laser welding techniques, including the novel Auxiliary Beam Stress Improved (ABSI) technique that proactively manages the stresses during laser repair welding is investigated to substantially reduce or avoid the occurrence of intergranular helium-induced cracking (HeIC). Here, laser weld cladding, with and without ABSI technique, was applied to deposit weld metal on irradiated stainless steel (Type 304 L) blocks with various helium concentrations up to approximately 20 atomic parts per million (appm). Welded samples were cross-sectioned and analyzed using optical and electron microscopes. It was found that, with the improved laser welding techniques, no macroscopic (millimeter-sized) cracks were observed in any of the samples. Microscopic (less than 50 μm) cracks were present in the heat affected zone. Statistical analysis on the sample with the highest helium concentration (~20 appm) reveals that the ABSI technique could effectively reduce the size and amounts of microscopic cracks.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Influence of helium dilution of silane on microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by HW-CVD

Highlights: ► nc-Si:H films synthesized using HW-CVD method from silane and helium gas mixture without hydrogen. ► Volume fraction of crystallites and its size in the films decreases with increase in He dilution of SiH{sub 4}. ► Increase in Urbach energy and defect density with increase in He dilution of SiH{sub 4}. ► Increasing He dilution, hydrogen bonding in the films shifts from Si-H{sub 2} and (Si-H{sub 2}){sub n} complexes to Si-H. ► Hydrogen content films were found to be <2.2 at.% but the bandgap remains as high as 2.0 eV or even more. -- Abstract: We report influence of helium dilution of silane in hot wire chemical vapor deposition for hydrogenated nano-crystalline silicon films. Structural properties of these films have been investigated by using Raman spectroscopy, low angle x-ray diffraction, Fourier transform infra-red spectroscopy and non-contact atomic force microscopy. Optical characterization has been performed by UV–visible spectroscopy. It has been observed that in contrast to conventional plasma enhanced chemical vapor deposition, the addition of helium with silane in hot wire chemical vapor deposition has an adverse effect on the crystallinity and the material properties. Hydrogen content in the films was found <2.2 at.% whereas the bandgap remain as high as 2 eV or more. Increase in Urbach energy and defect density also suggests the deterioration effect of helium on material properties. The possible reasons for the deterioration of crystallinity and the material properties have been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Monitoring of helium gas leakage from canister storing spent nuclear fuel: Radiological consequences and management

Chloride-induced stress corrosion cracking (CISCC) of canisters has been identified as a potential degradation mechanism requiring ageing management — especially in a marine air environment. Methods for monitoring helium gas leakage from canisters have been developed on the basis of a phenomenon that the bottom temperature of the canister increases as the top temperature of the canister decreases when helium gas leaks during storage. In this paper, we consider the radiological consequences of CISCC based on two scenarios of helium gas leakage and propose effective management that includes a timeline for monitoring and mitigation. Monitoring helium gas leakage by measuring the difference in temperature between canister surfaces in the dry cask storage of spent fuel (SF) can detect leakage before SF undergoes oxidation as a result of air ingress and before country-specific regulatory limits of off-site doses are exceeded, thereby enabling mitigatory action.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Temperature threshold for preferential bubble formation on grain boundaries in tungsten under in-situ helium irradiation

Understanding a material's radiation tolerance requires examining its performance under different irradiation conditions. In this work, we investigate the radiation tolerance in terms of helium bubble damage in tungsten irradiated in-situ with 16 keV helium at 1073 K and 1223 K. Damage evolution represented by helium bubble density, size and total change in volume in the grain matrices and the grain boundaries are quantified as a function of fluence. Preferential large bubble formation and a higher change in volume on the grain boundaries occurred at 1223 K, suggesting faster migration of certain helium-vacancy complexes as confirmed by a diffusion-reaction model.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Tribology of incoloy 800HT for nuclear reactors under helium environment at elevated temperatures

Nickel-based alloy 800HT is considered one of the main candidate alloys for nuclear reactors with gas cooled high-temperature environment and, therefore, it is necessary to have a thorough understanding of the alloy tribological response for obtaining optimum operating and loading conditions. The current study investigated the wear and friction behavior of alloy 800HT using a customized high temperature tribometer to simulate the environment of helium cooled reactor up to 750 °C. The effect of temperature, contact load, environment, sliding distance and sliding speed on the alloy friction and wear were studied. To generalize the study for other applications, an investigation is also performed in air environment. The friction and wear coefficients have the highest values at high temperature helium atmosphere where the formation and stability of the oxide scales play an important role. Optical and contact profiling, scanning electron microscopy, as well as energy dispersive spectroscopy techniques were utilized to study the surface oxide. The analysis showed the presence of Fe-Cr-Ni rich oxide both in air and helium. Finally, the protective glazed layer did not form in helium in any condition, whereas in air and under specific conditions a stable protective oxide layer was observed.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Helium irradiation effects in polycrystalline Si, silica, and single crystal Si

Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin ({approx_equal}55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change.

36 MATERIALS SCIENCE↗