Helium Accumulation Research Articles

Study of Radiation Resistance to Helium Swelling of Li2ZrO3/LiO and Li2ZrO3 Ceramics

The key aim of this paper is to study the presence effect of LiO impurity phases in Li2ZrO3 ceramics on the resistance to helium swelling and structural degradation during implanted helium accumulation in the near-surface layer structure. The hypothesis put forward is based on a number of scientific papers, in which it was reported that the presence of two or more phases in lithium-containing ceramics led to a decrease in the rate of radiation damage and gas swelling due to the presence of additional interfacial boundaries that prevent the point defect accumulation. As a result of the evaluation of the crystal structure deformation, it was found that the presence of the LiO impurity phase in the structure of Li2ZrO3 ceramics led to a threefold decrease in the deformation of the crystal lattice due to helium swelling at doses of 5 × 1017–5 × 1018 ion/cm2. At the same time, the nature of the crystal lattice deformation for different ceramic types is different: in the case of Li2ZrO3 ceramics, an anisotropic distortion of the crystal structure is observed, in the case of Li2ZrO3/LiO ceramics, the crystal lattice deformation has an isotropic nature.

The principles of helium exploration

Commercial helium systems have been found to date as a serendipitous by-product of petroleum exploration. There are nevertheless significant differences in the source and migration properties of helium compared with petroleum. An understanding of these differences enables prospects for helium gas accumulations to be identified in regions where petroleum exploration would not be tenable. Here we show how the basic petroleum exploration playbook (source, primary migration from the source rock, secondary longer distance migration, trapping) can be modified to identify helium plays. Plays are the areas occupied by a prospective reservoir and overlying seal associated with a mature helium source. This is the first step in identifying the detail of helium prospects (discrete pools of trapped helium). We show how these principles, adapted for helium, can be applied using the Rukwa Basin in the Tanzanian section of the East African Rift as a case study. A thermal hiatus caused by rifting of the continental basement has resulted in a surface expression of deep crustal gas release in the form of high-nitrogen gas seeps containing up to 10% 4 He. We calculate the total likely regional source-rock helium generative capacity, identify the role of the Rungwe volcanic province in releasing the accumulated crustal helium and show the spatial control of helium concentration dilution by the associated volcanic CO 2 . Nitrogen, both dissolved and as a free-gas phase, plays a key role in the primary and secondary migration of crustal helium and its accumulation into what might become a commercially viable gas pool. This too is examined. We identify and discuss evidence that structures and seals suitable for trapping hydrocarbon and CO 2 gases will likely also be efficient for helium accumulation on the timescale of the Rukwa Basin activity. The Rukwa Basin prospective recoverable P 50 resources of helium have been independently estimated to be about 138 BSCF (billion standard cubic ft: 2.78 × 10 9 m 3 at STP). If this volume is confirmed it would represent about 25% of the current global helium reserve. Two exploration wells, Tai 1 and Tai 2, completed by August 2021 have proved the presence of seal and reservoir horizons with the reservoirs containing significant helium shows. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

Comparative study on the radiation resistance of pure and He-doped Gd2Zr2O7 pyrochlore by DFT + U calculation

The helium accumulation in the pyrochlore ascribed to alpha decay has important effects on its performance during service. This work adopts first-principle method to investigate the structural evolution, discrepancy in mechanical properties and radiation resistance before and after He doping in Gd2Zr2O7 pyrochlore. By calculating each configuration of pure and He-doped Gd2Zr2O7, it is found that the unit cell expands and distorts due to the doping helium. Single helium atom prefers to occupy the octahedral interstitial site and Gd vacancy. The variation of elastic constants and elastic modulus implies that helium doping has no obvious damage to mechanical stability. He-Gd2Zr2O7 systems have lower antisite defects formation energy and higher vacancy defects formation energy, indicating better radiation resistance. Moreover, the electronic structure analysis shows that <Zr-O> bonds have a greater influence on radiation resistance than <Gd-O> bonds. This study demonstrates that Gd2Zr2O7 pyrochlore would exhibit excellent radiation resistance in long-term service.

Helium accumulation in Cr18Ni10Ti austenitic steel under different saturation methods

Investigation results of changes in the surface structure and helium accumulation in steel Cr18Ni10Ti at various saturation methods are presented. It is shown that different methods of helium introduction can lead to both different and identical changes in the structure and to forms of helium accumulation.

Effect of He on the hardness and incipient plasticity of W films during nanoindentation

In future fusion reactors, the production and accumulation of helium (He) will lead to a dramatic mechanical failure for the material. Tungsten (W) is considered as one of the most promising plasma facing materials. In order to study the effect of He on mechanical properties of W, the novel He-charged W films with a homogeneous He dose profile were fabricated by the magnetron sputtering device. The prepared samples in this study can well avoid the problems like damage gradient effect, indentation size effect, and soft substrate effect during the nanoindentation test, which will appear in the samples obtained by conventional high energy He+ implantation. Nanoindentation results showed that the hardness of the films increased with the increasing He content, and the hardness-depth relationship is in good agreement with the Nix-Gao model. The pop-in events can be observed in the pure W film, which were absent in the He-charged films. Results of molecular dynamics (MD) simulations proved that, the He bubble not only promotes the nucleation and propagation of dislocations, but also hinders the dislocation movement, which leads to a smoother transition from elastic to plastic deformation in the indentation process.

Microstructural effects of high dose helium implantation in ErD2

Metal hydrides can store hydrogen isotopes with high volumetric density. In metal tritides, tritium beta decay can result in accumulation of helium within the solid, in some cases exceeding 10 at.% helium after only 4 years of aging. Helium is insoluble in most materials, but often does not readily escape, and instead coalesces to form nanoscale bubbles when helium concentrations are near 1 at.%. Blistering or spallation often occurs at higher concentrations. Radioactive particles shed during this process present a potential safety hazard. This study investigates the effects of high helium concentrations on erbium deuteride (ErD2), a non-radioactive surrogate material for erbium tritide (ErT2). To simulate tritium decay in the surrogate, high doses of 120 keV helium ions were implanted into ErD2 films at room temperature. Scanning and transmission electron microscopy indicated spherical helium bubble formation at a critical concentration of 1.5 at.% and bubble linkage leading to nanoscale crack formation at a concentration of 7.5 at.%. Crack propagation occurred through the nanocrack region, resulting in spallation extending from the implantation peak to the surface. Electron energy loss spectroscopy was utilized to confirm the presence of high-pressure helium in the nanocracks, suggesting that helium gas plays a predominant role in deformation. This work improves the overall understanding of helium behavior in ErD2 by using modern characterization techniques to determine: the critical helium concentration required for bubble formation, the material failure mechanism at high concentration, and the nanoscale mechanisms responsible for material failure in helium implanted ErD2.

Actinides induced irradiation damage and swelling effect in irradiated Zircaloy-4 after 30 years of storage

Abstract. After several years in the reactor core, irradiated nuclear fuel is handled and subsequently stored for a few years under water next to the core, to achieve thermal cooling and decay of very short-lived radionuclides. Thereafter, it might be sent to dry-cask interim storage before final disposal in a deep geological repository. Here, the spent nuclear fuel (SNF) is subject to a series of physicochemical phenomena which are of concern for the integrity of the nuclear fuel cladding. After moving the SNF from wet to dry storage, the temperature increases, then slowly decreases, leading the hydrogen in solid solution in the cladding to precipitate radially with consequent hydride growth and cladding embrittlement (Kim, 2020). Another phenomenon affecting the physical properties of the cladding during interim dry storage is the irradiation damage produced in the inner surface of the cladding by the alpha decay of the actinides present at the periphery of the pellet, particularly when the burnup at discharge is high. SNF pellets with high average burnup present larger fuel volumes at the end of their useful life due to accumulation of insoluble solid fission products and noble gases, which leads to disappearance of the as-fabricated pellet–clad gap. Further swelling is expected as a consequence of actinide decay and the accumulation of helium. This leads to larger cladding hoop stress and larger alpha decay damage. The present work first investigates the variation in diameter caused by pellet swelling in an irradiated Zircaloy-4 cladding after chemical digestion of the uranium oxide (UOx) pellet. Second, the irradiation damage produced during the 30 years elapsed since the end of irradiation in terms of displacements per atom (dpa) is studied by means of the FLUKA Monte Carlo code. The irradiation damage produced by the decay of actinides in the inner surface of the cladding extends for less than 3 % in depth. The considered cladded UOx pellet was extracted from a pressurized water reactor (PWR) fuel rod consisting of five segments, with an average burnup at discharge of 50.4 GWd (tHM)−1.

Study of Resistance to Helium Swelling of Lithium-Containing Ceramics under High-Temperature Irradiation

The aim of this work was to study resistance to helium accumulation processes in the structure of the surface layer of lithium-containing ceramics and the subsequent destruction and embrittlement processes, depending on radiation fluence. The objects of study were Li2TiO3-type ceramics obtained by thermal sintering. The fluence dependency of changes in the structural and strength properties of ceramics was determined to be in the range from 1018 to 1022 ion/m2, which corresponded to the concentration of implanted helium from 0.01% to 0.8–1 at.%. Irradiation was carried out at a temperature of 700 °C, which made it possible to simulate the processes of radiation damage that were closest to the real conditions in the reactor core. During the studies carried out, it was found that, at irradiation fluences of 1018–1020 ion/m2, the formation of point radiation defects was equaled by the process of thermal annealing of defects, as a result of which the concentration of defects and their effect on the change in the structural and strength properties of ceramics were insignificant. An increase in the concentration of implanted helium in the structure of the surface layer to above 0.5 at.% led to the dominance of radiation damage processes over the annealing of defects and the formation of gas-filled cavities, which negatively affects the strength of ceramics.

Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten

We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022 to 4 × 1025 He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019 He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022 He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

New ideas and preliminary practice for searching helium resources

<p indent="0mm">Helium resources have irreplaceable applications in many high-tech fields such as aerospace, medical treatment, and basic physics research. Thus, helium resources have become an important strategic material to win future scientific and technological competition. Currently, the main international helium exporters are the United States, Russia and Qatar. For a long time, China has been regarded as a helium-poor country, and the helium resources mainly rely on imports. Helium has become one of the potential risk factors in China’s future high-tech development, and it is urgent to find new solutions. Through the analysis of the physical and chemical properties of helium, it can be seen that helium can only exist as a gas state after being produced by nuclear fission inside the Earth. As the most inert gas, helium does not react with any other elements. Therefore, helium is not associated with other compound gases, such as natural gas (CH₄), CO₂, H₂S, etc. Besides, helium has a molecular weight of 4 or 3, which is only heavier than hydrogen (molecular weight is 2). As the lightest gas, helium tends to get rid of the constraints of the Earth’s gravity. Thus, helium tends to accumulate in a dome-shaped stratum structure with higher altitude and good sealing characters, rather than a region with a basin structure. In previous studies, people have always believed that helium and natural gas have an accompanying relationship. Therefore, many geological theories of searching for helium gas often focus on the basin area. Here in this work, we propose a new theory on the migration and accumulation of helium, which indicates that the Tibet Plateau is expected to find helium resources with exploitable value. In June 2021, our team members went to Tibet and collected the gas in the hot spring. In order to ensure the purity of the gas, we use the drainage method to collect the gas from the hot spring. This method can avoid pollution caused by gas components in the air. By using a high-resolution mass spectrometer, we analyzed the gas composition in the hot springs in Tibet. All the gas samples from hot springs are detected with high concentration of helium, which is a good verification of the new-proposed model. The air in Beijing and Tibet was used as a reference sample, and the mass spectrometry measurement was also carried out, but no signal of helium gas was found. The concentration of helium in the hot springs in Tibet is comparable to the concentration of helium in the Hetian River gas in the Tarim Basin. The higher concentration of helium in the Qinghai-Tibet Plateau is worthy of further exploration. This point of view breaks the previous habit of thinking that the search for helium must be in the basin area. The plateau area provides a new option for searching for helium gas. It is of great significance for China to solve the shortage of helium resources and external dependence, and will also give a huge boost to the development of other scientific and technological fields in China.

DETERMINATION OF CRITICAL DOSES OF RADIATION DAMAGE TO ALN CERAMIC UNDER IRRADIATION OF HELIUM AND HYDROGEN IONS

The work is devoted to the study of the radiation damage kinetics to heat-conducting, insulating and mechanical properties in polycrystalline ceramics based on aluminum nitride under irradiation of helium and hydrogen ions, as well as the determination of critical doses that cause maximum irreversible consequences. The choice of ions for irradiation is due to the ability to simulate the radiation damage processes during the accumulation of helium and hydrogen ions in the structure of the near-surface layer with the subsequent formation of gas-filled bubbles. During the studies carried out, it was found that at doses of irradiation with helium ions above 1х1017 ion/cm2 , there is a sharp deterioration in thermal conductivity and a decrease in ceramic resistance, which is associated with the onset of the formation of helium bubbles and partial embrittlement of the near-surface layer. However, an increase in the radiation dose above 5x1017 ion/cm2 does not lead to significant changes in thermal conductivity and insulation characteristics, which indicates the effect of radiation damage accumulation and a decrease in the ceramic degradation rate. In contrast to irradiation with helium ions, irradiation with hydrogen ions to doses higher than 1- 3х1017 ion/cm2 does not lead to significant changes in the thermal insulation characteristics, which indicates the ceramic resistance to hydrogen absorption processes.

Ab Initio Investigation of Helium Mobility in La2Zr2O7 Pyrochlore

The α-decay of incorporated actinides continuously produces helium, resulting in helium accumulation and causing security concerns for nuclear waste forms. The helium mobility is a key issue affecting the accumulation and kinetics of helium. The energy barriers and migration pathways of helium in a potential high-level nuclear waste forms, La2Zr2O7 pyrochlore, have been investigated in this work using the climbing image nudged elastic band method with density functional theory. The minimum energy pathway for helium to migrate in La2Zr2O7 is identified as via La–La interstitial sites with a barrier of 0.46 eV. This work may offer a theoretical foundation for further prospective studies of nuclear waste forms.

Effects of grain boundaries and nano-precipitates on helium bubble behaviors in lanthanum-doped nanocrystalline steel

Helium accumulation in advanced nuclear systems would accelerate the failure of structural materials. Nanostructured materials have the potential to mitigate helium effects by providing more nucleation sites for He bubbles. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) possesses ultra-fine and stable nano-grains ~45 nm in diameter and high-density fine nano-precipitates. In this work, we implanted NC304-La with helium ions, then annealed at different temperatures after implantation. We found that combining the key features of nanocrystalline steels and oxide dispersion-strengthened steels, NC304-La showed excellent bubble swelling resistance. We characterized the interaction between He bubbles and the two features in NC304-La: grain boundaries and nano-precipitates. Through detailed study for the heat-treatment related behaviors of He bubbles in this multi-sink system, the mechanisms of He bubble coarsening resistance in NC304-La were proposed.

Effect of Sequential Helium and Nickel Ion Implantation on the Nano-Indentation Hardness of X750 Alloy

Abstract Sequential He+ and Ni+ implantations were performed to investigate their combined effect on the indentation hardness of heat-treated X750 alloy. The microstructure of the ion-implanted region was also characterized with transmission electron microscope (TEM). The X750 alloy displayed a pronounced softening with very low Ni+ implantation levels, ψ = 0.01–1.0 dpa, however it showed a clear increase in hardness when implanted with He+ up to CHe = 5000 appm. Samples subjected to sequential He+ and Ni+ implantations displayed hardness values between those presented by sole He+ or Ni+ implantation suggesting that the effects of ion-induced microstructural damage and helium accumulation on the hardness of this alloy can be considered as independent and additive over the range of conditions studied. This observation is in contradiction to previously reported TEM-based studies, which suggest that accumulated helium slows the dissolution/disordering of the γ′ hardening phase in this alloy. In our study, established theories were applied to assess the contribution of ion-induced defect clustering, γ′ precipitate disordering, and helium bubble accumulation to the hardness of the X750 alloy. It was observed that generation of ion-induced defect clusters and the formation of helium bubbles increased the indentation hardness slightly while the disordering of γ′ precipitates resulted in a dramatic decrease in the total hardness. Ni+ and He+ implantation also had different effects on the depth dependence of the indentation hardness indentation size effect (ISE). The ISE was pronounced in the samples subjected to only Ni+ implantation while it was almost absent in samples subjected to only He+ implantation.

Mechanisms of plastic deformation and fracture of austenitic chromium-nickel steel irradiated during 45 years in WWER-440

The test results of tensile round bars from austenitic stainless steel of 18Cr-10Ni-Ti grade (Russian analog of AISI 321 steel) irradiated during 45 years in Pressure Water Reactor of WWER-440 type with various damage doses are represented over wide temperature range. The standard mechanical properties including the fracture stress and the fracture strain are determined, and the fracture mechanisms are studied. A phenomenon of decrease in the fracture strain with decrease of temperature is revealed that is accompanied by transition to intergranular brittle fracture. This phenomenon is termed as low temperature helium embrittlement (LTHE). On the basis of the analysis of other published works it has been concluded that LTHE is sufficiently common phenomenon that is revealed for various austenitic steels irradiated with mixed neutron spectrum.It is shown that the strain-induced martensite and low cohesive strength of grain boundary (GB) due to the helium accumulation on GBs are two necessary factors which cause LTHE of the irradiated austenitic steels. Mechanism of microcrack nucleation on weakened GB is proposed that takes into account the morphology of the strain-induced martensite formed near GB.Some features of plastic deformation over the temperature range of LTHE are revealed and explained as follows: (i) a running neck and large yield drop; and (ii) a difference in the true stress-strain curves for the cross-section with the originally arisen neck and for other cross-sections of tensile round bar over initial plastic strain range.

DEPENDENCES OF HELIUM RETENTION IN TUNGSTEN AND TANTALUM COATINGS IRRADIATED WITH He+ IONS ON FLUENCE AND TEMPERATURE

The processes of helium accumulation and thermal desorption for tungsten and tantalum coatings deposited on a stainless-steel substrate with an intermediate titanium layer were studied at various temperatures of the samples when irradiated with He+ ions to various fluences. The dependences of the concentration of captured helium and the form of the spectra of its thermal desorption into vacuum were found both on the fluence of He+ ions and on the temperature of the samples upon irradiation. Possible mechanisms for the accumulation and thermal desorption of helium, as well as the formation of defects in the crystal lattice of the samples, are discussed.

Effects of elastic softening and helium accumulation kinetics on surface morphological evolution of plasma-facing tungsten

Based on a continuous-domain model, capable of accessing the spatiotemporal scales relevant to fuzz formation on the surface of plasma-facing component (PFC) tungsten, we report self-consistent simulation results that elucidate the effects of elastic softening and helium (He) accumulation kinetics on the surface morphological response of PFC tungsten. The model accounts for the softening of the elastic moduli in the near-surface region of PFC tungsten, including both thermal softening at high temperature and softening due to He accumulation upon He implantation. The dependence of the elastic moduli on the He content follows an exponential scaling relation predicted by molecular-dynamics simulations, while the He content in the near-surface region of PFC tungsten evolves according to a first-order saturation kinetics, consistent with experimental and simulation results reported in the literature. We establish that this elastic softening accelerates both nanotendril growth on the PFC surface and the onset of fuzz formation. We also explore the role of the rate of He accumulation to a saturation level in the near-surface region of irradiated tungsten in the onset of fuzz formation. For PFC tungsten surfaces such as W(110) where, under typical irradiation conditions, the characteristic time scale for stress-driven surface diffusion is comparable to the characteristic time scale for He accumulation, we find that accelerating the rate of He accumulation accelerates the growth rate of nanotendrils emanating from the surface. Additionally, we present a systematic parametric study of the PFC surface morphological response to explore its dependence on the He accumulation kinetics that is controlled by the irradiation conditions for low-energy implantation. Finally, we introduce an incubation time for nanotendril growth on the PFC surface, a concept equivalent to that of incubation fluence discussed in the literature, to predict and explain the minimum exposure time required to observe fuzz formation on PFC tungsten surfaces.

On the CFD modelling of hydrogen dispersion at low-Reynolds number release in closed facility

Hydrogen release inside closed facilities could cause explosions with harmful consequences. Safety assessment should be performed, in order to design prevention and mitigation measures in case of such an accident. A numerical study for helium (as hydrogen surrogate) accumulation inside a closed facility representative of a real-scale garage at low release rate is conducted. Due to the nature of the examined flow several turbulence modelling approaches (RANS and LES type) and the laminar approach are examined with the aim to evaluate their predictive capabilities in flows resulting from low-Reynolds number leaks. Best practice guidelines are followed in the simulations, several sensitivity studies are performed and different grid types are examined. The comparison of computational results with experimental data shows that RANS and LES approaches reproduce well the gas distribution inside the facility, while laminar approach predicts more enhanced stratification at the release phase. Statistical Performance Measures are used to evaluate the models and narrower acceptable ranges are suggested for releases in indoor configurations compared to open environments.

Experimental study on accumulation of helium released into a semi-confined enclosure with distributed leaks

This paper examines the helium dispersion behavior in a 16.6 m3 enclosure with a small opening in the floor and distributed leaks along the edges. Helium, a simulant for hydrogen, was injected near the center of the floor with an injection rate ranging from 2 to 50 standard liters per minute (Richardson number of 0.3–134) through an upward-facing nozzle. In a short-term transient, the helium distribution predicted with the models of Baines & Turner (1969) and Worster & Huppert (1983) matched the measured distributions reasonably well. In a long-term transient, the vertical helium profile always reached a steady state, which consisted of a homogenous layer at the top overlaying a stratified layer at the bottom. The helium transients in the uniform layer predicted with the models of Lowesmith (2009) and Prasad & Yang (2010), assuming a vent was located in the ceiling, were in good agreement with the measured transients.

Temperature Dependence of Deuterium and Helium Accumulation in W and Ta Coatings during D+- or He+-Ion Irradiation

Temperature Dependence of Deuterium and Helium Accumulation in W and Ta Coatings during D+- or He+-Ion Irradiation