Helium Content Research Articles

Is helium the key parameter in the extended colour spread of the first generation stars in M3?

The study of the "chromosome maps" of Galactic Globular Clusters has shown that the stars identified as `first generation' often define an extended sequence in the $m_{F275W}-m_{F814W}$ colour, whose straightforward interpretation, by comparison with synthetic spectra, is that they are inhomogeneous in helium content. The cluster M 3 (NGC 5272) is one of the most prominent example of this phenomenon, since its first generation is distributed on an extended colour range, formally corresponding to a large helium enhancement ($ \sim 0.1$). It is necessary to ask whether the bulk of photometric observations available for this cluster supports or falsifies this interpretation. For this purpose, we examine the horizontal branch morphology, the period and magnitude distributions of the RR Lyrae variables, and the main sequence colour distribution. Simulating the first generation stars with such internal variation of helium content we can not meet all the observational constraints at the same time, concluding that the origin of the first generation colour spread is still without a straightforward explanation.

Experimental Investigation of Natural Circulation During Air-Ingress Scenario in a Very High Temperature Reactor

In order to investigate air-ingress phenomena in a gas-cooled very high temperature reactor (VHTR), natural circulation experiments have been conducted in a helium flow loop after the injection of nitrogen into the lower plenum. A pair of helium analyzers were used to measure the nitrogen and helium concentrations in the lower plenum and upper plenum. The changes in the nitrogen concentration in the upper plenum were used to calculate the time required for the transport of nitrogen from the lower plenum to upper plenum through a riser flow channel made of graphite. The effect of system temperature and pressure on the rate of nitrogen transport has been studied extensively. Furthermore, a close examination of the graphite flow channel wall temperatures at different elevations showed small but sudden drops indicating the arrival of nitrogen at each elevation. From these data, the upward transport of nitrogen injected into the lower plenum under natural circulation conditions could be quantitatively investigated. The experimental findings indicate that the driving mechanisms for air transport through the reactor core of VHTR would result from both molecular diffusion and natural circulation. At low graphite temperatures in the riser, molecular diffusion is the dominating mechanism; however, as the riser temperature increases, natural circulation becomes dominant and the rate of nitrogen transport increases. Further, the time constants for these mechanisms have been calculated using a simplified species transport equation.

A flammability limit model for hydrogen-air-diluent mixtures based on heat transfer characteristics in flame propagation

Abstract Predicting lower flammability limits (LFL) of hydrogen has become an ever-important task for safety of nuclear industry. While numerous experimental studies have been conducted, LFL results applicable for the harsh environment are still lack of information. Our aim is to develop a calculated non-adiabatic flame temperature (CNAFT) model to better predict LFL of hydrogen mixtures in nuclear power plant. The developed model is unique for incorporating radiative heat loss during flame propagation using the CNAFT coefficient derived through previous studies of flame propagation. Our new model is more consistent with the experimental results for various mixtures compared to the previous model, which relied on calculated adiabatic flame temperature (CAFT) to predict the LFL without any consideration of heat loss. Limitation of the previous model could be explained clearly based on the CNAFT coefficient magnitude. The prediction accuracy for hydrogen mixtures at elevated initial temperatures and high helium content was improved substantially. The model reliability was confirmed for H2-air mixtures up to 300 °C and H2-air-He mixtures up to 50 vol % helium concentration. Therefore, the CNAFT model developed based on radiation heat loss is expected as the practical method for predicting LFL in hydrogen risk analysis.

Spontaneous Nucleation in Superheated Helium Solutions in Methane

The kinetics of spontaneous boiling of liquid methane saturated with helium has been studied in experiments on measuring the lifetime of the superheated liquid. The temperature dependence of nucleation frequency J has been determined in a range from 104 to 108 s–1 m–3 at pressures $$p$$ = 1.6 and 2.0 MPa and helium concentrations in liquid methane $$x$$ = 0.06 and 0.10 mol %. The experimental results have been compared with the data of the classical nucleation theory. As in the case of pure methane, the solution superheating temperatures reached in the experiments at J > 4 × 106 s–1 m–3 appear to be systematically lower (by 0.6–1.0 K) than their theoretical values. It has been shown that the discrepancy between the theoretical and experimental data is associated with the size dependence of the surface tension at the critical bubble–solution interface.

Heat Source Characteristics of Ternary-Gas-Shielded Tandem Narrow-Gap GMAW.

The characteristics of the welding heat source for tandem narrow-gap gas metal arc welding are examined for different ternary shielding gas (Ar-CO2-He) compositions. Results of previous calculations of arc properties for bead-on-plate geometry are adapted to the narrow-gap geometry to predict these characteristics. The heat source concentration factor decreases and the maximum heat flux density increases as the helium content increases, which leads to an increased welding heat efficiency. Addition of CO2 up to around 10% also increases the heat efficiency. When the CO2 content exceeds 10%, the heat source concentration factor increases significantly and the heat efficiency decreases. The shielding gas composition also affects the heat source distribution. The heat source characteristics are applied to a computational fluid dynamic model of the weld pool to predict the weld shape, and the predictions are verified by experiment. The results indicate that the appropriate addition of helium to the shielding gas can increase the heat transferred to the peripheral regions of the arc and increase the sidewall penetration.

Numerical study of the stratification erosion benchmark for NPPs containment using CFD code GASFLOW-MPI

During a severe accident involving a Nuclear Power Plant (NPP), a hydrogen-enriched stratification may occur in the dome of the containment. This is due to the gaseous density difference of a steam/hydrogen turbulent jet released from the break. This accident scenario is one of the high-ranking phenomena identified in the Phenomena Identification and Ranking Table (PIRT) for containment safety thermal hydraulics. In this work, the temporal evolution of the stratification erosion is analysed using the parallel CFD code GASFLOW-MPI with comparison to the experimental data from the THAI TH-20 benchmark, where helium is used as a hydrogen simulate. Identifying a suitable turbulence model is an important issue for a successful stratification erosion simulation due to turbulent mixing between the stratification and turbulent jet featured with the sharp velocity and density gradients. Three well-known turbulence models: 1. The classic k-ε model, 2. Large Eddy Simulation (LES) model and 3. Detached Eddy Simulation (DES) model are employed to evaluate their performances. Due to the negative buoyancy effect resulting from the light gas layer, the jet gradually loses its initial momentum and cannot penetrate the stratified layer in depth. The interaction Froude (Fr) number which is used to categorize the mixing mechanism is close to 1 in this work, indicating that the erosion process of the stratified layer is dominated by both the molecular diffusion and the momentum transport. The experimental data measured at different heights in THAI TH-20 benchmark, from the pure air region at the lower containment region up into the large helium concentration gradient region, and further up to high helium concentration region at the top of the containment. The calculated results show that the LES and DES turbulence models can capture time histories of helium concentration at all of the sensor points accurately, while the k-ε turbulence model predictions are always delayed due to its overestimating the turbulence mixing in the stratified layer. Moreover, compared with the k-ε model, more detailed turbulent eddy structure is resolved by the LES model and DES model, whose frequency satisfies the negative −5/3 energy decay rate for a wide region of frequency. Therefore, the LES and DES models are recommended for the stratification erosion simulation. This can provide guidance for nuclear engineers analysing hydrogen distribution phenomena during severe accidents.

Interaction between recrystallization and helium behavior in cold-rolled nickel

Introducing a high density of defect sinks is a promising approach to relieve helium embrittlement in structural alloys for future fusion energy applications. However, the interaction between preset defect evolution and helium behavior at elevated temperatures, which could lead to dramatic changes of microstructure and properties of alloys, is still not well understood. In this contribution, cold-rolled nickel subjected to He+ irradiation at 500 °C was investigated to uncover the interaction process and the consequent microstructure evolution. Our experimental results revealed that during the initial period partial recrystallization and grain growth were unaffected by He+ irradiation, leading to the formation of coarse grains. With the increasing of helium concentration, helium bubbles retarded dislocation rearrangement and grain boundary migration in the remaining recrystallization areas, and meanwhile the stabilized grain boundaries facilitated helium bubble coarsening and bubble-to-void transformation. Such interaction ultimately led to the retention of fine grains with severe swelling and hardening.

Evaluation of helium effect on irradiation hardening in F82H, ODS, SIMP and T91 steels by nano-indentation method

To access irradiation conditions of candidate materials used in advanced nuclear energy systems, simultaneous helium and iron-ion irradiation have been widely used to study on the synergistic effects of helium bubble nucleation and displacement damage. In present study, nano-indentation was used to study the irradiation hardening of F82H, T91, SIMP and ODS steels under dual Fe3+/He+-ion irradiation. Two different irradiation temperatures (300℃ and 450℃), three different helium injection ratios (60 appm He/dpa, 200 appm He/dpa and 600 appm He/dpa, displacements per atom: dpa), the same irradiation damage (5 dpa), were chosen. The effect of helium concentration on irradiation hardening was investigated. The irradiation hardness of these four kinds of common structural materials was compared. The research results are useful for the development of irradiation hardening resistant Fe-Cr alloy steels in future.

Dependence of the manifestation’s time of a hydrochemical earthquake forecast in the southern part of the Lake Baikal from their energy classes and epicentral distances

Have been studied the concentrations of dissolved helium in the water of a artesian well in the Southern Cis-Baikal region. It is established that on the eve of earthquakes they experience certain variations. Based on the obtained data, the correlation function lgτR (the logarithm of the time product of the harbinger of the seismic event at its epicentral distance) is calculated from the energy class of the earthquake. It is proposed to consider variations in the concentration of helium as a short-term precursor of earthquakes.

A determination of atmospheric helium, neon, argon, krypton, and xenon solubility concentrations in water and seawater

We have determined the concentrations of atmospheric helium, neon, argon, krypton, and xenon in distilled water and seawater equilibrated with moist marine air at one atmosphere over a temperature range from near freezing point to approximately 35 °C and a salinity range of zero to roughly 39.5 PSS78. In all, we made 74 sets of noble gas measurements at 34 distinct temperature and salinity combinations. The experiments included 35 replicate pairs of samples drawn from 35 separate equilibrations, which in turn had three pairs of repeat equilibrations run at close to identical temperatures. We fit the results to an eight-parameter function similar to one commonly used to compute solubility equilibrium concentrations of these gases for environmental waters. Based on an estimate of analytical accuracy, reproducibility of the comparison with secondary atmospheric standards, replicate sample reproducibility, and reproducibility of the repeat equilibrations, we estimate this function to predict equilibrium concentrations at a particular temperature and salinity within this range to overall precisions 0.10% or better. This includes the regression statistics associated with interpolation and analytical errors for fitting the 8-parameter smoothing function. There is an overall systematic uncertainty of 0.15% for all gases, based on our confidence in and experience with the integrity and cross-calibration of our air standards over recent years. Because our methods are calibrated using assumed and explicit atmospheric abundances of these noble gases, any subsequent uncertainties in atmospheric abundances cancel out in the determination, particularly when the oceanographic measurements utilize marine air as a primary standard. Furthermore, measurement at ambient atmospheric abundances avoids any potential co-solvency induced biases introduced by using ~1 atm pure noble gases, as has been done in most previous studies. We compare these determinations to those made by others in the past and find modest but significant systematic differences with those results.

Mass Loss of Different Stellar Populations in Globular Clusters: The Case of M4

Abstract In a Globular Cluster (GC), the mass loss during the red giant branch (RGB) phase and the helium content are fundamental ingredients to constrain the horizontal-branch (HB) morphology. While many papers have been dedicated to the helium abundance in different stellar populations, small efforts have been made to disentangle the effects of mass loss and helium content. We exploit the nearby GC NGC 6121 (M4), which hosts two well-studied main stellar populations, to infer both helium and RGB mass loss. We combine multi-band Hubble Space Telescope photometry of RGB and main-sequence (MS) stars of M4 with synthetic spectra to constrain the relative helium content of its stellar populations. We find that the second-generation stars in M4 are enhanced in helium mass fraction by with respect to the remaining stars that have pristine helium content. We then infer the mass of the HB stars by searching for the best match between the observations and HB populations modeled assuming the helium abundance of each population estimated from the MS. By comparing the masses of stars along the HB, we constrain the mass loss of first- and second-generation stars in M4. We find that the mass lost by the helium-enriched population is ∼13% larger than the mass lost by the first-generation stars ( ). We discuss the possibility that this mass-loss difference depends on helium abundance, the different formation environment of the two generations, or a combination of both.

Hubble Space Telescope analysis of stellar populations within the globular cluster G1 (Mayall II) in M 31

In this paper we present a multi-wavelength analysis of the complex stellar populations within the massive globular cluster Mayall II (G1), a satellite of the nearby Andromeda galaxy projected at a distance of 40 kpc. We used images collected with the Hubble Space Telescope in UV, blue and optical filters to explore the multiple stellar populations hosted by G1. The $m_{\rm F438W}$ versus $m_{\rm F438W}- m_{\rm F606W}$ colour-magnitude diagram shows a significant spread of the red giant branch, that divides $\sim 1$ mag brighter than the red clump. A possible explanation is the presence of two populations with different iron abundance or different C+N+O content, or different helium content, or a combination of the three causes. A similar red giant branch split is observed also for the Galactic globular cluster NGC6388. Our multi-wavelength analysis gives also the definitive proof that G1 hosts stars located on an extended blue horizontal branch. The horizontal branch of G1 exhibits similar morphology as those of NGC6388 and NGC6441, which host stellar populations with extreme helium abundance (Y>0.33). As a consequence, we suggest that G1 may also exhibit large star-to-star helium variations.

Is the additional increase of star luminosity due to partial mixing real?

The partial mixing of matter between the radiative envelope and convective core in an early B-type star produces an additional increase of star luminosity during main sequence evolution. High quality data on stellar mass and luminosity defined from studies of detached double-lined eclipsing binaries are used to check the existence of such additional increase. It is shown that the additional luminosity increase does not contradict observed high quality data, if the intensity of partial mixing is restricted by the observed increase in surface helium content.

Type Ib/Ic Supernovae: Effect of Nickel Mixing on the Early-time Color Evolution and Implications for the Progenitors

Abstract We investigate the effect of mixing of radioactive nickel (56Ni) on the early-time color evolution of Type Ib and Ic supernovae (SNe Ib/Ic) using multigroup radiation hydrodynamics simulations. We consider both helium-rich and helium-poor progenitors. Mixing of 56Ni is parameterized using a Gaussian distribution function. We find that the early-time color evolution with a weak 56Ni mixing is characterized by three different phases: initial rapid reddening, blueward evolution due to the delayed effect of 56Ni heating, and redward evolution thereafter until the transition to the nebular phase. With a strong 56Ni mixing, the color evolution is characterized by a monotonic reddening. We compare our models with the early-time color evolution of several SNe Ib/Ic (SN 1999ex, SN 2008D, SN 2009jf, iPTF13bvn, SN 1994I, SN 2007gr, SN 2013ge, and 2017ein) and find signatures of relatively weak and strong 56Ni mixing for SNe Ib and SNe Ic, respectively. This suggests that SNe Ib progenitors are distinct from SN Ic progenitors in terms of helium content and that 56Ni mixing is generally stronger in the carbon–oxygen core and weaker in the helium-rich envelope. We conclude that the early-time color evolution is a powerful probe of 56Ni mixing in SNe Ib/Ic.

A new equation of state for helium nanobubbles embedded in UO2 matrix calculated via molecular dynamics simulations

Molecular dynamics simulations have been carried out to determine the equation of state of helium inside nanobubbles embedded into a UO2 matrix. The parameters of the equation of state are fitted with the Brearley and MacInnes hard-sphere model based on the formalism of Carnahan-Starling used in fuel performance codes. This new equation of state takes into account the interactions between the surrounding UO2 matrix and the helium atoms. Four nanobubble sizes (diameters: 1, 2, 5, and 10 nm) have been investigated over four temperatures (300, 500, 700, and 900 K) and for initial helium concentration inside the bubble ranging from 0.33×105 to 3.9×105 mol.m−3 (corresponding to helium-to-vacancy ratio of 0.3–3.3, respectively). We find that helium atoms are inhomogeneously distributed inside the bubble. A boundary layer of 1 nm thickness appears at the bubble surface in which helium atoms are more concentrated and diffuse into the UO2 matrix. We also find a saturation concentration of the helium atoms that can be incorporated into the bubble. This concentration limit is equal to 1.6 helium atom per vacancy in UO2. It corresponds to an atomic volume of 7.8×10−30 m3, which is almost half of the value proposed with the original Brearley and MacInnes model (13×10−30 m3). For this threshold concentration and for bubble of diameter higher than 5 nm, nano-cracks and dislocations appear at the bubble surface. However, experimental observation is needed to confirm this finding. We calculated the critical pressures inside the bubble which yields to this onset of crack in UO2. These critical pressures are in good agreement with those calculated with the Griffith criterion for brittle fracture.

Detecting the signatures of helium in type Iax supernovae

Recent studies have argued that the progenitor system of type Iax supernovae must consist of a carbon-oxygen white dwarf accreting from a helium star companion. Based on existing explosion models invoking the pure deflagration of carbon-oxygen white dwarfs, we investigate the likelihood of producing spectral features due to helium in type Iax supernovae. From this scenario, we select those explosion models producing ejecta and 56Ni masses that are broadly consistent with those estimated for type Iax supernovae (0.014–0.478 M⊙ and ∼0.003–0.183 M⊙, respectively). To this end, we present a series of models of varying luminosities (−18.4 ≲ MV ≲ −14.5 mag) with helium abundances accounting for up to ∼36% of the ejecta mass, and covering a range of epochs beginning a few days before B-band maximum to approximately two weeks after maximum. We find that the best opportunity for detecting He I features is at near-infrared wavelengths, and in the post-maximum spectra of the fainter members of this class. We show that the optical spectrum of SN 2007J is potentially consistent with a large helium content (a few 10−2 M⊙), but argue that current models of accretion and material stripping from a companion struggle to produce compatible scenarios. We also investigate the presence of helium in all objects with near-infrared spectra. We show that SNe 2005hk, 2012Z, and 2015H contain either no helium or their helium abundances are constrained to much lower values (≲10−3 M⊙). For the faint type Iax supernova, SN 2010ae, we tentatively identify a small helium abundance from its near-infrared spectrum. Our results demonstrate the differences in helium content among type Iax supernovae, perhaps pointing to different progenitor channels. Either SN 2007J is an outlier in terms of its progenitor system, or it is not a true member of the type Iax supernova class.

Origin of helium and nitrogen in the Panhandle–Hugoton field of Texas, Oklahoma, and Kansas, United States

Helium and nitrogen variations in Panhandle–Hugoton field (PHF) gases are products of interaction between hydrocarbon gas from the Anadarko basin and at least two water masses with dissolved nitrogen and helium. The two most distinct water masses are from the Palo Duro basin (highest He/N2) and the Hugoton embayment (lowest He/N2). Geochemical data indicate several hundred million years of helium generation in porous rock. Helium migrated to the gas by diffusion through water-saturated rock and by west-to-east water flow. Sediment and basement helium generation and helium migration were modeled to validate timing and source of PHF helium. Models indicate a predominantly sedimentary helium source with some basement helium charge on the Amarillo uplift. Helium in the central and eastern PHF diffused from underlying rocks, whereas gases on the west and southwest sides were enriched in nitrogen and helium delivered by hydrodynamic water flow. Nitrogen in high-nitrogen gases was probably sourced as ammonium released from clays by cation exchange with brines derived from overlying salt units. The amount of mudrock (nitrogen and helium source) relative to other potential helium sources (arkose, radioactive dolomite) correlates to decreasing gas He/N2. The high helium concentrations in PHF gases result from multiple favorable circumstances. Old pore water accumulated dissolved helium during hundreds of millions of years of helium generation in sediment. High water/gas and low pressure favored higher helium concentrations in gas. Hydrodynamic flow delivered helium-rich pore water from basins west of the PHF.

Atomistic simulation study of the helium effects on the deformation behavior in nickel bicrystals

Experimental and theoretical studies have shown that helium impurities in nickel tend to precipitate into nano-sized bubbles. The formation of helium bubbles has been found to have detrimental effects on the mechanical properties of nickel and its alloys. However, the mechanisms that lead to the observed helium-induced embrittlement are still unclear. In the present work, we perform molecular dynamics simulations to study helium effects on the deformation behavior of nickel systems. The role of symmetric tilt grain boundary (STGB) structures in nickel is investigated for various helium concentration levels. The deformation behavior is investigated in relation to the STGB properties, the helium distribution, and the evolution of lattice defects in the nickel matrix. In pure nickel bicrystals, it is found that the tensile strength and failure strain depend on the misorientation angle and the GB energy, with the lowest-energy STGB exhibiting the highest strength. In bicrystals with helium, we observe that the emission of dislocation loops nucleated at helium clusters significantly reduces the strength. The mechanical properties were found to be dependent on the GB misorientation angle, with the GB energy having no significant effect. Reduction of the material strength, with the presence of helium, was found to be associated with heterogeneous nucleation of dislocation loops at helium bubbles, which was not limited to GB region.

Pathways in the molecular fragmentation for the C2H5OH/He electrical discharge

ABSTRACTThis study focuses on the glow discharge generated with a gases mixture of Ethanol (C2H5OH) and Helium (He), at different concentrations maintained at a total pressure of 2.0 Torr. We used optical emission spectroscopy (OES) to analyze the discharge mixture at different concentrations of Helium. Single Langmuir probe data was used to determine the Electron Energy Distribution Function (EEDF). For the total C2H5OH/He mixture plasma concentrations, the EEDF has a Maxwellian distribution function. A decrease in He concentration results in significant changes in the EEDF, this behavior is related to the increase in the C2H5OH percentage must increase the energy loses of the electrons in the inelastic collision with C2H5OH producing a significant change in the EEDF, therefore, the EEDF pattern results in an increase of electron–molecule reaction rates. The rise in electron temperature for increasing Helium percentage is explained by the decreasing electron energy loss in the inelastic collisions with C2H5OH molecule. It observes a decrease of electron density ne as a function of the Helium percentage, which can be related to the ratio between ionization cross sections of Helium and C2H6O molecule. The active species are generated in the electron-molecule processes, which are associated with electron impact dissociation of C2H5OH and Helium electronic impact excitation in the gas phase. The emission optical spectra (OES) show changes in the intensity of the most important peaks of the plasma mixture, which indicates the dependence in the formation of the plasma as a function of the percentage of the gases. The changes in the intensities of the same observed species are due to different processes of excitation and ionization energies of the system, in addition to the increase of He metastable states He I. Hydrogen is the main product obtained from the decomposition of C2H5OH.

Radiation damage assessment of the sixth SINQ target irradiation program based on MCNPX simulation

The 6th SINQ Target Irradiation Program (STIP VI) was conducted in SINQ Target 9 during 2011 and 2012. A radiation damage assessment based on a simulation using the MCNPX code was performed to obtain the irradiation parameters of displacement damage, helium and hydrogen concentrations in each specimen rod placed in the target.The gamma mapping performed on the proton beam entrance window after irradiation provided the proton fluence distribution used for implementing the source definition in the simulation. The proton and neutron fluences along the target as well as the energy deposition were calculated. Corresponding to the average beam current of 1.42 mA, the maximum neutron flux obtained is 5.95 × 1014 n/(cm2⋅ s) and the maximum energy deposition in the Zircaloy-2 cladding tubes is 568 W/cm3. The irradiation parameters of selected materials in each specimen rod were calculated by multiplying the proton and neutron fluences with the corresponding cross section data. The maximum displacement damage was obtained for Zircaloy-2 in Row 2 with a value of 72.1 dpa (displacement per atom). The maximum helium concentration was obtained for tantalum in Row 1 with a value of 3630 appm and the maximum hydrogen concentration was obtained for tungsten in Row 1 with a value of 17590 appm. All these values are the highest for the corresponding materials ever reached in a spallation target irradiation environment. The helium concentrations are also the highest ever obtained in bulk specimens.