Low Helium Concentrations Research Articles

Noble gas characteristics of microbial gas in the Qaidam Basin, China: Implications for helium enrichment processes

Helium is a critical, strategic and nonrenewable resource with limited reserves and unstable supplies. However, the helium sources and migration processes in petroliferous basins remain unclear. The Qaidam Basin features multiple reservoir layers, multiple genetic types of natural gas, and varying helium concentrations, providing an ideal natural laboratory for understanding the helium enrichment processes. Here, we present the original noble gas isotopic compositions and major gas components of 11 microbial gas samples from three gas layers in the Sebei No.1 gas field, eastern Qaidam Basin, China. The helium isotopic ratios range from 0.007 to 0.057 Ra, reflecting a predominantly crustal source. The Quaternary Sebei microbial gas features much lower helium concentration (0.0003–0.001%), 21Ne/22Ne ratio (0.031–0.034), and 40Ar/36Ar ratio (292–380) than thermogenic natural gases in the Qaidam Basin. The middle gas layer (II) of the Sebei gas field features higher 21Ne/22Ne and 40Ar/36Ar ratios than the top and bottom gas layers, displaying positive correlations with higher 4He groundwater concentrations before degassing, as calculated from 4He/20Ne ratios and 20Ne concentrations in air-saturated water. However, the correlations between the 4He contents in natural gas and the 21Ne/22Ne and 40Ar/36Ar ratios are unclear, suggesting that 4He dissolves into the groundwater system before degassing into a gas reservoir and that the variations in the 4He concentration in the gas phase are caused by the different levels of hydrocarbon dilution. Quantitative calculations of helium production revealed an external helium flux from the crustal basement except for the in-situ production from sedimentary strata even in this broadly-defined self-generating and self-preserving natural gas system. The helium resources in the Qaidam Basin are mainly distributed in tectonically active zones with uplifted pre-Jurassic basement rocks (providing sufficient helium sources), ancient groundwater systems (accumulating more helium) and suitable hydrocarbon production (acting as a carrier gas but diluting helium as slightly as possible).

A Threshold Helium Leakage Detection Switch with Ultra Low Power Operation.

Detecting helium leakage is important in many applications, such as in dry cask nuclear waste storage systems. This work develops a helium detection system based on the relative permittivity (dielectric constant) difference between air and helium. This difference changes the status of an electrostatic microelectromechanical system (MEMS) switch. The switch is a capacitive-based device and requires a very negligible amount of power. Exciting the switch's electrical resonance enhances the MEMS switch sensitivity to detect low helium concentration. This work simulates two different MEMS switch configurations: a cantilever-based MEMS modeled as a single-degree-freedom model and a clamped-clamped beam MEMS molded using the COMSOL Multiphysics finite-element software. While both configurations demonstrate the switch's simple operation concept, the clamped-clamped beam was selected for detailed parametric characterization due to its comprehensive modeling approach. The beam detects at least 5% helium concentration levels when excited at 3.8 MHz, near electrical resonance. The switch performance decreases at lower excitation frequencies or increases the circuit resistance. The MEMS sensor detection level was relatively immune to beam thickness and parasitic capacitance changes. However, higher parasitic capacitance increases the switch's susceptibility to errors, fluctuations, and uncertainties.

Effects of Helium Production and Displacement Damage on Microstructural Evolution and Mechanical Properties in Helium-Implanted Austenitic Stainless Steel and Ferritic/Martensitic Steel

The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

The influence of helium on cavity evolution in ion-irradiated T91

The effect of helium pre-implanted in concentrations spanning 0 to 1000 appm on the swelling evolution in ferritic-martensitic alloy T91 was explored. Irradiation with 5.0 and 4.4 MeV Fe2+ ions at 460 °C up to damage levels of 350 dpa were performed with pre-implanted helium levels of 0, 1, 10 and 1000 appm. Low concentrations of helium were found to enhance nucleation at low damage levels. However, at high damage levels, differences in nucleation became insignificant, and nearly identical cavity distributions were created. At low helium levels, the cavity evolution was facilitated by similar cavity sink strengths. High helium concentrations reduced the critical cavity radius and resulted in the formation of a high-density bubble population. The bubbles were <4 nm and remained stable even up to high damage levels, while the growth of larger cavities was suppressed. The suppression of swelling was found to be a result of helium trapping or alteration of cavity and dislocation bias. Dislocation and precipitate evolution was also studied and exhibited relative insensitivity to helium concentration, and did not significantly influence the cavity evolution.

Effects of helium concentration and radiation temperature on interaction of helium atoms with displacement cascades in bcc iron

Abstract In fusion applications, helium, implanted or created by transmutation, plays an important role in the response of reduced-activation ferritic/martensitic steels to neutron radiation damage. The effects of helium concentration and radiation temperature on interaction of interstitial helium atoms with displacement cascades have been studied in Fe-He system using molecular dynamics with recently developed Fe-He potential. Results indicate that interstitial helium atoms produce no additional defects at peak time and promote recombination of Frenkel pairs at lower helium concentrations, but suppress recombination of Frenkel pairs at larger helium concentrations. Moreover, large helium concentrations promote the production of defects at the end of cascades. The number of substitutional helium atoms increases with helium concentration at peak time and the end of cascades, but the number of substitutional helium atoms at peak time is smaller than that at the end of displacement cascades. High radiation temperatures promote the production at peak time and the recombination of defects at the end of cascades. The number of substitutional helium atoms increases with radiation temperature, but that at peak time is smaller than that at the end of cascades.

Radiation damage studies in fusion reactor steels by means of small-angle neutron scattering (SANS)

In this contribution, small-angle neutron scattering (SANS) studies of the micro-structural radiation damage in technical steels, developed for application in future fusion reactors, are presented. The effect of neutron irradiation at 250 °C–450 °C, up to dose levels of 16 dpa (displacement per atom), has been investigated in the European reference ferritic/martensitic steel Eurofer97 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%), both in its standard composition and mechanically alloyed with B (up to 1000 appm) to enhance the helium production, via transmutations, and reproduce fusion relevant helium/dpa ratios. The obtained SANS results suggest that in the irradiated standard Eurofer97 micro-voids distributions are present, with very small average radii (a few Å) and volume fractions (around 0.01); such low values, close to the SANS resolution limit, are consistent with the good resistance of this steel to radiation damage, for low helium concentrations. In the irradiated B-alloyed Eurofer97, more complex changes are observed: the helium bubble distributions appear strongly dependent on the irradiation temperature and on the helium concentration; furthermore, micro-structural effects possibly related to modifications in the steel matrix are detected.

Helium recovery using membrane processes

Abstract Helium is a key resource for many high-tech applications. The only feasible source of helium is natural gas, which can contain up to 2% helium. These low helium concentrations demand efficient separation and purification processes to be economically feasible. However, the currently wide-spread low-temperature distillation is energy intensive and only applied at high helium concentrations. Instead of being used, lower helium concentrations are usually vented to the atmosphere. Here, we compare different process alternatives for helium recovery for two cases: (A) A new natural gas plant is built (B) an existing plant is retrofitted for helium recovery. Process simulations are conducted in Aspen Plus and are combined with an economical evaluation. The results show that membrane processes can well compete with thermal separation technologies even at low helium concentrations.

Evolution of plasma parameters in a He-N2/Ar magnetic pole enhanced inductive plasma source

A magnetic pole enhanced inductively coupled He-N2/Ar plasma is studied at low pressure, to monitor the effects of helium mixing on plasma parameters like electron number density (ne), electron temperature (Te), plasma potential (Vp) , and electron energy probability functions (EEPFs). An RF compensated Langmuir probe is employed to measure these plasma parameters. It is noted that electron number density increases with increasing RF power and helium concentration in the mixture, while it decreases with increase in filling gas pressure. On the other hand, electron temperature shows an increasing trend with helium concentration in the mixture. At low RF powers and low helium concentration in the mixture, EEPFs show a “bi-Maxwellian” distribution with pressure. While at RF powers greater than 50 W and higher helium concentration in the mixture, EEPFs evolve into “Maxwellian” distribution. The variation of skin depth with RF power and helium concentration in the mixture, and its relation with EEPF are also studied. The effect of helium concentrations on the temperatures of two electron groups ( Tbulk and Ttail) in the “bi-Maxwellian” EEPFs is also observed. The temperature of low energy electron group ( Tbulk) shows significant increase with helium addition, while the temperature of tail electrons ( Ttail) increases smoothly as compared to ( Tbulk).

Two-phase coexistence for hydrogen-helium mixtures.

We use our quantum Gibbs ensemble Monte Carlo algorithm to perform computer experiments for the two-phase coexistence of a hydrogen-helium mixture. Our results are in quantitative agreement with the experimental results of Sneed, Streett, Sonntag, and Van Wylen. The difference between our results and the experimental ones is in all cases less than 15% relative to the experiment, reducing to less than 5% in the low helium concentration phase. At the gravitational inversion between the vapor and the liquid phase, at low temperatures and high pressures, the quantum effects become relevant. At extremely low temperature and pressure, the first component to show superfluidity is the helium in the vapor phase.

Modeling of thermodynamic conditions for the recovery of high-purity helium from gas mixtures by the hydrate formation method

The modern technology for the recovery of helium from heliumcontaining natural gas is based on the lowtemperature separation (fractional distillation) method, in which helium is separated from other gases in the course of their liquefaction. This method requires considerable energy and resource inputs. An energysaving route to separate gas mixtures can be enrichment of natural gas with helium upon clathrate hydrate formation (1). Gas (clathrate) hydrates are inclusion compounds consisting of water molecules, which form the host crystal lattice, and guest molecules occupying lattice cavities (2). Current interest in gas hydrates stems from their possible use as fuel, as well as from their possible effect on global climate changes (3) and the use of gas hydrate technologies for natural gas storage and transportation (4). The helium content in natural gas is usually low as compared with the major component methane, although there are some natural gas deposits contain� ing up to 8 vol % helium. Methane accounts for more than 70-80 vol % of such gas deposits. Commercially interesting for helium recovery are natural gas deposits containing more than 0.3 vol % helium (5). The exist� ence of clathrate helium hydrates is still debatable since no reliable experimental data supporting or refuting this fact have been available so far. However, experimental evidence for the possibility of helium hydrate formation has been reported in (6). It has been theoretically predicted that structure II helium hydrates can form at high pressures (beginning with 800 atm) and low temperatures (250 K) (7). It has been shown that, at the same temperatures, mixed structure I and II methane-helium clathrate hydrates form at lower pressures than pure helium hydrates even at small concentrations of methane in the gas phase (8). The present work is aimed to find a correlation between the composition of mixed methane-helium hydrates of cubic structures I and II and the composi� tion of the gas phase being at equilibrium with the hydrate, as well as to determine conditions of forma� tion of mixed methane-helium hydrates at low helium concentrations in the gas phase for developing meth� ods of deep purification of helium from methane via hydrate formation.

Helium and neon isotopes in São Miguel island basalts, Azores Archipelago: New constraints on the “low 3He” hotspot origin

Lavas from the São Miguel Island, Azores Archipelago, have peculiar isotopic compositions, including radiogenic lead and strontium and un-radiogenic neodymium. The peculiar isotopic trend of São Miguel is evident in the lead-lead diagram where both 207Pb/204Pb and 208Pb/204Pb ratios are high for a given 206Pb/204Pb ratio compared to other oceanic island basalts. This signature is unique among OIBs and is particularly evident in the Nordeste area, the oldest part of São Miguel island (≥1Ma). Only a few olivine samples from the Nordeste volcanic complex have been analyzed for helium. They show radiogenic helium signatures with 4He/3He up to 174,000 (R/Ra ~4) [Moreira et al., Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169: 189–205, 1999]. However, because the Nordeste volcano has an age between 1 and 4Ma and because these samples have low helium concentrations, these radiogenic helium isotopic ratios must be considered with caution as they can also reflect post eruptive radiogenic production. In this paper we present a detailed study of the helium and neon isotopic ratios obtained from 17 Nordeste samples in order to better constrain the helium isotopic signature of the São Miguel mantle source. By coupling helium and the other isotopic systems, we propose that the São Miguel source contains non-degassed material, enriched in U and Th, that was stored in the mantle for the last ~3Ga. As suggested by Elliot et al. [Elliott et al., The origin of enriched mantle beneath São Miguel, Azores, Geochim. Cosmochim. Acta 71: 219–240, 2007], underplated magma intruded into oceanic lithosphere and subducted ~3Ga ago is a possible explanation for the peculiar São Miguel source isotopic signatures.

CO2CRC Otway project–Soil gas baseline and assurance monitoring 2007–2010

A four-stage soil gas monitoring study was conducted from March 2007 to March 2010 in the western Port Campbell Embayment of the onshore Otway Basin, SE Australia. This study evaluated existing vadose zone soil gases over the CO2CRC Otway Project’s site for CO2 storage, and nearby deep natural CO2 accumulations. Carbon dioxide, methane and helium were the main gases analysed and much of the time CH4 concentrations were too low for accurate analysis. Carbon dioxide concentrations ranged from atmospheric background levels of 0.033% up to about 10% of the total soil gas. The δ13CCO2 values are typical for organic matter decomposition as the main source of the CO2 in the soil gas. Modern radiocarbon ages, determined from 14C analysis, and low helium concentrations support the conclusion that CO2 in the soil gas is predominantly from modern organic sources. There was a relative increase in CO2 concentration measured in locations where the Port Campbell Limestone is near the ground surface, which implies additional CO2 has been produced from the weathering of limestone.

High- 3He/ 4He, depleted mantle and low- δ18O, recycled oceanic lithosphere in the source of central Iceland magmatism

New helium and oxygen isotope data and trace element concentrations are reported for volcanic rocks from central Iceland. Basalts that are depleted in the most incompatible trace elements possess a wide range in 3He/ 4He but most ratios are similar to or higher than those of mid-ocean ridge basalt (MORB:∼8 R A [1] [D.W. Graham, Noble gas geochemistry of mid-ocean ridge and ocean island basalts: characterisation of mantle source reservoirs, in: D.P. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Noble gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, 2002, pp. 247–317]). The low concentrations of helium in these rocks suggest that significant degassing has made them susceptible to contamination by low- 3He/ 4He crust, therefore all measured 3He/ 4He are considered minimum estimates for their sources. Elevated helium isotope ratios in the source of these rocks result from interaction with high- 3He/ 4He mantle. The highest oxygen isotope ratios in the depleted rocks are similar to those in melts from typical depleted upper mantle and the range of δ 18O values is consistent with variable, limited amounts of contamination by Icelandic crust. Most of the incompatible trace element-enriched rocks possess 3He/ 4He ratios that are similar to or lower than those in MORB. These rocks were erupted close to the postulated centre of the Iceland plume. This observation contradicts models in which high- 3He/ 4He characterises the focus of mantle upwelling. A source with MORB-like 3He/ 4He ratios may also be common to other parts of the North Atlantic Igneous Province. The highest δ 18O values in the enriched rocks are lower than those in MORB and do not appear to have been affected by interaction with low- δ 18O Icelandic crust. Recycling of hydrothermally altered oceanic crust that has been subducted into the mantle provides a plausible mechanism for generating an 18O-poor source with the trace element and isotopic characteristics of the enriched lavas.

Helium and carbon isotope systematics of natural gases from Taranaki Basin, New Zealand

The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maui gas is over an order of magnitude higher in helium concentration (up to 190 μmol mol−1) and its 3He/4He ratio of 3.8 RA (where RA=the air 3He/4He ratio of 1.4×10−6) is approximately half that of upper mantle helium issuing from volcanic vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03–0.32 RA and low total helium concentrations of 10–19 μmol mol−1. The 3He/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The 3He/CO2 ratio of the Maui gases of 5 to 18×10−9 is higher than the MORB value of 0.2 to 0.5×10−9, a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO2/3He and CO2/C indicates δ13C(CO2) values between −7 and −5‰ close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with δ13C(CO2) c.−15‰. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth field. The radiogenic 4He content (Herad) varies across the Taranaki Basin with the highest Herad/C ratios occurring in the Maui field. δ13C(CH4) becomes more enriched in 13C with increasing Herad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and 3He/C is virtually constant in the Maui field, there is a correlation between RC/RA (where RC=air-corrected 3He/4He) and δ13C(CH4) in the Maui and New Plymouth fields, with the more negative δ13C(CH4) values corresponding to high 3He/4He ratios. A correlation between 3He/4He and δ13C(CO2) was also observed in the Maui field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and δ13C(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone.

Experiments on the instability modes of buoyant diffusion flames and effects of ambient atmosphere on the instabilities

Large scale dynamic behavior of buoyant diffusion flames were studied experimentally. It was found that buoyant diffusion flames originating from circular nozzles exhibit two different modes of flame instabilities. The first mode results in a sinuous meandering of the diffusion flame, characteristic of flames originating from small diameter nozzles. This instability originates at some distance downstream of the nozzle exit in the contraction region of the buoyant flame envelope and develops into a sinuous motion of the flame. The second mode is the varicose mode which develops very close to the nozzle exit as axisymmetric perturbations of a contracting flame surface. In this mode, flame oscillations result in the formation of toroidal vortical structures that convect through the flame and cause periodic burn out at the flame top resulting in the observed flame height fluctuations. The average flame heights are found to be typically shorter for these flames. The oscillation frequencies and their scaling for the two modes are also different with the sinuous mode having higher frequencies than the varicose mode. It was also observed that the instability can switch from one mode to the other and the probability of observing the varicose mode appears to increase with increasing Richardson number. Additionally, the feasibility of altering the behavior of buoyant diffusion flames was explored through variation of the oxidizer medium density. It was found that the flame oscillations can be completely suppressed for flames burning in helium rich helium–oxygen mixtures. At lower helium concentrations, the oscillation frequency can be significantly reduced. In order to enhance the buoyancy effect, CO2–O2 mixtures were also studied. However, the density increase and its effects on flame oscillation frequency were found to be small compared to those flames burning in air. These experiments point towards the feasibility of altering buoyant flame behavior under earth gravity and studying the large scale dynamical aspects of buoyant flames without the need of variable gravity environment.

Helium irradiation effects in single crystals of MgAl2O4

Magnesium aluminate spinel, (MgAl2O4), is a promising material as a uranium free matrix for the transmutation of americium. Fission products and α-particles are produced during the transmutation. The impact of α-particles is simulated by 30 keV 3He ion implantations at room temperature (RT) with the doses 6.2, 16, 20 and 53×1015cm−2. In another set of experiments a single crystal MgAl2O4 (100) sample is irradiated with α-particles (4.5 MeV) from a 241Am source at RT to a dose of 1.3×1012cm−2.Helium release from the implanted samples was studied by thermal desorption spectrometry (TDS). The numerical analysis of the experimental thermal desorption results of α-implanted samples to a very low helium concentration (0.0288 appm in the irradiation zone of 12.4 μm) show that helium release is dominated by helium interstitial diffusion with an activation energy of 1.8 eV. In the case of high dose implantation to 1.74 at.% in the implantation zone approximately of 100 nm, helium is released from He-vacancy clusters with the activation energy of 2.35 eV.The evolution of the helium concentration profile in the temperature range from RT to 1483 K is monitored by neutron depth profiling (NDP). It confirms that the release of helium is governed by dissociation from vacancy clusters.

The Statistics and Distribution of Helium in the Mantle

An unbiased estimate of the 3He/4He ratio (R) along the Earth's spreading ridge system is 9.14 ± 3.59 Ra (n = 503) where Ra is the atmospheric ratio (1.38 × 10∼6). By arbitrarily excluding all values with R > 11 Ra and from ocean depths less than 2500 meters, I obtain 7.91 ± 1.50 Ra (n = 212), which is close to previous “filtered” estimates based on the hypothesis that the excluded values have been influenced by plumes. These are biased estimates. Based on unbiased statistics, many of the socalled high-3He hotspot regions have isotopic ratios well within the MORB range, and all have absolute 3He concentrations much less than MORB. The high variance of some oceanic-island data compared to MORB reflects, in part, the difference between a small sample and a large sample, and magma-chamber processes. Values of 11 to 15 Ra are commonly attributed to deep mantle plumes and “indicative of lower mantle involvement,” but these values are within 2σ of the mean and are not exceptional. Much higher values combined with low absolute helium concentrations are commonly associated with the onset of rifting, or volcanism, and may reflect a shallow, or lithospheric, low-238U/3He (LONU) source. The temporal progression to average MORB-like values suggests that the bulk of the magmas at spreading ridges and large oceanic constructs comes from below the LONU level. The termination of spreading is associated with low ratios, 6 to 7 Ra, similar to values associated with the high-238U/204Pb (HIMU) mantle component and some oceanic islands.

Isotope Geochemistry of the Oceanic Mantle Near the Bouvet Triple Junction

We have measured the isotopes of helium and lead in three suites of dredged basalts from the Southwest Indian Ridge, the America-Antarctic Ridge, and the Mid-Atlantic Ridge near the Bouvet triple junction. The magmatic 3He/ 4He ratios range from 6.5 to 14.2 times atmospheric (R a), which encompasses the entire range of existing MORB data, and differs significantly from the commonly accepted value for depleted MORB of roughly eight times atmospheric (Ra). Several samples from near the Bouvet triple junction have extremely low helium concentrations, low 3He/ 4He ratios, and display internal isotopic disequilibrium between glass and vesicles. These basalt glasses are, therefore, unique among the sample suite in having a significant contribution from post-eruptive accumulation of radiogenic 4He. Although these data do not reflect the mantle source, they can be used to calculate eruption ages for the basalts, which fall between 7 and 100 kyr, and demonstrates the potential utility of He-U ages for MORB. Lead isotopic compositions from the dredged basalt glasses range from depleted MORB to those of Bouvet Island itself ( 206Pb/ 204Pb of 18.2–19.6). The results are consistent with a mantle plume origin of Bouvet Island in that high 3He/ 4He and 206Pb/ 204Pb ratios are observed both on Bouvet island and the ridge segment immediately adjacent to it. The American-Antarctic Ridge has lower He, Sr and Pb isotope ratios than those of the Southwest Indian Ridge, which suggests a lesser influence of the plume. The dredge samples from near the triple junction have relatively low 3He/ 4He ratios (7.4 ± 0.2 R a), suggesting that the plume is presently centered beneath Bouvet Island rather than beneath the triple junction. The data from the ridge segment adjacent to Bouvet Island, on the Southwest Indian Ridge, define a trend toward high helium, strontium, and lead isotope ratios, which could be explained by mixing processes. A single dredge from one ridge segment at 7°E, between the Islas Orcadas and Shaka Fracture Zones, defines most of the helium isotopic variability found throughout the region and defines an isotopic composition unique in the region. The highest 3He/ 4He ratios were obtained from this ridge segment (∼14 R a) and are associated with relatively unradiogenic strontium and lead isotopic compositions. The remarkable isotopic variability over short distances along the Southwest Indian Ridge and the America-Antarctic Ridge may be related to their slow spreading rates and the lack of steady state magma chambers which could homogenize geochemically distinct magma batches. Alternatively, there is a different scale of heterogeneity in the mantle beneath this region.

Welding irradiated stainless steel

Conventional welding processes produced severe cracking in irradiated stainless steel containing 1 to 12 appm helium from (n,α) reactions. A shallow penetration overlay technique was successfully demonstrated for welding irradiated stainless steel. The technique was applied to irradiated 304 stainless steel that contained 10 appm helium. Surface cracking, present in conventional welds made on the same steel at the same and lower helium concentrations, was eliminated. Underbead cracking was minimal compared to conventional welding methods. However, cracking in the irradiated material was greater than in tritium charged and aged material at the same helium concentrations. The overlay technique provides a potential method for repair or modification of irradiated reactor materials.

Creep properties and microstructures of helium implanted AISI 316L electron-beam weld and parent material

Creep properties and microstructures of the “as-received” electron-beam welds and its parent material of the former Next European Torus (NET) reference material AISI 316L (now a candidate material for the International Thermonuclear Experimental Reactor (ITER) program) have been investigated at 873 K as a function of applied stress and pre-implanted helium concentration. The results show that helium embrittlement effects are more serious in the parent material than in the welds. The creep properties of the welds are almost unaffected by helium concentrations below 300 appm. Weld specimens with low helium concentrations (CHe < 100 appm) ruptured in the welded zone with a ductile “transgranular” fracture. In those with higher helium concentrations some regions of “intergranular” fracture were also found. Parent material specimens always ruptured in a mixed fracture mode. TEM observation revealed that in the welded zone δ-ferrite occupies about half of the boundary area. δ-ferrite partly transforms into precipitates (molybdenum rich carbides, M23C6 etc.) and austenite during high temperature treatments. Preferential helium bubble sites are grain boundaries, δ-ferrite—matrix interfaces, incoherent parts of twin boundaries and disloactions. Quantitative helium bubble size and number distribution results indicate that in the matrix helium bubble sizes are lager but the bubble densities are lower than at boundaries or interfaces. With increasing helium concentration helium bubble densities at the different sites do not change much, while the sizes increase with slopes of about 0.3 in a power law fit.