Donald F. Cowgill

Helium Nano-Bubble Evolution in Aging Metal Tritides

A continuum-scale, evolutionary model of bubble nucleation, growth and He release for aging metal tritides is described which accounts for major features of the tritide database. Bubble nucleation, modeled as self-trapping of interstitially diffusing He atoms, occurs during the first few days following tritium introduction into the metal. Bubble growth by dislocation loop punching yields good agreement between He atomic volumes and bubble pressures determined from bulk swelling and 3He NMR data. The bubble spacing distribution determined from NMR is shown to remain fixed with age, justifying the separation of nucleation and growth phases and providing a sensitive test of the growth formulation. Late in life, bubble interactions are proposed to produce cooperative stress effects, which lower the bubble pressure. Helium generated near surfaces and surface-connected porosity accounts for the low-level early helium release. Use of an average ligament stress criterion predicts an onset of inter-bubble fracture in good agreement with the He/Metal ratio observed for rapid He release. From the model, it is concluded that He retention can be controlled through control of bubble nucleation.

Ion nitriding of titanium alpha plus beta alloy for fusion reactor applications

The titanium alpha plus beta alloy, Ti–6Al–4V, has been plasma nitrided at 1033 K in pure nitrogen to produce a 9 μm thick surface nitride zone, followed by a 5 μm thick Al-enriched zone and a 100 μm thick nitrogen diffusion zone. X-ray diffraction and Auger electron spectroscopy indicate that the nitride zone consists of two layers: a thin TiN layer at the surface followed by a Ti2N layer. In preliminary tests, the deuterium retention of nitrided titanium specimens was determined after exposure to fluences of 4×1025 m−2, 60 eV D+ ions at 440 K and of 2×1025 m−2, 800 eV D+ ions at 623 K. The results are compared to the retention found for similar exposure of the untreated alloy and to model simulations. Implications for the use of these materials in a fusion environment are discussed.

Uranium for hydrogen storage applications : a materials science perspective.

Under appropriate conditions, uranium will form a hydride phase when exposed to molecular hydrogen. This makes it quite valuable for a variety of applications within the nuclear industry, particularly as a storage medium for tritium. However, some aspects of the U+H system have been characterized much less extensively than other common metal hydrides (particularly Pd+H), likely due to radiological concerns associated with handling. To assess the present understanding, we review the existing literature database for the uranium hydride system in this report and identify gaps in the existing knowledge. Four major areas are emphasized: {sup 3}He release from uranium tritides, the effects of surface contamination on H uptake, the kinetics of the hydride phase formation, and the thermal desorption properties. Our review of these areas is then used to outline potential avenues of future research.

Dynamic implant profiling by low-energy nuclear reaction spectroscopy

Abstract A new method is presented for analyzing the nuclear-reaction charged-particle spectra emitted during ion bombardment of materials. The approach is based on energy rather than depth increments and includes explicitly the effects of multiple-scattering and straggling. The technique is applied to dynamic deuterium profiling of near-surface regions of materials during deuterium implantation. Theoretical sensitivities and depth resolutions are computed for deuteron energies of 40 and 200 keV. By detecting protons emitted in the backward direction, at 40 keV one can profile the top 0.2μm of a typical target, e.g., ScD 2 , with a fwhm depth resolution of 0.04μm, while maintaining a surface deuterium sensitivity of 800 counts per D/Sc atom ratio, per millicoulomb accumulated normal bombardment. At 200 keV, the profiled depth is 0.9 μm with a resolution of 0.07 μm and a sensitivity pf 2 × 10 4 counts/atom ratio·mC. Experimental profiles for deuterium occluding and non-occluding materials illustrate the techniques unique ability of observing profile development.

A multi-technique analysis of deuterium trapping and near-surface precipitate growth in plasma-exposed tungsten

In this work, we examine how deuterium becomes trapped in plasma-exposed tungsten and forms near-surface platelet-shaped precipitates. How these bubbles nucleate and grow, as well as the amount of deuterium trapped within, is crucial for interpreting the experimental database. Here, we use a combined experimental/theoretical approach to provide further insight into the underlying physics. With the Tritium Plasma Experiment, we exposed a series of ITER-grade tungsten samples to high flux D plasmas (up to 1.5 × 1022 m−2 s−1) at temperatures ranging between 103 and 554 °C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 °C and diminished rapidly thereafter for T > 300 °C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 μm over the surface with a clear correlation with grain boundaries. Electron back-scattering diffraction maps over a large area of the surface confirmed th...

High heat flux target for intense neutron source

A 200‐kV, 200‐mA deuterium ion accelerator has been developed to simulate the operation of an intense neutron source for use in cancer therapy. The thin‐film ScD2 target for the neutron source is supported on a water‐cooled copper substrate designed to dissipate 40 MW/m2 at a surface temperature of 450 °C. This paper describes the theoretical and experimental analysis of the target and the postmortem analysis of a target after 140 h of operation at power densities exceeding 40 MW/m2. Cooling channel erosion due to nucleate boiling of the cooling water was shown to be the life‐limiting feature of the target design.

Characterization and conditioning of SSPX plasma facing surfaces

Abstract The Sustained Spheromak Physics Experiment (SSPX) will examine the confinement properties of spheromak plasmas sustained by DC helicity injection. Understanding the plasma-surface interactions is an important component of the experimental program since the spheromak plasma is in close contact with a stabilizing wall (flux conserver) and is maintained by a high current discharge in the coaxial injector region. Peak electron temperatures in the range of 400 eV are expected, so the copper plasma facing surfaces in SSPX have been coated with tungsten to minimize sputtering and plasma contamination. Here, we report on the characterization and conditioning of these surfaces used for the initial studies of spheromak formation in SSPX. The high pressure plasma-sprayed tungsten facing the SSPX plasma was characterized in situ using β-backscattering and ex situ using laboratory measurements on similarly prepared samples. Measurements showed that water can be desorbed effectively through baking while the removal rates of volatile impurity gases during glow discharge and shot conditioning indicated a large source of carbon and oxygen in the porous coating.

Intense Neutron Source Development for Use in Cancer Therapy

A neutron production rate of 1.1 × 1011/s from the D(d,n)3He reaction has been measured from a 200 kV, 200 mA d. c. deuterium ion accelerator during the initial bombardment of a scandium deuteride target. Over a period of 40 h operation, the output decreased to 8 × 1010/s due to impurities in the ion beam being implanted in the target and displacing deuterium just below the target surface. Deuterium concentration depth profiles in the target have been measured which show the deuterium depletion near the surface. The initial neutron output scales to >1 × 10l3/s from the T(d,n)4He reaction for a D,T ion beam on a ScDT target, which is the output required for a clinically useful cancer therapy machine. Efforts to reduce the ion beam impurity content to acceptable levels will be evaluated in a new accelerator tube that is described.

Retention and release of deuterium implanted in copper coatings on Al-6061

Abstract To mitigate the problem of retention and permeation of tritium implanted in Al-6061, the use of copper coatings was investigated. Copper coatings (having weights of 0.03, 0.06 and 0.088 kg/m 2 ) deposited on Al-6061 substrates by the RF Magnetron sputtering method were implanted with deuterium (D) in an accelerator at 350 K, and the resulting D profiles were monitored using negative SIMS and the D( 3 He,p) 4 He nuclear reaction. The retention characteristics of deuterium were subsequently studied as a function of coating weight, D + fluence (varied in the 1−3×10 21 D + / m 2 range) and D + ion energy (40 and 120 keV). Under identical implantation conditions, deuterium retention in Al-6061 was higher than in Al-6061 coated with 0.088 kg/m 2 Cu. In the various coatings implanted under different conditions, deuterium retention ranged between 1.2% and 5.4% of the implanted amount. The deuterium retention decreased with increasing coating weight and then leveled off with further increases in the coating weight. The retention increased linearly with implantation fluence.

NMR determination of porosity and permeability of western tight gas sands

Samples of fine-grained sandstone from the Colorado Interstate Gas Exploration (CIGE), Natural Buttes No. 21 core, Uinta Basin, Utah were studied using pulsed nuclear magnetic resonance (NMR) and standard mineralogical techniques. Brine-saturated rock porosities varied from 1 to 13% and were found deducible from the magnitude of the proton NMR. The complex pore geometry and presence of authigenic carbonate and clay minerals in these samples precluded the use of standard flow models for predicting brine permeabilities from T/sub 1/ decays. A network model of the pore system is proposed and shown capable of accurately reproducing measured rock permeabilities, which varied from 10/sup -4/ to 1 millidarcy.