Le béryllium, utilisé comme matériau de première paroi dans la chambre à vide d'ITER, peut piéger une fraction significative de tritium provenant du plasma. Du point de vue de l'analyse de sûreté, il s'agit d'évaluer l'efficacité des dispositifs mis en place par l'exploitant pour maîtriser l'inventaire en tritium durant le fonctionnement nominal d'ITER et de prédire le comportement du tritium en situation accidentelle. L'objectif de ces travaux de thèse est de déterminer, par le biais de la Théorie de la Fonctionnelle de la Densité, le comportement du tritium dans le béryllium en termes de mécanismes de rétention et de désorption. La stabilité des défauts ponctuels dans le béryllium a d'abord été évaluée et analysée au regard des données expérimentales, mettant en évidence la prédominance du défaut lacunaire dans le béryllium. Ensuite, tous les sites d'occupation stables du tritium et les chemins de diffusion les plus favorables ont été déterminés. Cette analyse a été étendue au processus de multi-piégeage, mettant en évidence que cinq atomes de tritium peuvent être piégés dans une lacune. Enfin, les données obtenues ont été introduites dans un code Monte-Carlo Cinétique permettant d'évaluer le coefficient de diffusion du tritium et un modèle couplant mécanismes de réaction et de diffusion pour l'interprétation de spectres expérimentaux de thermo-désorption (TDS) obtenus à l'IEK-4 (Jülich -- Allemagne). Les spectres modélisés reproduisent de manière satisfaisante les spectres expérimentaux pour les faibles fluences. A forte fluence, la mise en évidence expérimentale d'un pic à basse température pourrait s'expliquer par la formation d'hydrure de béryllium. / Beryllium will be used as a plasma-facing material for the ITER vacuum vessel. Due to high plasma fluxes, significant amounts of hydrogen isotopes could be retained in the beryllium walls. From the safety point of view, it is important to assess the capability of devices which will be used to limit the tritium inventory in the tokamak in nominal conditions and secondly, to predict the behaviour of tritium in case of accident. The objective of this work is to evaluate within the framework of the Density Functional Theory the behaviour of tritium in beryllium in terms of retention and desorption mechanisms. Firstly, the stability of point defects in beryllium has been evaluated and compared to experiments. Vacancies are shown to be the dominant defect in beryllium. Then, the most stable interstitial sites for tritium atoms and the most favorable migration pathways have been determined. This study has been extended to multiple-trapping phenomenon in monovacancy, in which up to five atoms can be trapped. These data have been used in a kinetic Monte-Carlo code to calculate the diffusion coefficient of tritium and a reaction-diffusion based model, which provides a good agreement with experimental thermal desorption spectra made at IEK-4 (Jülich -- Germany). The emergence of desorption peak at low temperature under high fluence could be explained by the hydride formation.

Made available in DSpace on 2014-10-09T12:44:28Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:11Z (GMT). No. of bitstreams: 1 06874.pdf: 5138428 bytes, checksum: e22d5fad1a78f9c1d2ced36a4c194d23 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:97/05621-9

Ongoing retreat of Antarctica’s marine-based glaciers is associated with warm (~2° C) modified Circumpolar Deep Water intrusion onto the continental shelf, suggesting that Southern Ocean temperatures may influence Antarctic ice sheet stability. Understanding past cryosphere response to environmental forcing is crucial to modeling future ice sheet behavior. Of particular interest is the response of the East Antarctic Ice Sheet (EAIS), which stands to contribute ~20 m to global sea level. However, marine sediment sequences recording timing and variability of EAIS fluctuations through the last major climate shift, the Last Glacial Maximum (LGM), are either missing from the margin or have poor chronological control. Here we present three marine sediment cores that contain a record of pre-LGM fluctuations of the marine-based Lambert Glacier-Amery Ice Shelf (LG-AIS) system into Prydz Channel, East Antarctica. Analyses of core lithology, physical properties, cosmogenic nuclide concentration and diatom assemblage demonstrate that Prydz Channel was characterized by alternating open-marine and sub-shelf deposition, implying repeated LG-AIS fluctuations through the LGM. Our radiocarbon chronology demonstrates that LG-AIS fluctuations occurred on millennial timescales. Our record corroborates regional marine and terrestrial records, which demonstrate millennial scale variability in Antarctic Circumpolar Current strength, ice-rafted debris deposition, sea ice extent, Antarctic atmospheric temperature, and Southern Ocean sea surface temperature. This evidence suggests that the EAIS was sensitive to sub-orbital climate forcing in the past, and has implications for modeling future EAIS behavior.

Cette thèse est consacrée à l'étude du conditionnement des parois des réacteurs de fusion, en particulier ITER. Le conditionnement est nécessaire pour contrôler l'état de surface de l'enceinte à vide et donc les performances des plasmas d'ITER.Le conditionnement du tokamak JET, ayant une paroi représentative de celle d'ITER, et son impact sur l'opération est étudié de manière approfondie.Un modèle 2D des décharges luminescentes de conditionnement est validé par des données expérimentales. Il prédit des décharges raisonnablement uniformes dans ITER.Des expériences de conditionnement sur JET montrent que l'échange isotopique est un moyen efficace pour contrôler l'inventaire de tritium dans ITER, l'efficacité d'élimination étant potentiellement comparable à la rétention prédite dans un plasma nominal.Un modèle 1D de l'hydrogène échange isotopique en béryllium est élaboré et validé. Il montre que la fluence et la température de surface déterminent l'efficacité de l'échange isotopique. / Thesis is devoted to studies of performance and efficiency of wall conditioning techniques in fusion reactors, such as ITER. Conditioning is necessary to control the state of the surface of plasma facing components to ensure plasma initiation and performance. Conditioning and operation of the JET tokamak with ITER-relevant material mix is extensively studied. A 2D model of glow conditioning discharges is developed and validated; it predicts reasonably uniform discharges in ITER. In the nuclear phase of ITER operation conditioning will be needed to control tritium inventory. It is shown here that isotopic exchange is an efficient mean to eliminate tritium from the walls by replacing it with deuterium. Extrapolations for tritium removal are comparable with expected retention per a nominal plasma pulse in ITER.A 1D model of hydrogen isotopic exchange in beryllium is developed and validated. It shows that fluence and temperature of the surface influence efficiency of the isotopic exchange.

Thesis advisor: Jeremy D. Shakun / The deglacial extent chronology of the southeastern Laurentide Ice Sheet as it retreated through the northeastern United States and southern Quebec has been well constrained by multiple lines of evidence. By comparison, few data exist to constrain the thinning history of the southeastern Laurentide, resulting in lingering uncertainty about volume changes and dynamics of this ice mass during the deglacial period. To address the lack of thinning information, my team collected 120 samples for in-situ `10Be exposure dating from various elevations at numerous mountains in New England and southern Quebec. Monte Carlo regression analyses using the analytical uncertainties of exposure ages from each mountain are used to determine the most-likely timing and rate of ice thinning for that location, a technique known as the ‘dipstick approach’. While this larger project is ongoing, I have processed and measured 10Be concentrations of 42 samples for this thesis and present my preliminary results and interpretation here. Exposure ages from Peekamoose Mt. in southern NY suggest ice thinning early in the deglacial period (~19.5 – 17.5 ka), near the onset of the Heinrich Stadial I cold period. Samples from Franconia Notch, NH, and Mt. Mansfield, VT, suggest ice thinning from approximately 15 – 13 ka in northern New England, roughly coincident with the Bølling-Allerød warm period. Exposure ages from each of the northern New England dipsticks are nearly identical within 1σ internal uncertainty, indicating that ice thinning was rapid. Higher elevation (>1200 m a.s.l.) samples from the northern New England mountains appear to contain inherited 10Be from previous periods of exposure, indicating a lack of glacial erosion on these surfaces. My high-elevation samples with inherited 10Be suggest that these summit landscapes were preserved beneath cold-based, non-erosive ice during the last glacial and deglacial periods. 40 samples that have yet to be processed will provide more information on ice thinning around Killington Mt., VT, Mt. Greylock, MA, Mt. Bigelow, ME, and Mt. Jacques-Cartier, Quebec. Ultimately, this information will be used to create probabilistic reconstructions of the lowering southeastern Laurentide ice surface during its retreat. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Harriet Hardy, protégé of Alice Hamilton, spent 1948 in the Health Division of Los Alamos Scientific Laboratory. The contemporary campaign for federal legislation to compensate nuclear workers brought to the fore living retirees in whose cases of occupational illness Hardy had a role in diagnosis or case management. A third case is documented in archival records. Methods of participatory action research were used to better document the cases and strategize in light of the evidence, thereby assisting the workers with compensation claims. Medical and neuropsychological exams of the mercury case were conducted. Hardy’s diary entries and memoirs were interpreted in light of medicolegal documentation and workers’ recollections. Through these participatory research activities, Harriet Hardy’s role and influence both inside and outside the atomic weapons complex have been elucidated. An important lesson learned is the ongoing need for a system of protective medical evaluations for nuclear workers with complex chemical exposures.

Currently, our ability to predict the flux of fine sediment at the watershed scale is limited by the precision of erosion rate estimates for the many potential sources distributed throughout a landscape as well as our understanding of the connectivity of sediment pathways during transport. In absence of a robust predictive model which can be validated by measurements of sediment fluxes and use of geochemical tracers. Predicting fine sediment yield at the watershed scale requires multiple redundant lines of information. This thesis outlines the methods used, and the data sets collected in the Root River watershed in Southeastern Minnesota, all of which are multiple lines of evidence to the sediment dynamics in the Root River. The research indicates that the Root River is a very dynamic watershed. The hydrologic regime of the watershed has shifted over the last half century. Due to this shift sediment fluxes are very dependent of the magnitude and sequence of events. Geomorphic analysis of the landforms and the use of a developed tool, TerEx, indicate that many reaches of the river have easily accessible near-channel sources of sediment. Sediment fingerprinting results illustrate that source tracer concentrations are variable across the landscape, that as a whole, upland sources are still a major contributor to the suspended sediment load, and that in some sub-watersheds near-channel sources are dominate in the suspended load. Over all the channel-floodplain exchange exerts strong control on the flux of sediment through this river system.

No description available.

We have designed a new type of plasma gun ion source for a Malmberg-Penning trap based on Metal Vapor Vacuum Arc (MeVVA) ion source designs. Our primary intent with this MeVVA-type source is to create a confinable beryllium-7 (7-Be) plasma. 7-Be is a peculiar isotope due to its varying radioactive decay half-life in different electro-chemical configurations. It is also found in an unexpected abundance at high altitudes of the Earth's atmosphere. It is possible ioniziation affects the radioactivity of the isotope, partly explaining this discrepancy with atmospheric models. The short half-life of 7-Be requires us to replace the sample inside the ion source on a regular basis. Our design makes it possible to easily remove the cathode of the ion source from an ultra-high vacuum trap and exchange 7-Be samples while only needing to repressurize a small chamber rather than the entire trap. This design has an added benefit of being capable of generating plasmas from a wide variety of metals by simply exchanging the source target in the removable cathode. Because of this wide compatibility, we will be able to use our trap for studying any number of different plasmas, including other radioactive types. Testing of the ion source design shows we are able to extract more than a sufficient number of ions at reasonable energies for confinement.

The material properties of low stress silicon nitride make it a possible replacement material for beryllium in X-ray windows. In this study, X-ray windows made of LPCVD deposited low stress silicon nitride are fabricated and characterized. The Young's modulus of the LPCVD low stress silicon nitride are characterized and found to be 226±23 GPa. The residual stress is characterized using two different methods and is found to be 127±25 MPa and 141±0.28 MPa. Two support structure geometries for the low stress silicon nitride X-ray windows are used. X-ray windows with thicknesses of 100 nm and 200 nm are suspended on a silicon rib support structure. A freestanding circular geometry is used for a 600 nm thick X-ray window. The 100 nm and 200 nm thick low stress silicon nitride X-ray windows with a silicon support structure are burst tested, cycling tested and leak rate tested. The average burst pressure for the 100 and 200 nm films on a silicon support structure are 1.4 atm and 2.2 atm respectively. Both 100 nm and 200 nm windows are able to withstand a difference in pressure of 1 atm for over 100 cycles with a leak rate of less than 10-10 mbar-L/s.The low stress silicon nitride with 100 nm and 200 nm thicknesses, the 600 nm freestanding low stress silicon nitride windows and freestanding 8 micron thick beryllium windows are mechanical shock resistance tested. The support structure low stress silicon nitride and beryllium windows are tested with an applied vacuum. The freestanding 600 nm thick low stress silicon nitride windows burst at 0.4 atm and are therefore mechanical shock wave tested without an applied vacuum. The support structure low stress silicon nitride windows fractured when subjected to an acceleration of roughly 5,000 g. The 8 micron thick beryllium windows are subjected to accelerations of over 30,000 g without fracturing. A quasistatic model is used to show that for low stress silicon nitride with a freestanding circular geometry, an acceleration of 106 g is required to have the same order of magnitude of stress caused by a pressure differential of 1 atm. Low stress silicon nitride can act as a replacement for beryllium in X-ray windows, but the support geometry, residual stress, and strength of the material need to be optimized.