Applications of in situ cosmogenic nuclides to problems in Quaternary geology require increasingly accurate and precise knowledge of nuclide production rates. Production rates depend on the terrestrial cosmic-ray intensity, which is a function of the elevation and geomagnetic coordinates of a sample site and the geomagnetic field intensity. The main goal of this dissertation is to improve the accuracy of cosmogenic dating by providing better constraints on the spatial variability of production rates.In this dissertation I develop a new scaling model that incorporates the best available cosmic-ray data into a framework that better describes the effects of elevation and geomagnetic shielding on production rates. This model is based on extensive measurements of energetic nucleon fluxes from neutron monitor surveys and on more limited data from low-energy neutron surveys. A major finding of this work is that neutron monitors yield scaling factors different from unshielded proportional counters. To verify that the difference is real I conducted an airborne survey of low-energy neutron fluxes at Hawaii (19.7° N 155.5° W) to compare with a nearby benchmark neutron monitor survey. Our data confirm that the attenuation length is energy dependent and suggest that the scaling factor for energetic nucleons is 10% higher between sea level and 4000 m than for low-energy neutrons at this location. An altitude profile of cosmogenic 36Cl production from lava flows on Mauna Kea, Hawaii, support the use of neutron flux measurements to scale production rates but these data do not have enough precision to confirm or reject the hypothesis of energy-dependent scaling factors.

The decommissioning of nuclear power stations will lead to the production of a number of contaminated components. The materials produced will consist of process and structural wasteforms. Before this waste can be disposed it will be necessary to obtain a full inventory of those radionuclides contained in those wastes. This will be necessary to ensure that the waste is handled in a suitable manner and that full information is available so that reliable risk assessment can be carried out on the proposed disposal site. A number of radionuclides have been highlighted as significant in terms if their halflife, terrestrial mobility and predicted levels in decommissioning wastes. A selection of these radionuclides have been investigated, and methods developed for their determination in concrete. It was a requirement of this work that the methods developed were robust and that the minimum detectable amount was below that of the de minimis of O.4Bq g-1.

Le Site Pilote de Tchernobyl (CPS) est voué à l’étude de la migration des radionucléides à partir de l’une des tranchées creusée pour enfouir la matière contaminée à la suite de l'accident de Tchernobyl (1986). L’objectif de cette thèse est d’étudier les processus de migration des éléments dans la nappe phréatique.D’abord, l’extension maximale du panache de contamination est étudiée à l’aide du traceur conservatif 36Cl. Dans la nappe, les rapports 36Cl/Cl sont 1 à 4 ordres de grandeur supérieurs au rapport naturel théorique (particulièrement en aval de la tranchée), signifiant une importante contamination de la nappe par le 36Cl.Ensuite, un modèle conceptuel des processus géochimiques est proposé à partir de l’étude des variations en [Cl-], [HCO3-], [SO42-], [NO3-], [Na+], [Ca2+], [K+] [Mg2+], [Si], [Fe2+], [Mn2+] et des rapports δ18O et δ2H. Certains éléments sont très influencés par la présence de la tranchée mais des processus naturels peuvent aussi avoir une influence sur géochimie des eaux (eau-roche ou de drainage).Puis, les rapports isotopiques 238U/235U, 86Sr/88Sr et 87Sr/86Sr sont mesurés pour étudier la migration d’U et du Sr. La dissolution des particules de combustible enfouies dans la tranchée et le lessivage des radionucléides qui leur sont associés pourraient avoir un impact significatif sur les rapports isotopiques dans la nappe. Une augmentation des concentrations en [238U] est observée en aval de la tranchée mais les rapports 238U/235U mesurés sont naturels. La procédure d'analyse des rapports 86Sr/88Sr et 87Sr/86Sr ne permet pas d’observer un changement de ces rapports dans la nappe mais une diminution du rapport est observée en profondeur. / The Chernobyl Pilot Site (CPS) was set up to study the migration of radionuclides from one of the trenches dug in situ to bury materials contaminated after Chernobyl accident. The aim of this study is to investigate migration processes in groundwater. At first, the maximal extent of the contaminant plume is investigated based on the study of conservative tracer 36Cl. High contamination of groundwater by 36Cl is shown, with 36Cl/Cl ratios 1 to 4 orders of magnitude higher than the theoretical natural ratio (specifically downgradient of the trench). Then, a conceptual model of the main geochemical processes in groundwater is proposed based on the study of variantions in [Cl-], [HCO3-], [SO42-], [NO3-], [Na+], [Ca2+], [K+] [Mg2+], [Si]), [Fe2+], [Mn2+], and δ18O and δ2H. Some element concentrations are mainly governed by migrations from the trench. However, natural geochemical processes are also assessed to have an influence on groundwater geochemistry (water-rock interaction, leakage). Next, uranium and strontium migrations are investigated based on measurements of 238U/235U, 86Sr/88Sr, 87Sr/86Sr ratios. Indeed, dissolution fuel particles buried in the trench and the release of associated radionuclides is supposed to have a significant impact on those ratios in groundwater. However, in spite of an increase of [238U] concentrations downgradient of the trench, measured 238U/235U ratios in groundwater are in the natural range. Analytical procedure for 86Sr/88Sr and 87Sr/86Sr ratio measurement does not allow observing some trend downgradient of the trench; however, 87Sr/86Sr ratios clearly decrease with the depth.

Accelerator Mass Spectrometry (AMS) studies of rare isotopes with abundant isobars that form negative ions often require the use of large accelerators to achieve high sensitivity measurements. The Isobar Separator for Anions (ISA) is a radiofrequency quadrupole (RFQ) reaction cell system that provides selective isobar suppression for many of these isotopes in the low energy system, prior to injection into an accelerator. The ISA can then facilitate the measurement of these ions using smaller accelerators. A commercial version from Isobarex Corp. was installed in a separate low energy injection line of the 3 MV accelerator system at the A. E. Lalonde AMS Laboratory in the University of Ottawa and was tested using the measurement of 36Cl, suppressing its stable isobar 36S. The ISA includes a DC deceleration region, a combined cooling and reaction cell, and a DC acceleration region. The deceleration region reduces the beam energy from the ion source (20-35 keV) to a level that chemical reactions can occur, scattering is minimized, and that the reaction cell can accept and contain. RFQ segments along the length of the cell create a potential well, which limits the divergence of the traversing ions. DC offset voltages on these RFQ segments maintain a controlled ion velocity through the cell. Helium was used as a cooling gas to further decelerate the ions, facilitating charge exchange between 36S and a reaction gas. Helium provided the highest transmission of 30-80% for chlorine anions. The reaction gas NO2 was chosen to preferentially react with sulfur. Over seven orders of magnitude reduction of sulfur to chlorine was observed. After exiting the cell, the beam is reaccelerated prior to injection into the tandem accelerator for AMS analysis. Using 36Cl reference materials, it was determined that linear transmission results could be obtained for a 36Cl/Cl ratio ranging from 10−11 to 10−15. The measurements were stable over more than 24 hours of continuous measurement. A blank level on the order of 10−15 was observed. The ISA was used to measure unknown 36Cl /Cl ratio groundwater samples and the results are compared to external AMS measurements.

Two long-lived halogen radionuclides (36Cl, T1/2 = 301 kyr, and 129I, T1/2 = 15.7 Myr) have been studied by means of Accelerator Mass Spectrometry (AMS) at the Uppsala Tandem Laboratory. The 36Cl measurements in natural samples using a medium-sized tandem accelerator (~1 MeV/amu) have been considered. A gas-filled magnetic spectrometer (GFM) was proposed for the separation of 36Cl from its isobar, 36S. Semi-empirical Monte-Carlo ion optical calculations were conducted to define optimal conditions for separating 36Cl and 36S. A 180° GFM was constructed and installed at the dedicated AMS beam line. 129I has been measured in waters from the Arctic and North Atlantic Oceans. Most of the 129I currently present in the Earth's surface environment can be traced back to liquid and gaseous releases from the nuclear reprocessing facilities at Sellafield (UK) and La Hague (France). The anthropogenic 129I inventory in the central Arctic Ocean was found to increase proportionally to the integrated 129I releases from these reprocessing facilities. The interaction and origin of water masses in the region have been clearly distinguished with the help of 129I labeling. Predictions based on a compartment model calculation showed that the Atlantic Ocean and deep Arctic Ocean are the major sinks for the reprocessed 129I. The variability in 129I concentration measured in seawater along a transect from the Baltic Sea to the North Atlantic suggests strong enrichment in the Skagerrak–Kattegat basin. The 129I inventory in the Baltic and Bothnian Seas is equal to ~0.3% of the total liquid releases from the reprocessing facilities. A lake sediment core sampled in northeastern Ireland was analyzed for 129I to study the history of the Sellafield releases, in particular the nuclear accident of 1957. High 129I concentration was observed corresponding to 1990 and later, while no indication of the accident was found. The results of this thesis research clearly demonstrate the uniqueness and future potential of 129I as a tracer of processes in both marine and continental archives.

La caractérisation géochimique et isotopique des aquifères du Complexe Terminal (CT) du sud tunisien a été effectuée sur la base d'une approche multi isotopique. Cette étude a mis en évidence la continuité hydrogéologique des aquifères du CT en dépit de la variabilité lithologique latérale des formations aquifères et à travers les grands accidents tectoniques. La minéralisation des eaux est acquise par les processus d’interactions eau / roche, la dissolution des évaporites associés à la précipitation de la calcite. Les teneurs en δ18O et δ2H, montrent un fort contraste entre les eaux du CT. La signature isotopique traduit l’effet de l’évaporation manifesté par un enrichissement en δ18O et δ2H. Le marquage isotopique atteste aussi d’un effet de paléorecharge. En outre, les teneurs en 14C couplées aux teneurs en δ13C, ont soutenu la recharge de la nappe au cours de l’Holocène. Le déséquilibre 234U / 238U a été aussi utilisé. Les eaux du CT ont enregistré des teneurs qui varient de 1.5 à 19.5 ppb et des rapports 234U / 238U qui varient de 1.1 à 3.2. La mise en solution de l’uranium est contrôlée par la dissolution de la matrice aquifère. Cette étude a mis en évidence aussi des conditions oxydantes régnant au sein des aquifères du CT. Ces conditions sont propices au lessivage préférentiel de l’234U et dont l’effet marquant les rapports 234U / 238U qui sont >1. L’étude des teneurs en 36Cl et du rapport 36Cl / Cl montre que leur variation est liée aux processus d’évaporation et de dissolution. Les taux d’évaporation et de dissolution calculés sont > 50% traduisant l’effet marquant de ces deux processus sur la variation des teneurs en chlore des eaux du CT. / The geochemical and isotopic characterization of CT aquifers of southern Tunisia was conducted on the basis of a multi isotopic approach. This study highlighted the hydrogeological continuity of the CT aquifers despite the lateral lithological variability of aquifers and through major tectonic accidents. Groundwater mineralization is controlled by water rock interaction and the dissolution of evaporate minerals associated with the calcite precipitation. The δ18O and δ2H composition, show a strong contrast. The isotopic signature reflects the evaporation effect shown by stable isotope enrichment. Isotopic composition also demonstrates a paleorecharge effect. In addition, the 14C content coupled with the contents δ 13C, supported the groundwater recharge in the Holocene period. 234U / 238U disequilibrium was also used. Results indicate a range in 238U concentration and 234U/238U activity ratios of 1.5 to 19 ppb and 1.1 to 3.2 respectively. Our results show that groundwater geochemistry is ultimately a result of lithologic variations associated with water / rock interaction. Results elucidate also the mixing relationships between the different aquifer horizons.In this study, we used also chlorine-36 (36Cl) as a groundwater tracer. The variation in 36Cl content and 36Cl / Cl ratio is essentially related to both evaporation and dissolution processes. Evaporation and dissolution rates recorded values that exceed 50 % showing the major effect of these processes on the change of chloride contents in CT groundwater.

Au cours des prochaines années, neuf centrales nucléaires de type UNGG (Uranium Naturel Graphite Gaz) devront être démantelées en France. Ces centrales utilisent le graphite comme modérateur et réflecteur de neutrons. Pendant leur exploitation, celui-ci est activé. Leur démantèlement conduira à 23000 tonnes de déchets de graphite irradiés à gérer. Ce travail focalise sur deux radionucléides contenus dans ces déchets : le 36Cl et le 3H. Le 36Cl a l'une des demi-vies les plus longues (301 000 ans). Par contre, le 3H a une demi-vie plus courte (12 ans), mais contribue beaucoup à l'activité initiale des déchets. Différentes données expérimentales suggèrent que le 36Cl et le 3H sont piégés à différents endroits du graphite, comme les boucles de dislocation, les surfaces ou les joints de grains. Le seul mécanisme de migration des radionucléides est le relâchement. Pour cette raison, il est important de comprendre quels sont les pièges et les différentes conditions du relâchement.Le graphite UNGG a une structure complexe, hétérogène et multi-échelle qui diffère du monocristal idéal du graphite. Cependant, pour comprendre les données macroscopiques, les études théoriques à l'échelle nanoscopique et microscopique sont des outils importants, même si elles reposent sur des modèles plus simples. Dans cette thèse, une approche multi-échelle a été utilisée afin d’étudier les interactions des radionucléides avec le graphite ainsi que les mécanismes de diffusion et de piégeage à l'échelle du nm-μm.Les interactions du 3H et du 36Cl avec différents défauts du graphite ont été étudiées dans le cadre de la théorie fonctionnelle de la densité (DFT). L'hydrogène forme une liaison covalente avec le graphite massique ainsi qu'avec ses surfaces (001), (100) et (110). Plusieurs reconstructions de surface ont été explorées. Les résultats montrent que les hypothèses existantes sur le piégeage de l'hydrogène doivent être affinées. Le comportement du Cl est plus complexe. Sa chimisorption est observée sur les surfaces (100) et (110). Cependant, sur la surface (001), le Cl interagit par transfert de charge. Le Cl2 n'interagit que par interactions de van der Waals avec celle-ci. Le Cl2 se dissocie dans le graphite massique.Les diffusions du H et du Cl dans le graphite irradié ont été étudiées en effectuant des simulations de dynamique moléculaire. Les résultats ab initio ont été utilisés pour développer des potentiels de type « bond order » afin de modéliser l'interaction des radionucléides avec la matrice de graphite, qui possède des contributions à court et à long portée. Pour le Cl, un nouveau potentiel a été paramétré qui reproduit toutes les données obtenues au niveau DFT. Pour les interactions 3H-graphite, les potentiels AIREBO/M, pour les interactions C-H, et LCBOP, pour les interactions C-C, ont été utilisés.Pour évaluer l'influence de la structure complexe du graphite UNGG sur le comportement des radionucléides, plusieurs modèles atomiques ont été utilisés pour rendre compte de cette diversité, tels que les surfaces, les joints de grains et les nanopores.Pour le Cl, des simulations d'irradiation ont été réalisées pour une gamme d’énergie allant de 1 à 10 keV et une gamme de température de 200 à 500ºC. Les dépendances à la température et à la direction d'irradiation ont été étudiées. D’une façon générale, les dommages causés par l'irradiation perpendiculaire aux surfaces augmentent avec la température. L'irradiation à des angles d’incidence <90º aux surfaces peut causer plus ou moins de dommages par rapport à l'irradiation perpendiculaire selon le type de surface.Les diffusions du H et du Cl montrent que tous les bords de cristallites avec des liaisons pendantes sont des pièges. Pour le Cl, la diffusion dans le graphite nanoporeux a révélé deux emplacements préférés: les bords des cristallites où le Cl forme une liaison covalente et les coins des microfissures où le Cl interagit par transfert de charge. / In the upcoming years, nine nuclear UNGG (Uranium Naturel Graphite Gaz) power plants will have to be dismantled in France. In these power plants, nuclear graphite was used as a neutron moderator and reflector, and was activated during operation. The dismantlement will lead to 23000 tons of irradiated graphite waste, which will have to be managed. The graphite is classified as a nuclear waste containing radionuclides with low activity and long half-life. Two radionuclides are the focus of this work: 36Cl and 3H. 36Cl has one of the longest half-lives (about 301000 years) among the waste's radionuclides. 3H has a shorter half-life (12 years), but contributes significantly to the waste’s initial activity. Previous experiments suggest that both, 36Cl and 3H, are mainly fixed at different traps in graphite, which are defective structures, such as dislocation loops, surfaces, or grain boundaries. Since the only significant migration mechanism of these radionuclides is release, it is important to understand where the traps are located and the conditions of the release.UNGG graphite has a complex heterogeneous multi-scale structure which differs substantially from an ideal monocrystal of graphite. However, in order to understand macroscopic data, theoretical studies at the nano- and microscopic scale are an important tool to explain underlying phenomena even though they rely on simpler models due to the limitations of computation power. A multi-scale approach was therefore applied to study the local interactions of the radionuclides with graphite as well as diffusion and trapping mechanisms on the nm-μm length scale.First, the interaction of 3H and 36Cl with defects in graphite was studied with density functional theory (DFT). Hydrogen interacts covalently with bulk graphite as well as with the studied surfaces (001), (100), and (110). Several surface reconstructions were investigated: arch-type reconstructions and in-plane reconstructions. The results show that the existing hypothesis on the trapping of hydrogen needs to be refined. The behavior of Cl is more complex. On the (100) and (110) surface chemisorption is observed. However, on the (001) surface a strong charge transfer interaction is observed for Cl. In contrast to that, Cl2 only interacts via weak van der Waals interactions with this surface. In bulk graphite Cl2 dissociates.The diffusion of H and Cl in irradiated graphite has been investigated by performing molecuar dynamics simulations. The ab initio results were used to develop bond order potentials to model the interaction of radionuclides and the graphite matrix, which attributes for short and long range interactions. For Cl, a new potential has been parameterized which is able to describe all aspects obtained with DFT. For the 3H-graphite interactions, the bond order potential AIREBO/M was used for C-H interactions. For C-C interactions the LCBOP potential was used.To evaluate the influence of the complex heterogeneous structure of the UNGG graphite on the radionuclide's behavior, several different atomic models were studied to account for this diversity such as surfaces, grain boundaries and nanopores.For Cl, irradiation simulations of different systems were performed up to an energy of 10 keV for the primary knock-on atom (PKA), and in a temperature range of 200 to 500ºC. The dependence on temperature and irradiation direction was investigated. In general, direct irradiation damage increases with temperature. Irradiation at incident angles <90º can create more or less damage compared to the perpendicular one depending on the surface type.Diffusion of H and Cl along surfaces shows that all crystallite edges with dangling bonds can serve as traps. For Cl, diffusion in nanoporous graphite revealed two preferred locations : First, the crystallite edges where Cl forms strong covalent; second, the corners of microcracks where Cl interacts via charge transfer.

L'exploitation des réserves d'eau douce, principalement contenues dans les aquifères est une préoccupation croissante dans les zones où elle se traduit par une baisse des niveaux piézométriques.Ce travail de thèse propose de caractériser l'évolution du Système Aquifère du Saharan Septentrional (SASS), et plus particulièrement le Continental Intercalaire (CI) à l'aide d'un couplage entre des outils de modélisation hydrogéologique et de traceurs géochimiques (36Cl, 234U/238U). Situé dans une région semi-aride à aride et s'étendant sur plus d'un million de km2, le CI présente un potentiel d'enregistrement d'une variabilité climatique à grande échelle en plus des baisses piézométriques anthropiques.Nous avons amélioré la définition de la recharge moderne du SASS à l'aide d'une méthode basée sur des données de télédection (entre 0 et 6.75 mm an−1 sur la période 2003-2010) ainsi que de la localisation zones de recharge du Sud Tunisien à l'aide de traceurs isotopiques (14C, 36Cl, 234U). Ces traceurs fournissant un lien évident mais complexe entre les conditions hydrodynamiques présentes et passées, nous nous sommes efforcés d'interpréter les informations paléoenvironnementales apportées par le 36Cl et le 4He sur l'ensemble de la zone tunisienne du CI. Introduites dans un modèle hydrogéologique du CI, les données de 36Cl nous ont permis d'améliorer des hypothèses sur des scénarios de recharge (variant entre 0.5 et 60 mm an−1) pendant les huit derniers cycles interglaciaires-glaciaires (0-775 ka).A partir d'informations complémentaires (échantillons d'eau, affleurements, zone climatique) nous avons finalement construit une représentation "hydro-climato-geochimique" du CI. / Concerns about freshwater availability increase with the rise of populations demands and decrease of piezometric levels in some aquifers. However, this "instantaneous" variability is complicated by a long-term climatic variability that impacts hydrodynamics on timescales of thousands to hundred of thousands of years.This study characterizes the evolution of a large aquifer (extending over more than a million of km2), the Continental Intercalaire (CI), from hydrogeological and geochemical viewpoints, over several hundreds thousand years. For this purpose, we combined hydrogeologic modeling and geochemical tracing methods (36Cl, 234U/238U. . .).The CI is the main aquifer of the North Western Saharan Aquifer System (NWSAS). Located in a semi-arid to arid region, its presence suggests potential records of climatic variability on large temporal and spatial scales. Our results corroborate previous assessment of the modern recharge, which fluctuates between 0 and 6.75 mm yr−1, over the entire NWSAS. We also improve the understanding of previously poorly characterized recharge areas in the South of Tunisia. Isotopic tracers can provide a strong but complex link between past and present hydrodynamic conditions, so we struggled to investigate chlorine-36 and helium-4 paleorecords. In our hydrogeologic model of the CI, chlorine-36 data allowed us to propose some hypotheses about recharge scenarios (varying between 0.5 and 60 mm yr−1) during height of the last interglacial-glacial cycles (0-775 ky). By using multi-scale information (groundwater samples, outcrops and climatic areas), we developed a "hydro-climatic-geochemical" representation of the CI.

<div> <p>Despite its location in the rain shadow of the southern Sierra Nevada, the Panamint Range within Death Valley National Park, CA hosts a complex aquifer system that supports numerous springs. These springs, in turn, support unique groundwater-dependent ecological communities. Spring emergences range in elevation from 2434 m above sea level (within the mountain block) to 77 m below sea level (in the adjacent basins). Waters were collected from representative Panamint Range springs and analyzed for environmental isotopes and geochemical tracers to address the following questions: 1) What is the primary source of recharge for the springs? How much recharge occurs on the Panamint Range? 2) What groundwater flowpaths and geologic units support springflow generation? and 3) What are the residence times of the springs? The stable isotopic composition (δ¹⁸O and δ²H) of spring water and precipitation indicate that localized high-elevation snowmelt is the dominant source of recharge to these perennial springs, though recharge from rainfall is not wholly insignificant. Geochemical evolution was evaluated using principle component analysis to compare the concentrations of all major spring cations and anions in a multidimensional space and group them according to dominant geochemical signatures. These resulting geochemical groups are controlled primarily by topography. The Noonday Dolomite and other carbonate units in the range are identified as the water-bearing units in the mountain block based on the ⁸⁷Sr/⁸⁶Sr of spring waters and rock samples. These units also offer higher hydraulic conductivities than other formations and are chemically similar. Radiocarbon- and ³H derived residence times of these spring waters range from modern to approximately 1840 years, with the shortest residence times at higher altitudes and Hanaupah Canyon and increasing residence times with decreasing altitude. This residence time-altitude relationship is likewise likely topography-driven though there are significant disparities in mountain block storage between the various canyons of the range resulting in variable residence times between drainages. Lower Warm Springs A and B, however, are the exceptions to this trend as they emerge at lower altitudes (750m above sea level) and are likely driven by the transport of groundwater to the surface along faults which increases both the temperature and groundwater residence times of waters from these springs. Benthic macroinvertebrates and benthic and planktonic microbes were also sampled for each spring studied. BMI and microbial community structure in the Panamint Range is likewise topography-controlled with more tolerant communities at lower elevations (within more chemically evolved waters) and less tolerant species in the unevolved waters at higher elevations.</p></div>