Certos ambientes subterrâneos da Terra possuem acumulados, naturalmente, compostos de radionuclídeos de longa vida, como 238U, 232Th 40K, próximo à presença de água líquida. O mesmo é esperado que aconteça em corpos planetários, no Sistema Solar, que possua quantidades apreciáveis de água. Nestes ambientes radioativos naturais, a radiólise da água produz espécies químicas e desequilíbrios químicos importantes para a vida. Apesar da proposta do decaimento radioativo como fonte alternativa de energia para sistemas vivos existir há mais de trinta anos, isto se mostrou realmente concreto após descoberta de um ecossistema peculiar cuja sobrevivência é dependente de espécies químicas produzidas por radiólise aquosa. Neste trabalho, avaliamos e quantificamos os desequilíbrios químicos gerados localmente pela radiólise aquosa e a possível contribuição destes para a emergência da vida, tendo como referência os estudos em ambientes de fontes hidrotermais alcalinas, consideradas promissores ambientes para esse evento. Também foram avaliados seus efeitos na habitabilidade de possíveis ambientes análogos na lua gelada Europa. Procuramos quantificar a diversidade química formada nessas condições e a associar aos desequilíbrios parâmetros termodinâmicos. As estimativas realizadas para ambientes radioativos na Terra primitiva apontaram para a similaridade entre o desequilíbrio causado por radiólise aquosa e o encontrado em fontes hidrotermais alcalinas. Confirmando e detalhando a análise preliminar que motivou o trabalho. Não obstante, considerando Europa, chegamos a valores de densidade de células do extremófilo Candidatus Desulforudis audaxviator que sobreviveriam em um conjunto de candidatos a análogos geológicos de possível ambiente radioativo na lua gelada. A partir deste estudo pudemos analisar o potencial para a emergência da vida e de protometabolismos nestes ambientes radioativos naturais na Terra primitiva, bem como levantar parâmetros mensuráveis para futuras missões espaciais que buscam vida ou habitabilidade em Europa. / Certain subterranean environments of the Earth has naturally accumulated compounds of longlived radionuclides, such as 238U, 232Th 40K, near the presence of liquid water. The same is estimated in wet planetary bodies in the Solar System. In these natural radioactive environments, water radiolysis produces chemical species and chemical disequilibria, which are important for life. Although the proposal of radioactive decay as an alternative source of energy for deep biospheres has existed for more than thirty years, this proved to be really feasible after the discovery of a peculiar ecosystem whose survivor is dependent on chemical species produced by water radiolysis. In this work, we evaluate and quantify the chemical disequilibria generated locally by water radiolysis and the possible contribution of these to the emergence of life, having as reference the studies alkaline hydrothermal vents, which is considered highly promising environment for this event. It is also evaluated their effects on the habitability of possible analogous environments on the Jupiter icy moon Europa. It was aimed to quantify the chemical diversity formed under these conditions and to calculate disequilibria using thermodynamic parameters. The estimates made for natural radioactive environments in early Earth pointed to the similarity between the disequilibrium caused by water radiolysis and those found in alkaline hydrothermal vents. What confirms and details the preliminary analysis that motivated this work. In addition, it was calculated values for cell density of the Candidatus Desulforudis audaxviator extremophile, that would survive in Europa comparable to the in situ analysis of some terrestrial radioactive environments, using a set of possible scenarios for possible local natural radioactive environments. From this study, we were able to analyze the potential for the emergence of life and protometabolisms in these natural radioactive environments in early Earth, as well as to provide measurable parameters for future space missions that seek for life or habitability in Europa.

Sellafield is home to the UK's largest repository of nuclear waste, including reprocessed uranium and plutonium, as well as a backlog of unprocessed used fuel and waste kept in outdated storage facilities; commonly referred to as "legacy waste". For this reason, Sellafield has often been called the most hazardous place in Western Europe and as such, is currently undergoing a multi-billion pound decommissioning and clean-up operation. Each on-site facility has unique challenges associated with it, many of them presenting situations where the radiation chemistry aspects of the material degradation are not well understood. The key factors that can affect water radiolysis processes in the Sellafield challenges are a high pH environment, the presence of magnesium hydroxide, the presence of iron oxide, and the presence of organic materials. This work examines the effect each of these factors has on H2 and H2O2 production in water radiolysis as well as developing a computational model to offer some understanding to the kinetic behaviour of water radiolysis under such conditions. The computational model was able to replicate experimental measurements of radiolytic H2 and H2O2 production in both aerated and deaerated water at neutral pH, and provide a further understanding of the role of dissolved oxygen in water radiolysis. Measurements of H2O2 from solutions containing NaOH have shown that an increase in pH generally results in a higher steady state of H2O2, while measurements of H2 show a similar increase with a maximum production rate at pH ~11. The model was also able to closely replicate these experimental measurements with some over prediction, which highlights a gap in our understanding of high pH radiolysis and also brings into question the validity of the estimated rate constant for the reaction: O- + O2- → 2OH- + O2 k= 6.0×10^8 M^-1 s^-1 which was originally determined from kinetic model calculations designed to describe the decay of ozonide (O3ˉ) during pulse-radiolysis studies of high pH solutions conducted byK. Sehested et al in 1982.The radiolysis of magnesium hydroxide slurry also resulted in an increased yield of hydrogen gas but had little effect on the yield of hydrogen peroxide. The hydrogen yield was 0.52 molecules per 100eV while a NaOH solution of equivalent pH gave a yield of 0.27, however interference from carbonate may be the cause of the increased yield. A surface effect was also estimated to contribute 0.05 molecules per 100 eV to the hydrogen gas yield. Hydrogen gas and hydrogen peroxide was measured from the radiolysis of aqueous methanol. This was modelled with a near agreement, but modifications to the model were necessary; highlighting areas of the model that need improvement, as well as providing a reaction scheme from which a more comprehensive model for aqueous methanol radiolysis could be developed.

Certos ambientes subterrâneos da Terra possuem acumulados, naturalmente, compostos de radionuclídeos de longa vida, como 238U, 232Th 40K, próximo à presença de água líquida. O mesmo é esperado que aconteça em corpos planetários, no Sistema Solar, que possua quantidades apreciáveis de água. Nestes ambientes radioativos naturais, a radiólise da água produz espécies químicas e desequilíbrios químicos importantes para a vida. Apesar da proposta do decaimento radioativo como fonte alternativa de energia para sistemas vivos existir há mais de trinta anos, isto se mostrou realmente concreto após descoberta de um ecossistema peculiar cuja sobrevivência é dependente de espécies químicas produzidas por radiólise aquosa. Neste trabalho, avaliamos e quantificamos os desequilíbrios químicos gerados localmente pela radiólise aquosa e a possível contribuição destes para a emergência da vida, tendo como referência os estudos em ambientes de fontes hidrotermais alcalinas, consideradas promissores ambientes para esse evento. Também foram avaliados seus efeitos na habitabilidade de possíveis ambientes análogos na lua gelada Europa. Procuramos quantificar a diversidade química formada nessas condições e a associar aos desequilíbrios parâmetros termodinâmicos. As estimativas realizadas para ambientes radioativos na Terra primitiva apontaram para a similaridade entre o desequilíbrio causado por radiólise aquosa e o encontrado em fontes hidrotermais alcalinas. Confirmando e detalhando a análise preliminar que motivou o trabalho. Não obstante, considerando Europa, chegamos a valores de densidade de células do extremófilo Candidatus Desulforudis audaxviator que sobreviveriam em um conjunto de candidatos a análogos geológicos de possível ambiente radioativo na lua gelada. A partir deste estudo pudemos analisar o potencial para a emergência da vida e de protometabolismos nestes ambientes radioativos naturais na Terra primitiva, bem como levantar parâmetros mensuráveis para futuras missões espaciais que buscam vida ou habitabilidade em Europa. / Certain subterranean environments of the Earth has naturally accumulated compounds of longlived radionuclides, such as 238U, 232Th 40K, near the presence of liquid water. The same is estimated in wet planetary bodies in the Solar System. In these natural radioactive environments, water radiolysis produces chemical species and chemical disequilibria, which are important for life. Although the proposal of radioactive decay as an alternative source of energy for deep biospheres has existed for more than thirty years, this proved to be really feasible after the discovery of a peculiar ecosystem whose survivor is dependent on chemical species produced by water radiolysis. In this work, we evaluate and quantify the chemical disequilibria generated locally by water radiolysis and the possible contribution of these to the emergence of life, having as reference the studies alkaline hydrothermal vents, which is considered highly promising environment for this event. It is also evaluated their effects on the habitability of possible analogous environments on the Jupiter icy moon Europa. It was aimed to quantify the chemical diversity formed under these conditions and to calculate disequilibria using thermodynamic parameters. The estimates made for natural radioactive environments in early Earth pointed to the similarity between the disequilibrium caused by water radiolysis and those found in alkaline hydrothermal vents. What confirms and details the preliminary analysis that motivated this work. In addition, it was calculated values for cell density of the Candidatus Desulforudis audaxviator extremophile, that would survive in Europa comparable to the in situ analysis of some terrestrial radioactive environments, using a set of possible scenarios for possible local natural radioactive environments. From this study, we were able to analyze the potential for the emergence of life and protometabolisms in these natural radioactive environments in early Earth, as well as to provide measurable parameters for future space missions that seek for life or habitability in Europa.

Water is a major component of living organisms, which can be 70-85% of the weight of cells. For this reason, water is a main target of ionizing radiations and plays a central role in radiobiology. Heavy ions, electrons and photons interact with water molecules; mainly by ionization and excitation. Neutrons interact with water molecules by elastic interactions, which generate recoil ions that will create ionizations and excitations in water molecules. These fast events (~10[superscript -12] s) lead to the formation of Reactive Oxygen Species (ROS). The ROS, in particular the hydroxyl radical (¨OH), interact with neighbour molecules such as proteins, lipids and nucleic acids by chemical interaction. Microbeams can irradiate selectively either the external membrane, the cytoplasm and the cell nucleus. These studies have shown that cell survival is greatly reduced when the nucleus is irradiated, but that this is not the case when cytoplasm or cell membrane is irradiated. Thus, DNA is a very sensitive site to ionizing radiation and ROS. For this reason, DNA has long been considered the most important molecule to explain radiobiological effects such as cell death. However, this concept has been challenged recently by new experimental results that have shown that cells which have not been directly in contact with radiation are also affected. This is called the bystander effect. Further studies have shown that a group of cells and their environment reacts collectively to radiation. A hypothesis put forward to explain this radiobiological phenomenon is that a irradiated cell will secrete signalling molecules that will affect non-irradiated cells. The implicated phenomenon and molecules are poorly understood at this moment. The purpose of this work is to improve our comprehension of the phenomenon in the microsecond that follows the irradiation. To these ends, a new Monte-Carlo simulation program of water radiolysis by photons has been generated. For photons of energy <2 MeV, they interact with water mainly by Compton and photoelectric effects, which create energetic electrons in water. The created electrons are then followed by our existing programs to simulate the radiolysis of water by photons. Similarly, a new code has been built to simulate the neutrons interaction with water. This code simulates the elastic collisions of a neutron with water molecules and calculates the number and energy of recoil protons and oxygen ions. The main part of this Ph.D. work was the generation of a non-homogeneous Monte-Carlo Step-By-Step (SBS) simulation code of non-homogeneous radiation chemistry. This new program has been used successfully to simulate radiolysis of water by ions of various LET, pH, ion types ([superscript 1]H[superscript +], [superscript 4]He[superscript 2+], [superscript 12]C[superscript 6+]) and temperature. The program has also been used to simulate the dose-rate effect and the Fricke and Ceric dosimeters. More complex systems (glycine, polymer gels and HCN) have also been simulated.

En France, les matériaux à base de ciment sont utilisés comme matrice de conditionnement des déchets nucléaires de faible et moyenne activité. La radiolyse de l'eau est due aux déchets nucléaires stockés dans les matériaux. La formation de ses produits radiolytiques tels que le gaz H₂ doit être évaluée pour des raisons de sécurité. Les silicates de calcium hydratés (C-S-H) représentent le principal produit (50%) d'hydratation du ciment Portland (CP). L’objectif de cette étude est de comprendre les mécanismes radiolytiques de la production d'hydrogène dans les C-S-H, d'étudier l'effet d'impuretés (telles que des ions alcalins, hydroxydes ou nitrates supplémentaires) sur la production de gaz H₂ dans les C-S-H et d'examiner l'existence d'interactions entre les phases principales (C-S-H et portlandite) du ciment Portland. Après avoir caractérisé les échantillons par diverses techniques, ils ont été soumis à différents types d'irradiation (faisceaux gamma et électrons et ions lourds (HI) pour déterminer leur rendement radiolytique en H₂, G(H₂). Dans les C-S-H, il a été démontré que, sous irradiation gamma, la production d’H₂ est indépendante de la teneur en eau et que les C-S-H produisaient autant d’H₂ que la même masse d’eau. Ainsi, le mécanisme de production d’hydrogène est très efficace dans les C-S-H. La comparaison entre les résultats obtenus sous rayons gamma et ceux obtenus sous HI implique qu’il n'y a pas ou peu d'effet de transfert d’énergie linéique (TEL) dans les C-S-H. Ainsi, les réactions de recombinaison semblent limitées. L’introduction d’ions nitrates dans la structure des C-S-H induit une diminution importante du G(H₂). L’irradiation des hydrates de C2S et C3S constitués majoritairement de C-S-H et de portlandite indique qu’il n’y a pas de phénomènes de transfert d’énergie entre ces phases. Enfin, les expériences de spectroscopie par résonance paramagnétique électronique (RPE) ont permis de proposer des mécanismes radiolytiques dans les C-S-H. L’ensemble de ces résultats nous permettent de mieux comprendre les effets d’irradiation dans les ciments. / In France, cementitious materials are used as conditioning matrix of low level and intermediate level nuclear wastes. Water radiolysis occurs due to the nuclear wastes stored in the materials. The formation of its radiolytic products such as H₂ gas must be evaluated for safety reasons. Calcium silicate hydrate (C-S-H) is the main product (50%) of hydration of Portland Cement (PC). The aim of this study is to understand the radiolytic mechanisms of the hydrogen production in C-S-H, to investigate the effect of impurities (such as alkali ions, additional hydroxides or nitrates ions) on H₂ gas production in C-S-H and to examine if interactions exist between different main phases (C-S-H and portlandite) in cement matrix. After using various characterization techniques, samples were submitted to different types of irradiation (gamma rays and electrons and heavy ions (HI) beams) to determine their H₂ radiolytic yield, G(H₂). In C-S-H system, it has been shown, under gamma irradiation, that G(H₂) does not depend on water content, moreover, C-S-H system itself produce efficiently H₂ gas. The comparison between the results obtained under gamma rays and that obtained under HI implies: there is no/ low LET effect in C-S-H. While with nitrate ions in C-S-H, a large decrease of G(H₂) is observed. Irradiation of C2S and C3S hydrates mainly composed of C-S-H and portlandite shows that here is no energy transfer phenomena between these two phases. Finally, the electron paramagnetic resonance (EPR) spectroscopy experiments have enabled proposing radiolytic mechanisms. All these results help us to understand the radiation effects in cements.

Résumé : Le contrôle de la chimie de l'eau dans un réacteur nucléaire refroidi à l'eau nécessite une compréhension détaillée des effets de la radiolysede l'eau afin de limiter la corrosion et la dégradation des matériaux par oxydation générée par les produits de cette radiolyse. Toutefois, la mesure directe de la chimie dans le cœur des réacteurs est extrêmement difficile, sinon impossible, en raison des conditions extrêmes de haute température et haute pression, et les champs d’irradiation mixtes neutrons/γ, qui ne sont pas compatibles avec l'instrumentation chimique normale. Pour ces raisons,des modèles théoriques et des simulations sur ordinateur sont essentielles pour la prédiction de la chimie sous rayonnement de l'eau de refroidissement dans le cœur et son impact sur les matériaux. Dans ce travail, des simulations Monte Carlo ont été utilisées pour calculer les rendements des principales espèces (e[indice supérieur -][indice inférieur aq], H[indice supérieur •], H[indice inférieur 2], [indice supérieur •]OH et H[indice inférieur 2]O[indice inférieur 2]) formées lors de la radiolyse de l’eau liquide neutre par des neutrons mono-énergétiques de 2 MeV à des températures entre 25 et 350 °C. Le choix des neutrons de 2 MeV comme énergie d'intérêt est représentatif du flux de neutrons rapides dans un réacteur. Pour l'eau légère, la contribution la plus significative à la radiolyse vient des quatre premières collisions des neutrons qui produisent, dans la majorité des cas, des protons avec des énergies de recul de ~1.264, 0.465, 0.171 et 0.063 MeV et des transferts d’énergie linéique (TEL) moyens respectivement de ~22, 43, 69et 76 keV/[micro]m. Par ailleurs, nous avons négligé les effets des radiations dus aux ions de recul de l’oxygène. Les rendements moyens finaux peuvent alors être estimés comme étant la somme des rendements résultant de l’action de ces protons après pondérations en fonction de l’énergie déposée. Les rendements ont été calculés à 10[indice supérieur -7], 10[indice supérieur -6] et 10[indice supérieur -5] s. Les valeurs obtenues sont en accord avec les données expérimentales disponibles. En comparant nos résultats avec les données obtenues pour les rayonnements à faible TEL (rayons γ de [indice supérieur 60]Co ou électrons rapides), nos rendements calculés pour les neutrons rapides ont montré une dépendance en température essentiellement similaire, mais avec des valeurs plus faibles pour les rendements en radicaux libres et des valeurs plus élevées pour les rendements moléculaires. Nous avons également utilisé les simulations Monte Carlo pour étudier l'existence de la chute rapide de la constante de vitesse de réaction de l'électron hydraté (e[indice supérieur -][indice inférieur aq]) sur lui-même – l’une des principales sources de formation de H[indice inférieur 2] – au-dessus de 150 °C. Cette dépendance en température a été observée expérimentalement en milieu alcalin par divers auteurs, mais jamais en milieu neutre. Lorsque cette baisse de la constante de vitesse d’auto-réaction de e[indice supérieur -][indice inférieur aq] est incluse dans nos codes de simulation, tant pour des rayonnements de bas TEL (grappes isolés) que de haut TEL (trajectoires cylindriques), g(H[indice inférieur 2]) montre une discontinuité marquée à la baisse à ~150°C, ce qui n'est pas observée expérimentalement. Les conséquences de la présence de cette discontinuité dans le rendement en H[indice inférieur 2] pour les rayonnements à bas et haut TEL sont discutées. Enfin, nous avons tenté d’expliquer l'augmentation – considérée comme anormale – du rendement en H[indice inférieur 2] en fonction de la température au-dessus de 200 °C par l’intervention de la réaction des atomes H[indice supérieur •] avec l'eau, préalablement proposée par Swiatła-Wojcik et Buxton en 2005. La constante de vitesse de cette réaction est toujours controversée. // Abstract : Controlling the water chemistry in a water-cooled nuclear power reactor requires understanding and mitigating the effects of water radiolysis to limit the corrosion and degradation of materials by oxidizing radiolysis products. However, direct measurement of the chemistry in reactor cores is extremely difficult due to the extreme conditions of high temperature, pressure, and mixed neutron/γ-radiation fields, which are not compatible with normal chemical instrumentation. For these reasons, theoretical models and computer simulations are essential for predicting the detailed radiation chemistry of the cooling water in the core and the impact on materials. Monte Carlo simulations were used to calculate the yields for the primary species (e[superscript -][subscript aq], H[superscript •], H[subscript 2], [superscript •]OH, and H[subscript 2]O[subscript 2]) formed from the radiolysis of neutral liquid water by mono-energetic 2-MeV neutrons and the mechanisms involved at temperatures between 25 and 350 °C. In this work, we chose 2-MeV neutron as our energy of interest since it is known as representative of a fast neutron flux in a nuclear reactor. For light water, for that chosen energy, the most significant contribution to the radiolysis comes from the first four neutron collisions that generate mostly ejected protons with energies of ~1.264, 0.465, 0.171, and 0.063 MeV, which had, at 25 °C, mean linear energy transfers (LETs) of ~22, 43, 69, and 76 keV/[micro]m, respectively. In this work, we simply neglected the radiation effects due to oxygen ion recoils. The average final fast neutron yields could be estimated as the sum of the yields for these protons after allowance was made for the appropriate weightings (by using the Eq (2) in Chapter 4) according to their deposited energy. Yields were calculated at 10[superscript -7], 10[superscript -6] and 10[superscript -5] s. Our computed yield agreed reasonably well with the available experimental data. By comparing our results with data obtained for low-LET radiation ([superscript 60]Co γ-rays or fast electrons), our computed yields for fast neutron radiation showed essentially similar temperature dependences over the range of temperature studied, but with lower values for yields of free radicals and higher values for molecular yields. In this work, we also used our Monte Carlo simulation to investigate the existence of drop of hydrated electron (e[superscript -][subscript aq]) self-reaction rate constant at 150 °C. One of the main sources of H[subscript 2] formation is the self-reaction of hydrated electrons. The temperature dependence of the rate constant of this reaction (k[subscript 1]), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150 °C. However, when this drop in the e[superscript -][subscript aq] self-reaction rate constant is included in our code for low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET), g(H[subscript 2]) shows a marked downward discontinuity at ~150 °C which is not observed experimentally. The consequences of the presence of this discontinuity in H[subscript 2] yield for both low and high LET radiation are discussed. Another reaction that might explain the anomalous increasing of H[subscript 2] yield with temperature is the reaction of H[superscript •] atoms with water previously proposed by Swiatla-Wojcik and Buxton (2005) whose rate constant is still in controversial.

Ce travail, réalisé dans le cadre du projet Geant4-DNA, consiste à concevoir un prototype pour la simulation des effets chimiques précoces des rayonnements ionisants. Le modèle de simulation étudié repose sur la représentation particule-continuum où toutes les molécules sont explicitement simulées et où le solvant est traité comme un continuum. La méthode proposée par cette thèse a pour but d'améliorer les performances de ce type de simulation. Elle se base sur (1) la combinaison d'une méthode de pas en temps dynamiques avec un processus de pont Brownien pour la prise en compte des réactions chimiques et afin d'éviter une simulation à pas en temps fixe, coûteuse en temps de calcul, et (2) sur la structure de données k-d tree pour la recherche du voisin le plus proche permettant, pour une molécule donnée, une localisation rapide du réactif le plus proche. La précision de l'algorithme est démontrée par la comparaison des rendements radiochimiques en fonction du temps et en fonction du transfert d'énergie linéaire avec des résultats d'autres codes Monte-Carlo et des données expérimentales. A partir de ce prototype, une tentative de prédiction du nombre et du type d'interactions radicaux-ADN a été entreprise basée sur d'une description simplifiée du noyau cellulaire. / The purpose of this work, performed under the Geant4-DNA project, is to design a prototype for simulating early chemical effects of ionizing radiation. The studied simulation model is based on the particle-continuum representation where all the molecules are explicitly simulated, and where the solvent is treated as a continuum. The method proposed by this thesis aims at improving the performance of this type of simulation. It is based on (1) a dynamical time steps method with a Brownian bridge process, to account for chemical reactions, which avoids the costly fixed time-step simulations, and (2) on the k-d tree data structure for quickly locating, for a given molecule, its closest reactants. The accuracy of the algorithm is demonstrated by comparing radiochemical yields over time and depending on the linear energy transfer with results obtained from other Monte Carlo codes and experimental data. Using this prototype, an attempt to predict the number and type of radical attacks on DNA has been performed using a simplified description of the cell nucleus.

L’objectif de cette thèse est de contribuer à la compréhension du mécanisme d’oxydation de Br- dans lequel le radical HO• intervient. Le rendement du radical HO• étant alors intimement lié au rendement d’oxydation de Br-, c’est sur lui que l'influence de différents paramètres physicochimiques a été étudiée : température, TEL, débit de dose, pH, nature du gaz saturant. Les solutions ont été irradiées avec 4 types de rayonnement : rayons X de 13 à 18 keV, électrons de 7 et 10 MeV, faisceaux d’ions C6+ de 975 MeV et He2+ de 70 MeV. Le développement d’un autoclave optique avec circulation de solution compatible avec le rayonnement de TEL élevé a permis de réaliser les premières expériences à TEL élevé constant et à température élevée. Cette cellule s’est avérée être aussi compatible avec les expériences pompe-sonde picoseconde réalisées avec l’accélérateur ELYSE.Le rendement de capture du radical hydroxyle a donc été estimé à TEL élevé mais aussi à haute température. Une meilleure compréhension du mécanisme d’oxydation de Br- en est issue, notamment en milieu acide et en comparant les résultats cinétiques avec les simulations Monte Carlo pour les temps inférieurs à la µs, et Chemsimul pour les produits stables (formation de Br2•- et de Br3-). / The purpose of this thesis is to contribute to the understanding of the oxidation mechanism of Br- in which the HO• radical is involved. The HO• radiolytic yield is strongly connected with the oxidation yield of Br-, and therefore we have studied the influence of different physical and chemical parameters on this global yield: temperature, LET, dose rate, pH, saturation gas. The solutions have been irradiated with 4 types of ionizing rays: X- rays (from 13 to 18 keV), electrons (from 7 to 10 MeV), C6+-ions beam of 975 MeV and He2+-ions beam of 70 MeV.The development of an optical autoclave with solution flow, compatible with high LET ionizing rays has allowed us conduct the first experiments at constant high LET and high temperature. This cell has turned out to be compatible with the picosecond pump-probe experiments performed with the ELYSE accelerator.The HO• scavenging yield has been, therefore, estimated at both high LET and high temperature. A better understanding of the Br- oxidation mechanism has been achieved, in acid medium, in particular, by comparing the kinetics results with Monte Carlo Simulations for time scales inferior to the microsecond and with Chemsimul for the stable products (Br2•- and Br3- formations).