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The development of an economical method for determining accurately group constants of hexagonal and rectangular cells is considered in this dissertation. The mathematical model constructed for this purpose has the capability to characterize the group constants for the entire range of the neutron spectrum. Furthermore, this model is also rigorous enough to predict the group constants with the required accuracy for a specific range of interest in the energy spectrum and for a variety of energy group configurations. The model is implemented separately for the fast and thermal energy regions. These regions are subsequently coupled via the source term. The construction of the model for the fast energy range has been pursued by implementing the transport equation specialized in a two-region cell. The regions are coupled via the escape probability functions. The model for the thermal energy range has been attained by implementing the appropriate Nelkin and Honeck amplitude functions within the kernels of the transport equation. The Nelkin amplitude function is utilized for treating light water moderated systems, and the Honeck amplitude function relates to heavy water moderated systems. The group constants calculated with the economical model have been benchmarked with those computed by the VIM Monte Carlo code. The values obtained for the group constants agree within 1-2% with those computed by VIM for the fast energy region. The agreements for the thermal energy region are within 2-3%. The CPU running time of the implemented model is about 3 1/2 minutes for a four group configuration. On the other hand a typical VIM run comprising 25,000 neutron histories and a four-group structure expends about 30 minuts of CPU time for light water moderated systems. Moreover, similar VIM runs utilizing heavy water as moderator require over one hour of CPU time. Therefore, the implemented model makes utilization of computer resources with a cost advantage of a factor of 10 or better as compared to VIM. This economical benefit of the implemented model enables it to be coupled directly with fuel depletion codes, whereby the group constants and the fuel isotopics are updated at relatively short time intervals. On the other hand, the coupling of VIM with burnup codes would result in prohibitively expensive CPU costs. / Ph. D.

Made available in DSpace on 2014-10-09T12:35:04Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:44Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:35:04Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:44Z (GMT). No. of bitstreams: 0 / Este trabalho apresenta a realização de experimentos no reator nuclear IPEN/MB-01 submetido à presença de um refletor com água pesada instalado na sua face oeste. Após a instalação do refletor no reator foram conduzidos três tipos de experimentos: A calibração das barras de controle, a verificação da influência do aumento da temperatura do moderador na reatividade e a medição das taxas de reações por meio da irradiação de fios e folhas de ativação. Devido às propriedades nucleares de interação de nêutrons com água pesada, notadamente sua elevada capacidade de espalhamento e sua baixa capacidade de absorção, é possível notar alterações no funcionamento do reator observadas pelas mudanças dos padrões de retiradas e inserções de barras de controle no núcleo. Essas alterações são apresentadas no decorrer do trabalho. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Des mesures de radiolyse impulsionnelle avec des paquets d’électron de 7 picosecondes ont été effectuées dans des systèmes D₂O et D₂SO₄ / D₂O purs afin d'étudier l'effet isotopique sur la réactivité de l'électron solvaté (es⁻), du radical hydroxyle (OH•) et de son radical cation cationique (H₂O•⁺). Dans le système D₂O, le spectre d'absorption molaire de l’électron solvaté dans D₂O a été déterminé entre 250 et 1500 nm en utilisant la méthode du point isosbestique. Sur la base de ce coefficient d'extinction, le rendement radiolytique de es⁻ dans D₂O juste après l'impulsion d'électrons de 7 ps a été déterminé à (4,4 ± 0,2) × 10⁻⁷ mol J⁻¹, ce qui coïncide avec celui dans H₂O. Le rendement du radical OD• juste après l'impulsion d'électrons s'est révélée être (5,0 ± 0,2) × 10⁻⁷ mol J⁻¹. Dans le cas de l’électron solvaté la vitesse de déclin dans D₂O est plus lent que dans H₂O, tandis que la vitesse de déclin de OD• est similaire à celui de OH•. Dans le système D₂SO₄ /D₂O, la réactivité de D₂O•⁺ a été sondée dans des solutions d'acide sulfurique deutérées de diverses concentrations et comparée à celle de H₂O•⁺. Le soluté oxydé, le radical sulfate a été observé à 7 ps et des rendements plus élevés ont été trouvés dans les solutions deutérées. Les effets isotopiques révèlent la compétition entre deux réactions ultra-rapides : le transfert de protons vers H₂O (D₂O) et le transfert d'électrons de H₂O•⁺ (D₂O•⁺). Les simulations par la théorie de la densité fonctionnelle décryptent le mécanisme du transfert d’électrons : elles se font par migration de charge sub-femtoseconde et ne sont pas affectées par la substitution isotopique. / Picosecond pulse radiolysis measurements were performed in neat D₂O and D₂SO₄/D₂O system in order to study the isotopic effect on the reactivity of the solvated electron(es⁻), the hydroxyl radical (OH•) and its precursor—water cation radical (H₂O•⁺). In D₂O system, the molar absorption coefficient spectra of spectrum in D₂O was determined between 250 to 1500 nm using the isosbestic point method. Based on this extinction coefficient, the radiolytic yield of es⁻ in D₂O just after the 7 ps electron pulse was determined to be (4.4 ± 0.2) × 10⁻⁷ mol J⁻¹, which coincides with the one for in H₂O. The yield of the radical OD• just after the 7 ps electron pulse was found to be (5.0 ± 0.2) × 10⁻⁷ mol J⁻¹. In the spurs of ionization, the decay rate of es⁻ is slower in D₂O than that in H₂O, whereas the decay rate of OD• is similar to the one of OH•. In D₂SO₄/D₂O system, the reactivity of and D₂O•⁺ was probed in deuterated sulfuric acid solutions of various concentrations and compared with that of H2O•⁺. The oxidized solute sulfate radical was observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H₂O (D₂O) and electron transfer from H₂SO4⁻ to H₂O•⁺ (D₂O•⁺). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution.

In the atomic energy pile at Chalk River, heavy water (deuterium oxide, D2O) is to be used as a moderator. Early in 1944, it became evident that frequent determinations of the isotopic composition of D2O samples would be required. For this purpose, there was obtained from the American authorities a mass spectrometer specially designed for routine hydrogen isotope analysis. This instrument was assembled and put into operation at McMaster in the summer of 1944. During the course of subsequent work, two other deuterium mass spectrometers were built in this laboratory, following the design of the instrument sent from the United States; one of these has now been installed at the plant site, and a second is to follow at an early date. The studies reported below involved the determination of operating conditions, the measurement of grid leaks, of the order of 10^10 ohms, and the investigation of further applications of the deuterium mass spectrometer. / Thesis / Master of Science (MSc)

Pour les structures de génie civil avec un rôle d’étanchéité, lors d’un accident grave, la perméabilité du béton est une question clé. Les mesures de perméabilités actuelles ne permettent d'avoir que des grandeurs moyennes (structurelles) qui ne conviennent pas pour des éprouvettes hétérogènes (fissures et/ou armatures). La compréhension des détails de l'écoulement des fluides est cruciale en raison des implications sur les voies préférentielles (interface acier/béton, fissures, etc.). Le but de cette thèse est de proposer une nouvelle méthodologie et de tester un nouveau dispositif expérimental par imagerie neutronique à la ligne de faisceaux D50 à l’Institut Laue Langevin à Grenoble. Le test consiste à injecter de l’eau normale (H2O), sous haute pression, dans un échantillon de béton coulé et saturé en eau lourde (D2O), afin de suivre la progression d’un front d’eau dans le temps par différence d’atténuation entre ces deux eaux. Une campagne expérimentale a été lancée sur des éprouvettes de béton sous différentes configurations (béton sain, béton fissuré et béton avec une armature) et des mesures de perméabilité locale dans les singularités (zone fissurée, interface armature-béton, etc.) ont été faites. Les essais ont montré que les mesures classiques de perméabilité sont sous-estimées et les écoulements dans le béton sont contrôlés par les défauts. / For civil engineering structures, especially containment buildings during a severe accident, the permeability of concrete is a key issue. Current permeability measurements allow only average (structural) magnitudes, which are not suitable for heterogeneous samples (cracks and/or reinforcements). Understanding the details of fluid flow is crucial because of the implications of preferred pathways (steel/concrete interface, cracks, etc.). The aim of this thesis is to propose a new methodology, and to test a new experimental setup, using neutron imaging at the D50 beam line at the Institute Laue Langevin in Grenoble to measure flow directly. The test consists of injecting normal water (H2O) under high pressure, into a concrete sample casted and saturated with heavy water (D2O), in order to follow the evolution of a waterfront over time by difference of attenuation between these two waters. An experimental campaign was launched on concrete specimens in different configurations (proper concrete, cracked concrete and concrete with reinforcement) and measurements of local permeability in singularities (cracked zone, reinforcement-concrete interface, etc.) were done. Tests have shown that classical permeability measurements are underestimated and flows in concrete are controlled by defects.

Este trabalho apresenta a realização de experimentos no reator nuclear IPEN/MB-01 submetido à presença de um refletor com água pesada instalado na sua face oeste. Após a instalação do refletor no reator foram conduzidos três tipos de experimentos: A calibração das barras de controle, a verificação da influência do aumento da temperatura do moderador na reatividade e a medição das taxas de reações por meio da irradiação de fios e folhas de ativação. Devido às propriedades nucleares de interação de nêutrons com água pesada, notadamente sua elevada capacidade de espalhamento e sua baixa capacidade de absorção, é possível notar alterações no funcionamento do reator observadas pelas mudanças dos padrões de retiradas e inserções de barras de controle no núcleo. Essas alterações são apresentadas no decorrer do trabalho. / This work presents the accomplishiment of experiments in the IPEN/MB-01 research nuclear under presence of a heavy water reflector installed in its western wall. After reflectors assembly in the reactor, three types of experiments were conducted: The control rod calibration, the check of temperature\'s influence increasing under reactivity and the measurement of reaction rates by means wires and foil\'s activation. Due to the properties of neutron interaction with heavy water, especially their high scattering and low absorption cross sections, pattern changes operation could be observed by the control rods moving inside the core. These observations will be the focus of this work

Este trabalho apresenta a realização de experimentos no reator nuclear IPEN/MB-01 submetido à presença de um refletor com água pesada instalado na sua face oeste. Após a instalação do refletor no reator foram conduzidos três tipos de experimentos: A calibração das barras de controle, a verificação da influência do aumento da temperatura do moderador na reatividade e a medição das taxas de reações por meio da irradiação de fios e folhas de ativação. Devido às propriedades nucleares de interação de nêutrons com água pesada, notadamente sua elevada capacidade de espalhamento e sua baixa capacidade de absorção, é possível notar alterações no funcionamento do reator observadas pelas mudanças dos padrões de retiradas e inserções de barras de controle no núcleo. Essas alterações são apresentadas no decorrer do trabalho. / This work presents the accomplishiment of experiments in the IPEN/MB-01 research nuclear under presence of a heavy water reflector installed in its western wall. After reflectors assembly in the reactor, three types of experiments were conducted: The control rod calibration, the check of temperature\'s influence increasing under reactivity and the measurement of reaction rates by means wires and foil\'s activation. Due to the properties of neutron interaction with heavy water, especially their high scattering and low absorption cross sections, pattern changes operation could be observed by the control rods moving inside the core. These observations will be the focus of this work