Depuis les années 60 les modèles de structure nucléaire prédisent l'existence d'un îlot de stabilité des noyaux super-lourds, situé autour du prochain noyau doublement magique, attendu selon les modèles à N=172 ou 184 et entre Z=114 et 126. Ces noyaux posséderaient de très hautes barrières de fission, de l'ordre de quelques MeV, générées par les effets microscopiques, et responsables de larges distributions de temps de fission s'étendant à des temps très longs. Ainsi nous avons entrepris des mesures de temps de fission du noyau superlourd Z=124 par fluorescence X, une méthode basée sur le remplissage des lacunes électroniques internes crées durant la collision conduisant à la formation du noyau composé. Cette expérience repose sur la détection des fragments de fission en coïncidence avec les raies XK caractéristiques du Z=124, formé lors de la réaction U+70,76Ge. La difficulté majeure a été d'identifier ces raies XK, du fait de la présence de raies gamma émises par des fragments de fission dans la gamme d'énergie des XK du 124 entachant les multiplicités de photons mesurées pour différentes sélections de fragments. Cette difficulté met clairement en évidence une limitation importante à la méthode de fluorescence X pour certains systèmes super-lourds. Des simulations de spectres d’émission XK, réalisées à partir d'un calcul MCDF (Multi-Configuration-Dirac-Fock) ont été comparées aux spectres expérimentaux. Une limite maximale de multiplicité d'XK compatible avec les données, de l'ordre de 6 à 7 % pour le 76Ge et de 12 à 14 % pour le 70Ge ont ainsi pu être extraites. Ces valeurs restent compatibles avec les signatures de temps longs obtenues sur le même système par la technique de blocage cristallin. / Since the 1960s nuclear structure model have predicted the existence of an island of stability of superheavy elements. It should be located around the next magic numbers expected at N=172 or 184 and between Z=114 and 126 depending on the model. Very high fission barrier of a few MeV are predicted to be generated by microscopic effects for those nuclei for which large fission times distributions extended to very high fission times are induced. Fission time measurements of the superheavy element Z=124 have been made by us using the X-ray fluorescence technique, a method based on the filling of inner-shell electronic vacancies created during the collison leading to the formation of the compound nucleus. The aim of this experiment was to detect in coincidence both fission fragments and characteristic X-rays from the Z=124, created by the reaction U+70,76Ge.The main difficulty was to identify those X-rays due to the fact that gamma-rays from fission fragments were emitted in the same energy range, which affected our photon multiplicities for any fragment selection. This new difficulty brings an mportant limitation to the study of some particular superheavy elements by the X-ray fluorescence method. K X-rays spectra have been simulated using MCDF (Multi-Configuration-Dirac-Fock) and then compared to the experimental ones in order to get a maximal K X-ray multplicity compatible with our data. The extracted results were about 6-7% for 76Ge and from 12 to 14% for 70Ge. Those values remain compatible with the experimental signature of long lifetime component observed for the same system but using a blocking tehcnique in single crystals.

The SABR fusion-fission hybrid concept for a fast burner reactor, which combines the IFR-PRISM fast reactor technology and the ITER tokamak physics and fusion technology, is adapted for a fusion-fission hybrid reactor, designated SABrR. SABrR is a sodium-cooled 3000 MWth reactor fueled with U-Pu-10Zr. For the chosen fuel and core geometry, two configurations of neutron reflector and tritium breeding structures are investigated: one which emphasizes a high tritium production rate and the other which emphasizes a high fissile production rate. Neutronics calculations are performed using the ERANOS 2.0 code package, which was developed in order to model the Phenix and SuperPhenix reactors. Both configurations are capable of producing fissile breeding ratios of about 1.3 while producing enough tritium to remain tritium-self-sufficient throughout the burnup cycle; in addition, the major factors which limit metal fuel residence time, fuel burnup and radiation damage to the cladding material, are modest.

A series of fuel cycle simulations were performed using CEA's reactor physics code ERANOS 2.0 to analyze the transmutation performance of the Subcritical Advanced Burner Reactor (SABR). SABR is a fusion-fission hybrid reactor that combines the leading sodium cooled fast reactor technology with the leading tokamak plasma technology based on ITER physics. Two general fuel cycles were considered for the SABR system. The first fuel cycle is one in which all of the transuranics from light water reactors are burned in SABR. The second fuel cycle is a minor actinide burning fuel cycle in which all of the minor actinides and some of the plutonium produced in light water reactors are burned in SABR, with the excess plutonium being set aside for starting up fast reactors in the future. The minor actinide burning fuel cycle is being considered in European Scenario Studies. The fuel cycles were evaluated on the basis of TRU/MA transmutation rate, power profile, accumulated radiation damage, and decay heat to the repository. Each of the fuel cycles are compared against each other, and the minor actinide burning fuel cycles are compared against the EFIT transmutation system, and a low conversion ratio fast reactor.

No description available.

Ferritic ODS steels show improved high temperature strength and irradiation tolerance compared with conventional ferritic steels, and are one of the key potential materials for fusion blanket structural applications. The processing of ODS steels is critical to their subsequent performance; however knowledge of the optimum processing approaches for these alloys is not complete. The microstructural evolution of ODS steels containing Y₂O₃ and other additions during manufacture has been investigated and the processing conditions optimised based on microstructural and mechanical investigations. Ferritic powders with Y₂O₃ and other additions were investigated, primarily using analysis on the micro- and nano-scale, with an emphasis on identifying the requirements for homogenization of the Y within the steel matrix. The Y₂O₃ dispersion and subsequent development of the nano-precipitates during thermal treatment was investigated using in-situ neutron diffraction. The nano-precipitates were resolved at approximately 900◦C after 1hr, with coarsening and/or re-precipitation progressively increasing at higher temperatures. A significantly increased number density of nano-precipitates (∼2x10²³m−3 to ∼7x10²³m−3) was established by hot isostatically pressing an Fe-14Cr-3W-0.2Ti0.25Y₂O₃ alloy at 950◦C compared with more traditional temperatures at 1150◦C, attributed to the increased coarsening and/or re-precipitation of the nano-precipitates at the higher temperatures. The influence of the mechanical alloy (MA)ing conditions on bulk mechanical properties was investigated using four point bend. The highest fracture toughness of ∼55MN/m^3/2 and ultimate strength of ∼1450MPa was achieved under conditions that minimised the mechanical alloying time and increased the average final size of the powders. An Fe-14Cr-3W-0.2Ti-0.25Y₂O₃ (wt%) ODS alloy manufactured under optimised conditions showed a bi-modal grain structure size distribution and had a comparatively high yield strength of >1200MPa at 20◦C and >330MPa at 700◦C. The grain structure and high yield strength were attributed to the random distribution of 25nm radius of gyration (R_g) Y, Ti and O rich nano-precipitates randomly dispersed throughout the alloy. Long term thermal ageing (750hr at 1000◦C) reduced the room temperature yield strength and increased the proportion of larger grains in the bi-modal distribution, but high temperature yield strength was remarkably stable.

Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2016-08-12T19:39:07Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_versão_final_biblioteca_português.pdf: 2265965 bytes, checksum: 707851ccf48e28513fc34178bca7c739 (MD5) / Made available in DSpace on 2016-08-12T19:39:07Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_versão_final_biblioteca_português.pdf: 2265965 bytes, checksum: 707851ccf48e28513fc34178bca7c739 (MD5) Previous issue date: 2016-03-04 / Capes / Cromossomos holocêntricos apresentam atividade cinetocórica difusa e essa organização favorece, em teoria, rápidas variações cromossômicas numéricas e o acúmulo de DNA satélite (DNAsat) predominantemente nas regiões terminais dos cromossomos. O gênero de plantas Rhynchospora (Cyperaceae), um dos diversos grupos com esse tipo cromossômico, apresenta espécies com cariótipos entre 2n = 4 e 2n = 58, cuja variação é atribuída à poliploidia e a eventos de quebra/fusão, levando a disploidias. Quanto à distribuição de DNAsat, o único relato até o momento revelou uma baixa proporção dessas sequências, com o único repeat identificado (Tyba) associado aos holocentrômeros. Com o intuito de entender como a estrutura centromérica difusa interfere na organização de sequências ao longo do cromossomo e na evolução do cariótipo como um todo, foram realizadas uma análise de reconstrução dos números cromossômicos ancestrais de Rhynchospora em um contexto filogenético e a caracterização de DNAsats em três espécies do gênero. O complemento cromossômico 2n = 10 foi indicado como o mais provável para o ancestral do gênero, tendo sido mantido em diferentes taxa. A maioria dos clados mostrou números estáveis e a homoplasia de cariótipos foi observada em uma frequência relativamente baixa. Os genomas de R. ciliata/R. globosa e R. tenuis apresentaram duas e uma família(s) de DNAsat, respectivamente, com um padrão de condensação típico (blocos condensados em intérfase). Uma localização preferencial nos terminais cromossômicos foi observada apenas para os DNAsat de R. globosa. Três tipos de cromatina foram revelados pela distribuição dessas sequências: (1) associadas à heterocromatina e presente na forma de cromocentros em intérfase e blocos nos cromossomos metafásicos (R. ciliata e R. globosa); (2) compactados em interfase mas parcialmente descondensados em metáfase e não diretamente associados à heterocromatina (R. ciliata e R. tenuis); ou (3) associados aos holocentrômeros (R. ciliata e R. tenuis). De forma geral em Rhynchospora, os eventos de fusão e fissão parecem atuar localmente no remodelamento dos cariótipos e as sequências satélites não mostram uma tendência única de distribuição. A estrutura centromérica difusa, portanto, não determina em larga escala a dinâmica evolutiva dos cromossomos do gênero. / Holocentric chromosomes show diffuse kinetochore activity, what would lead to fast evolution of chromosome numbers and a biased distribution of satellite repeats. The plant genus Rhynchospora (Cyperaceae) possesses holocentric chromosomes and shows a large chromosome number variation (2n = 4 to 2n = 58) attributed to polyploidy and frequent fusion/fission events, leading to dysploidy. Regarding satellite repeats (satDNA), the only investigated species showed a low proportion of these sequences, with the single family identified associated to the holocentromeres. In the present work, aiming to better understand how the diffuse centromere organisation could interfere with the distribution of satellite repeats along the chromosomes and with the karyotype evolution as a whole, we combined a reconstruction of Rhynchospora chromosome numbers in a phylogenetic framework and the characterisation of satellite repeats in three selected species. The karyotype with 2n = 10 was suggested as the ancestral state and was maintained in different lineages. Most of the clades showed stable chromosome number and recurrent karyotypes changes (leading to homoplasies) were detected in low frequency. All Rhynchospora species analysed (R. ciliata, R. globosa and R. tenuis) showed a higher diversity of satellite repeats than R. pubera, with most of the repeats showing a typical condensation profile (clustered in interphase). A preferential terminal location on chromosomes was only observed for R. globosa satDNAs. These sequences, however, might represent different chromatin types, organized in distinct ways: (1) associated to the heterochromatin and clustered in interphase and metaphase (identified in R. ciliata and R. globosa only); (2) clustered in interphase but partially decondensed in metaphase and not associated to heterochromatin domains (R. ciliata e R. tenuis); (3) associated to the holocentromeres (R. ciliata e R. tenuis). Taken together, at least for Rhynchospora, fusion/fission events may not act in a broader way in the reshuffling of karyotypes and satellite repeats distribution do not appeared to be biased towards the chromosome termini. A non-localized centromere, therefore, must not constrain, in a large scale, the chromosome evolution of the genus.

Medidas das seções de choque de fusão, de processos fortemente amortecidos e de espalhamento elástico foram realizadas para os sistemas 16,17,18O + 10,11B e 19F + 9Be no intervalo de energia de bombardeio compreendido entre 22 ELAB64 Mev. São apresentadas evidências de que processos binários fortemente amortecidos observados nestes sistemas originam-se preferencialmente de um processo de fusão-fissão e não de um mecanismo de orbiting dinuclear. A importância relativa do processo de fusão-fissão nestes sistemas muito leves é comprovada pelos resultados experimentais, que indicam a ocorrência de um processo de fissão de um núcleo composto estatisticamente equilibrado, assim como por cálculos de modelos teóricos. Os números atômicos e os de massa dos produtos de reação e dos resíduos de evaporação foram identificados usando-se duas câmaras de ionização e um sistema de tempo de voo. / Cross sections for fusion, strongly energy-damped processes and elastic scattering were measured for the 16 ,17, 18O + 10 11B and 19F + 9Be systems in the energy range 22 ELAB 64 HeV. Evidence that the fully energy-damped binary products observed in these reactions originate from a fusion-fission process, rather than through a deep-inelastic orbiting mechanism, is presented. The relative importance of the fusion-fission process in these very light systems is supported by experimental results which points towards the fission of a statistically equilibrated compound nucleus and also by model calculations. Charges and masses of the reaction products and evaporation residues were indentified using two ionization chamber and a time-of-flight system.

Medidas das seções de choque de fusão, de processos fortemente amortecidos e de espalhamento elástico foram realizadas para os sistemas 16,17,18O + 10,11B e 19F + 9Be no intervalo de energia de bombardeio compreendido entre 22 ELAB64 Mev. São apresentadas evidências de que processos binários fortemente amortecidos observados nestes sistemas originam-se preferencialmente de um processo de fusão-fissão e não de um mecanismo de orbiting dinuclear. A importância relativa do processo de fusão-fissão nestes sistemas muito leves é comprovada pelos resultados experimentais, que indicam a ocorrência de um processo de fissão de um núcleo composto estatisticamente equilibrado, assim como por cálculos de modelos teóricos. Os números atômicos e os de massa dos produtos de reação e dos resíduos de evaporação foram identificados usando-se duas câmaras de ionização e um sistema de tempo de voo. / Cross sections for fusion, strongly energy-damped processes and elastic scattering were measured for the 16 ,17, 18O + 10 11B and 19F + 9Be systems in the energy range 22 ELAB 64 HeV. Evidence that the fully energy-damped binary products observed in these reactions originate from a fusion-fission process, rather than through a deep-inelastic orbiting mechanism, is presented. The relative importance of the fusion-fission process in these very light systems is supported by experimental results which points towards the fission of a statistically equilibrated compound nucleus and also by model calculations. Charges and masses of the reaction products and evaporation residues were indentified using two ionization chamber and a time-of-flight system.