In Pressurised Water Reactors, the reactor pressure vessel (RPV) is considered a life limiting component due to the degradation of its mechanical properties. Nano-scale Cu-enriched precipitates are known to cause embrittlement in the form of increases in hardness and the ductile-to-brittle transition temperature. The effect of irradiation on the RPV is the dominant contributor to this embrittlement. This is due to the increased mobility of Cu from the high number of vacancies, and the matrix damage providing many heterogeneous nucleation sites. However, there are also thermal effects which may be difficult to separate from the irradiation effects. To understand the contribution of the long term thermal ageing to RPV embrittlement a series of weld and plate materials containing systematic variations of Ni and Cu has been thermally aged for times up to 100,000 hrs at 330 degrees C, 365 degrees C and 405 degrees C. Microstructural characterisation using Atom Probe Tomography has been performed. Complimentary Monte-Carlo simulations have been used to investigate the early stages of formation of Cu-enriched precipitates. Thermal ageing produces a high number density of nano-scale Cu-enriched precipitates. These nanometre precipitates have a Ni-Mn-Si rich interface which was found to be wider with increased precipitate size, lower ageing temperature and higher bulk Ni content. This interface reduces the interfacial energy of the Cu-enriched precipitates through a combination of the minimising of unfavourable Fe-Cu bonds and reduction in lattice strain. The matrix Cu levels after ageing for 90,000 - 100,000 hrs were found to be around 0.06 - 0.07 at.%, close to the expected solubility limits for Cu in Fe. The Fe content of the precipitates has been characterised and found to be higher at lower ageing temperature and for smaller precipitate sizes. Cu precipitation and solute segregation at dislocations were observed, particularly in the SG steels aged at higher temperatures where the supersaturation is lowest. Movies were produced from the Kinetic Monte-Carlo modelling (see accompanying DVD) and along with other analyses indicated that sub-critical Ni and Mn clusters may be active in the formation of Cu-enriched precipitates. This mechanism appears to occur in both the high and low Ni steels. Thus, the higher number density of larger precipitates observed with increased bulk Ni content is thought to be a consequence of the greater number of sub-critical Ni-Mn clusters providing more nucleation sites. A small number of grain boundaries were examined. Segregation of many solute species to them was observed, which is thought to result from a multi-element co-segregation process. Ni-Mn-Si precipitates were observed at grain boundaries and dislocations in the high Ni steels with high and low Cu levels. These are consistent with similar phases in found in irradiated high Ni steels. In the high Cu steels these particles were much larger and associated with Cu-enriched precipitates.

No presente trabalho foram estudados os fenômenos de encruamento e, principalmente, transformação de fases, recuperação e recristalização, presentes na liga U-7,5Nb-2,5Zr (Mulberry alloy) e no urânio não ligado. Realizou-se a fusão da liga por dois métodos: plasma (menor massa) e indução (maior massa). A caracterização microestrutural das ligas resultantes nos estados bruto de fundição e homogeneizado (tratado termicamente na região da fase γ seguido de resfriamento rápido em água), assim como do urânio em seu estado inicial, foi realizada com auxílio de várias técnicas complementares de análise microestrutural. No estado gama estabilizado, a liga U-7,5Nb-2,5Zr foi deformada na temperatura ambiente por dois métodos: laminação a frio, dividida em vários estágios (20%, 50%, 60% e 80%), e limagem, sendo o pó resultante de alto grau de deformação. As amostras deformadas foram posteriormente recozidas em tratamentos isócronos (1 hora) e isotérmicos (200ºC, 450ºC e 700ºC). O urânio não ligado foi deformado em aproximadamente 60% e 80% de redução em espessura, e em seguida submetido a tratamentos isócronos (1 hora) e isotérmicos (400ºC e 650ºC). Os fenômenos de encruamento, recuperação, recristalização e transformação de fases foram estudados predominantemente por microscopia óptica, dureza e difração de raios X, com auxílio do método de Rietveld. Adicionalmente, técnicas de análise térmica (dilatometria e calorimetria diferencial) foram utilizadas para acompanhamento da cinética de transformação de fase e energia armazenada na deformação. Com relação à deformação, a liga U-7,5Nb-2,5Zr mostrou ser capaz de sofrer reduções da ordem de 70% na temperatura ambiente, sem necessidade de recozimentos intermediários e com um baixo grau de encruamento. Similarmente, o urânio não ligado mostrou ser capaz de sofrer graus de deformação mais altos na temperatura ambiente, entretanto, este material apresentou alto grau de encruamento e, mesmo após considerável grau de deformação, ainda apresentava muitas heterogeneidades de deformação, como bandas de deformação e maclas. Foi observado que a recristalização do urânio não ligado teve início a aproximadamente 454ºC. Para a liga no estado deformado e supersaturado, a precipitação de fases tende a ocorrer antes da recristalização. Assim, o comportamento desta liga sob aquecimento pós-deformação pode ser resumido da seguinte forma: ~200°C (Recuperação) ---> 300-575°C (Precipitação de fases) ---> 575°C (Recristalização). O rápido aquecimento para temperaturas acima de 650ºC, ou a manutenção desta temperatura por longos tempos, gera uma estrutura γ recristalizada com grãos equiaxiais. Uma estrutura de grãos finos (~8,3µm) foi obtida no recozimento a 700ºC/1h tanto para baixo como para alto grau de deformação. Uma taxa de aquecimento lenta, ou recozimento na faixa de 300-575ºC, gera precipitação da fase antes da recristalização. Consequentemente, a transformação eutetóide γ→α+γ₃ ocorre de modo a herdar a orientação do grão γ deformado, o que pode gerar uma textura de transformação. Na faixa de temperaturas de 575-650ºC ocorre a interação entre os fenômenos de precipitação de fase e recristalização. Em recozimentos a 200ºC foi possível observar a predominância da recuperação para graus de deformação intermediários (60%) e altos (80%), mas para grau de deformação baixo (20%) prevaleceu endurecimento por precipitação da fase α\'\'. Com auxílio da análise em um calorímetro diferencial (DSC) foi observado que a energia armazenada na deformação e liberada durante o processo de recristalização da liga U-7,5Nb-2,5Zr foi de 6,5J/g. Tal valor é relativamente alto se comparado aos metais comuns, o que leva à suposição de que uma linha de discordância no urânio representa uma maior energia. Este fato tem influência direta no processo recristalização. Este experimento demonstrou também que os fenômenos de precipitação de fase e recristalização interagem entre si, com relação à energia disponível para o processo. A textura da liga U-7,5Nb-2,5Zr foi estudada por difração de raios X (DRX) nas condições com fase γ estabilizada (obtida através de fusão, coquilhamento e homogeneização seguida de têmpera) e no estado deformado (laminado a temperatura ambiente). A liga na condição com γ estabilizado apresentou textura moderada com apenas as componentes (023) e (032). Após a deformação de 80%, o material apresentou uma textura de fibra (001)<uvw>, pouco comum nos metais CCC, além da fibra γ (111)<uvw>, com intensidade intermediária. / In this work it was studied the phenomena of work hardening, mainly phase transformation, recovery and recrystallization in the U-7.5Nb-2.5Zr alloy (Mulberry alloy) and unalloyed uranium. The alloy was melted by two methods: plasma (smaller mass) and induction (larger mass). Microstructural characterization of the samples in the as-cast and homogenized states (the last one was heat treated in the γ phase region and then quenched in water), as well as uranium in its initial state, was performed using several complementary techniques for microstructural analysis. In the gamma stabilized state, the U-7.5Nb2.5Zr alloy was deformed at room temperature by two methods: cold rolling in several stages (20%, 50%, 60% and 80%), and then filed, resulting in a powder with high degree of deformation. Deformed samples were subsequently annealed by isochronal (1 hour) and isothermal (200°C, 450°C, 700°C) treatments. Unalloyed uranium was deformed by approximately 60% and 80% reduction in thickness, and then subjected to isochronous (1 hour) and isothermal (400°C and 650°C) treatments. The phenomena of work hardening, recovery, recrystallization and phase transformation were studied by optical microscopy, hardness testing and X-ray diffraction, using the Rietveld method. Additionally, thermal analysis techniques (differential calorimetry and dilatometry) were used to measure the kinetics of phase transformation and energy stored during deformation. With regard to deformation, the U-7.5Nb-2.5Zr alloy was reduced of approximately 70% at room temperature without intermediate annealing and with a low degree of work hardening. Similarly, unalloyed uranium was reduced of high degrees of deformation at room temperature. However, this sample showed a higher degree of work hardening, and even after significant deformation still showed lots of inhomogeneities of deformation, such as deformation bands and twins. It was observed that recrystallization of unalloyed uranium started at about 454°C. For the alloy in the supersaturated and deformed states, the phase precipitation tends to occur before recrystallization. Thus, the behavior of this alloy under heat treatments after deformation can be summarized as follows: ~200°C (Recovery) ---> 300-575°C (Phase precipitation) ---> 575°C (Recrystallization). Rapid heating to temperatures above 650°C, or maintain this temperature for a long time, generates a γ recrystallized structure with equiaxed grains. Fine grain structure (~8.3 µm) was obtained for annealing at 700°C/1 h for both lower and higher deformation degrees. Slow heating rate or annealing treatment in the range of 300 to 575ºC, causes precipitation before recrystallization. Consequently, the eutectoid transformation γ→α+γ₃ occurs in order to inherit orientation from the γ deformed grain, which may generate a transformation texture. The interaction between the phenomena of phase precipitation and recrystallization was observed in the temperature range of 575-650°C. At the annealing temperature of 200°C it was possible to observe the predominance of recovery at intermediate (60%) and higher (80%) degrees of deformation, while at lower deformation degree (20%) α phase precipitation hardening has predominated. The results obtained using a differential calorimeter (DSC) showed that the energy stored during deformation and released during the recrystallization of the U-7.5Nb-2.5Zr alloy was 6.5 J/g. That value is relatively high compared to common metals, which leads to the conclusion that dislocation lines in uranium alloys possess higher energy. This fact has a direct influence in the recrystallization process. This experiment also demonstrated that the phenomena of phase precipitation and recrystallization interact with each other with regard to energy available for the process. The texture of the U-7.5Nb-2.5Zr alloy was studied by X-ray diffraction (XRD) in the γ-phase stabilized condition (obtained by melting, casting, homogenization and then quenching) and in deformed state (rolled at room temperature). The first condition generated moderate texture with the components (023) e (032). After 80% of deformation, the samples showed a fiber texture (001)<uvw>, uncommon in the BCC metals, as well the γ fiber (111)<uvw> with intermediate intensity.

Foi efetuada a caracterização microestrutural de 10 ligas dos sistemas urânio-nióbio (U-10Nb; U-15Nb; U-20Nb), urânio-zircônio (U- 10Zr; U-15Zr, U-20Zr) e urânio-nióbio-zircônio (U-2,5Nb-2,5Zr; U-5Nb- 5Zr; U-7,5Nb-7,5Zr; U-10Nb-10Zr), no canto rico em urânio. As ligas estudadas são candidatas ao uso como elementos combustíveis tipo placa, utilizados tanto em reatores nucleares de pesquisa como em reatores nucleares de potência. As ligas foram preparadas por fusão a plasma em forno com eletrodo não consumível de tungstênio. Após várias fusões, as amostras sofreram tratamento térmico de homogeneização a 1000ºC por 96 horas, com resfriamento em água. Em seguida, as amostras homogeneizadas foram recozidas a 700 e a 500ºC, com resfriamento em água. No total, foram estudadas 40 amostras de 10 ligas diferentes em 4 condições diferentes: bruto de fundição, homogeneizadas a 1000ºC e envelhecidas a 700 e a 500ºC. Foram utilizadas várias técnicas complementares de caracterização microestrutural: microscopia óptica, microscopia eletrônica de varredura com auxilio de microanálise por dispersão de energia de raios X, difração de raios X com auxílio do método de análise de Rietveld, e medidas de microdureza Vickers. Os resultados mostraram que os elementos de liga Nb e Zr estabilizam a fase alotrópica &#947 do urânio e atrasam a transformação de γ para β. Neste aspecto, o Nb é mais eficaz que o Zr. Além disto, podem ocorrer durante o resfriamento transformações martensíticas γ→α\', β→α′ e possivelmente γ→γ°. A temperatura de início de transformação martensítica (Ms) formadora da fase diminui com a adição dos elementos de liga estudados. Ms intercepta a temperatura ambiente entre as composições U-5Nb-5Zr e U-7,5Nb-7,5Zr. Foi verificado também que a reação peritetóide α + γ2→ δ do sistema U-Zr possui uma cinética lenta e não pode ser detectada nos tempos e temperaturas estudados. Em algumas ligas foi possível reter na temperatura ambiente ligas com microestrutura martensítica dúcteis, que permitem a conformação mecânica a frio, o que é de significativo interesse tecnológico. / The microstructures of 10 uranium-rich alloys of the uraniumniobium (U-10Nb; U-15Nb; U-20Nb), uranium-zirconium (U-10Zr; U- 15Zr;U-20Zr) and uranium-niobium-zirconium (U-2.5Nb-2.5Zr; U-5Nb-5Zr; U-7.5Nb-7.5Zr; U-10Nb-10Zr)systems have been characterized. The studied alloys are considered for plate-type nuclear fuels fabrication used both in nuclear research reactors and in nuclear power reactors. The alloys were melted by arc plasma methods employing nonconsumable tungsten cathode. After several fusions, samples were subjected to homogenizing heat treatment at 1000ºC for 96 hours and then quenched in water. Then the samples were annealed at 700 and 500ºC. The microstructural characterization encompassed 40 samples of 10 different alloys composition in four different conditions: as cast, homogenized at 1000°C and aged at 700 and 500ºC. Microstructural characterization was performed using several complementary techniques: optical microscopy; scanning electron microscopy with energy-dispersive X-ray analysis; X-ray diffraction with the aid of the Rietveld analysis method; and Vickers microhardness measurements. The results showed that the Nb and Zr additions have stabilized the uranium γ-phase and delayed the γ and β phase transformation. In this regard, Nb was more effective than Zr. However, during cooling martensitic transformations γ→α\', β→α\' and possibly γ→γ° may occur. The martensitic transformation start temperature (Ms), which produces the phase , decreased with Nb and Zr additions. Ms intersected room temperature between the compositions U-5Nb-5Zr e U- 7,5Nb-7,5Zr. It was found that the peritectoid reaction α + γ2 → δ of the U-Zr system showed a very slow kinetics and could not be detected in the range of the studied times and temperatures. An important result of the technological point of view is that in some alloys it was possible to retain at room temperature a ductile martensitic microstructure, allowing cold forming.

Foi efetuada a caracterização microestrutural de 10 ligas dos sistemas urânio-nióbio (U-10Nb; U-15Nb; U-20Nb), urânio-zircônio (U- 10Zr; U-15Zr, U-20Zr) e urânio-nióbio-zircônio (U-2,5Nb-2,5Zr; U-5Nb- 5Zr; U-7,5Nb-7,5Zr; U-10Nb-10Zr), no canto rico em urânio. As ligas estudadas são candidatas ao uso como elementos combustíveis tipo placa, utilizados tanto em reatores nucleares de pesquisa como em reatores nucleares de potência. As ligas foram preparadas por fusão a plasma em forno com eletrodo não consumível de tungstênio. Após várias fusões, as amostras sofreram tratamento térmico de homogeneização a 1000ºC por 96 horas, com resfriamento em água. Em seguida, as amostras homogeneizadas foram recozidas a 700 e a 500ºC, com resfriamento em água. No total, foram estudadas 40 amostras de 10 ligas diferentes em 4 condições diferentes: bruto de fundição, homogeneizadas a 1000ºC e envelhecidas a 700 e a 500ºC. Foram utilizadas várias técnicas complementares de caracterização microestrutural: microscopia óptica, microscopia eletrônica de varredura com auxilio de microanálise por dispersão de energia de raios X, difração de raios X com auxílio do método de análise de Rietveld, e medidas de microdureza Vickers. Os resultados mostraram que os elementos de liga Nb e Zr estabilizam a fase alotrópica &#947 do urânio e atrasam a transformação de γ para β. Neste aspecto, o Nb é mais eficaz que o Zr. Além disto, podem ocorrer durante o resfriamento transformações martensíticas γ→α\', β→α′ e possivelmente γ→γ°. A temperatura de início de transformação martensítica (Ms) formadora da fase diminui com a adição dos elementos de liga estudados. Ms intercepta a temperatura ambiente entre as composições U-5Nb-5Zr e U-7,5Nb-7,5Zr. Foi verificado também que a reação peritetóide α + γ2→ δ do sistema U-Zr possui uma cinética lenta e não pode ser detectada nos tempos e temperaturas estudados. Em algumas ligas foi possível reter na temperatura ambiente ligas com microestrutura martensítica dúcteis, que permitem a conformação mecânica a frio, o que é de significativo interesse tecnológico. / The microstructures of 10 uranium-rich alloys of the uraniumniobium (U-10Nb; U-15Nb; U-20Nb), uranium-zirconium (U-10Zr; U- 15Zr;U-20Zr) and uranium-niobium-zirconium (U-2.5Nb-2.5Zr; U-5Nb-5Zr; U-7.5Nb-7.5Zr; U-10Nb-10Zr)systems have been characterized. The studied alloys are considered for plate-type nuclear fuels fabrication used both in nuclear research reactors and in nuclear power reactors. The alloys were melted by arc plasma methods employing nonconsumable tungsten cathode. After several fusions, samples were subjected to homogenizing heat treatment at 1000ºC for 96 hours and then quenched in water. Then the samples were annealed at 700 and 500ºC. The microstructural characterization encompassed 40 samples of 10 different alloys composition in four different conditions: as cast, homogenized at 1000°C and aged at 700 and 500ºC. Microstructural characterization was performed using several complementary techniques: optical microscopy; scanning electron microscopy with energy-dispersive X-ray analysis; X-ray diffraction with the aid of the Rietveld analysis method; and Vickers microhardness measurements. The results showed that the Nb and Zr additions have stabilized the uranium γ-phase and delayed the γ and β phase transformation. In this regard, Nb was more effective than Zr. However, during cooling martensitic transformations γ→α\', β→α\' and possibly γ→γ° may occur. The martensitic transformation start temperature (Ms), which produces the phase , decreased with Nb and Zr additions. Ms intersected room temperature between the compositions U-5Nb-5Zr e U- 7,5Nb-7,5Zr. It was found that the peritectoid reaction α + γ2 → δ of the U-Zr system showed a very slow kinetics and could not be detected in the range of the studied times and temperatures. An important result of the technological point of view is that in some alloys it was possible to retain at room temperature a ductile martensitic microstructure, allowing cold forming.

Made available in DSpace on 2014-10-09T12:42:16Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:15Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:42:16Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:15Z (GMT). No. of bitstreams: 0 / A amostragem ambiental por esfregaço ou swipe samples é uma metodologia que vem sendo empregada rotineiramente pela Agência Internacional de Energia Atômica (AIEA) para verificar se os Estados signatários do Acordo de Salvaguardas estão realizando atividades nucleares não declaradas. A análise dessas amostras ambientais é complementar aos procedimentos de salvaguardas convencionais para a verificação das informações cedidas pelos Estados. Neste trabalho, foi descrita uma metodologia que visa a fortalecer os processos de salvaguarda nuclear e da ciência forense nuclear no país. A proposta é estudar e avaliar a técnica de ablação à laser acoplada a um espectrômetro de massa de alta resolução com fonte de plasma indutivamente acoplado (LA-HR-ICP-MS Laser Ablation High Resolution Inductively Coupled Plasma Mass Spectrometry), como técnica alternativa para a análise das amostras de esfregaço. Adicionalmente, foram empregadas outras técnicas complementares (atividade radioativa, microscopia eletrônica de varredura e espectroscopia de energia dispersiva) com o intuito de garantir maior segurança ao processo de inspeção de salvaguardas e investigações forenses nucleares. A precisão, medida através do desvio padrão relativo (DPR),dos resultados obtidos para as razões isotópicas 234U/238U, 235U/238U e 236U/238U do padrão analisado (CRM -125A) para a medida do fator de discriminação de massa foram, respectivamente, 1,3 %, 0,2 % e 7,6 %. Já as incertezas percentuais (u %), que também contemplam o DPR das medições, variaram de 3,5 % a 29,8 % para as medições da razão isotópica 235U/238U e de 16,6 % a 42,9 % para a razão isotópica 234U/238U. Esses valores mostraram-se compatíveis com outros estudos que utilizaram a mesma técnica para análise de amostras reais coletadas em uma instalação nuclear. As amostras coletadas apresentaram nível de enriquecimento que variou de (2,3 ± 0,7) % (amostra 3 corredor de acesso à instalação) a (17,3 ± 2,8) % (amostra 18 bancada de recuperação de UF4). A partir da coleta de amostras nesses diversos pontos, desde a entrada da instalação até a bancada em que se manuseia o urânio enriquecido, foi possível detectar diversos níveis de enriquecimento no isótopo 235U. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

No presente trabalho foram estudados os fenômenos de encruamento e, principalmente, transformação de fases, recuperação e recristalização, presentes na liga U-7,5Nb-2,5Zr (Mulberry alloy) e no urânio não ligado. Realizou-se a fusão da liga por dois métodos: plasma (menor massa) e indução (maior massa). A caracterização microestrutural das ligas resultantes nos estados bruto de fundição e homogeneizado (tratado termicamente na região da fase γ seguido de resfriamento rápido em água), assim como do urânio em seu estado inicial, foi realizada com auxílio de várias técnicas complementares de análise microestrutural. No estado gama estabilizado, a liga U-7,5Nb-2,5Zr foi deformada na temperatura ambiente por dois métodos: laminação a frio, dividida em vários estágios (20%, 50%, 60% e 80%), e limagem, sendo o pó resultante de alto grau de deformação. As amostras deformadas foram posteriormente recozidas em tratamentos isócronos (1 hora) e isotérmicos (200ºC, 450ºC e 700ºC). O urânio não ligado foi deformado em aproximadamente 60% e 80% de redução em espessura, e em seguida submetido a tratamentos isócronos (1 hora) e isotérmicos (400ºC e 650ºC). Os fenômenos de encruamento, recuperação, recristalização e transformação de fases foram estudados predominantemente por microscopia óptica, dureza e difração de raios X, com auxílio do método de Rietveld. Adicionalmente, técnicas de análise térmica (dilatometria e calorimetria diferencial) foram utilizadas para acompanhamento da cinética de transformação de fase e energia armazenada na deformação. Com relação à deformação, a liga U-7,5Nb-2,5Zr mostrou ser capaz de sofrer reduções da ordem de 70% na temperatura ambiente, sem necessidade de recozimentos intermediários e com um baixo grau de encruamento. Similarmente, o urânio não ligado mostrou ser capaz de sofrer graus de deformação mais altos na temperatura ambiente, entretanto, este material apresentou alto grau de encruamento e, mesmo após considerável grau de deformação, ainda apresentava muitas heterogeneidades de deformação, como bandas de deformação e maclas. Foi observado que a recristalização do urânio não ligado teve início a aproximadamente 454ºC. Para a liga no estado deformado e supersaturado, a precipitação de fases tende a ocorrer antes da recristalização. Assim, o comportamento desta liga sob aquecimento pós-deformação pode ser resumido da seguinte forma: ~200°C (Recuperação) ---> 300-575°C (Precipitação de fases) ---> 575°C (Recristalização). O rápido aquecimento para temperaturas acima de 650ºC, ou a manutenção desta temperatura por longos tempos, gera uma estrutura γ recristalizada com grãos equiaxiais. Uma estrutura de grãos finos (~8,3µm) foi obtida no recozimento a 700ºC/1h tanto para baixo como para alto grau de deformação. Uma taxa de aquecimento lenta, ou recozimento na faixa de 300-575ºC, gera precipitação da fase antes da recristalização. Consequentemente, a transformação eutetóide γ→α+γ₃ ocorre de modo a herdar a orientação do grão γ deformado, o que pode gerar uma textura de transformação. Na faixa de temperaturas de 575-650ºC ocorre a interação entre os fenômenos de precipitação de fase e recristalização. Em recozimentos a 200ºC foi possível observar a predominância da recuperação para graus de deformação intermediários (60%) e altos (80%), mas para grau de deformação baixo (20%) prevaleceu endurecimento por precipitação da fase α\'\'. Com auxílio da análise em um calorímetro diferencial (DSC) foi observado que a energia armazenada na deformação e liberada durante o processo de recristalização da liga U-7,5Nb-2,5Zr foi de 6,5J/g. Tal valor é relativamente alto se comparado aos metais comuns, o que leva à suposição de que uma linha de discordância no urânio representa uma maior energia. Este fato tem influência direta no processo recristalização. Este experimento demonstrou também que os fenômenos de precipitação de fase e recristalização interagem entre si, com relação à energia disponível para o processo. A textura da liga U-7,5Nb-2,5Zr foi estudada por difração de raios X (DRX) nas condições com fase γ estabilizada (obtida através de fusão, coquilhamento e homogeneização seguida de têmpera) e no estado deformado (laminado a temperatura ambiente). A liga na condição com γ estabilizado apresentou textura moderada com apenas as componentes (023) e (032). Após a deformação de 80%, o material apresentou uma textura de fibra (001)<uvw>, pouco comum nos metais CCC, além da fibra γ (111)<uvw>, com intensidade intermediária. / In this work it was studied the phenomena of work hardening, mainly phase transformation, recovery and recrystallization in the U-7.5Nb-2.5Zr alloy (Mulberry alloy) and unalloyed uranium. The alloy was melted by two methods: plasma (smaller mass) and induction (larger mass). Microstructural characterization of the samples in the as-cast and homogenized states (the last one was heat treated in the γ phase region and then quenched in water), as well as uranium in its initial state, was performed using several complementary techniques for microstructural analysis. In the gamma stabilized state, the U-7.5Nb2.5Zr alloy was deformed at room temperature by two methods: cold rolling in several stages (20%, 50%, 60% and 80%), and then filed, resulting in a powder with high degree of deformation. Deformed samples were subsequently annealed by isochronal (1 hour) and isothermal (200°C, 450°C, 700°C) treatments. Unalloyed uranium was deformed by approximately 60% and 80% reduction in thickness, and then subjected to isochronous (1 hour) and isothermal (400°C and 650°C) treatments. The phenomena of work hardening, recovery, recrystallization and phase transformation were studied by optical microscopy, hardness testing and X-ray diffraction, using the Rietveld method. Additionally, thermal analysis techniques (differential calorimetry and dilatometry) were used to measure the kinetics of phase transformation and energy stored during deformation. With regard to deformation, the U-7.5Nb-2.5Zr alloy was reduced of approximately 70% at room temperature without intermediate annealing and with a low degree of work hardening. Similarly, unalloyed uranium was reduced of high degrees of deformation at room temperature. However, this sample showed a higher degree of work hardening, and even after significant deformation still showed lots of inhomogeneities of deformation, such as deformation bands and twins. It was observed that recrystallization of unalloyed uranium started at about 454°C. For the alloy in the supersaturated and deformed states, the phase precipitation tends to occur before recrystallization. Thus, the behavior of this alloy under heat treatments after deformation can be summarized as follows: ~200°C (Recovery) ---> 300-575°C (Phase precipitation) ---> 575°C (Recrystallization). Rapid heating to temperatures above 650°C, or maintain this temperature for a long time, generates a γ recrystallized structure with equiaxed grains. Fine grain structure (~8.3 µm) was obtained for annealing at 700°C/1 h for both lower and higher deformation degrees. Slow heating rate or annealing treatment in the range of 300 to 575ºC, causes precipitation before recrystallization. Consequently, the eutectoid transformation γ→α+γ₃ occurs in order to inherit orientation from the γ deformed grain, which may generate a transformation texture. The interaction between the phenomena of phase precipitation and recrystallization was observed in the temperature range of 575-650°C. At the annealing temperature of 200°C it was possible to observe the predominance of recovery at intermediate (60%) and higher (80%) degrees of deformation, while at lower deformation degree (20%) α phase precipitation hardening has predominated. The results obtained using a differential calorimeter (DSC) showed that the energy stored during deformation and released during the recrystallization of the U-7.5Nb-2.5Zr alloy was 6.5 J/g. That value is relatively high compared to common metals, which leads to the conclusion that dislocation lines in uranium alloys possess higher energy. This fact has a direct influence in the recrystallization process. This experiment also demonstrated that the phenomena of phase precipitation and recrystallization interact with each other with regard to energy available for the process. The texture of the U-7.5Nb-2.5Zr alloy was studied by X-ray diffraction (XRD) in the γ-phase stabilized condition (obtained by melting, casting, homogenization and then quenching) and in deformed state (rolled at room temperature). The first condition generated moderate texture with the components (023) e (032). After 80% of deformation, the samples showed a fiber texture (001)<uvw>, uncommon in the BCC metals, as well the γ fiber (111)<uvw> with intermediate intensity.

<p>  </p> <p>Metallic uranium-zirconium (U-Zr) nuclear fuel is a primary candidate for future fast reactors. The U-Zr system has been studied for decades with thousands of fuel pins being irradiated, yet the phase boundaries and lattice evolution with respect to temperature and composition remain poorly quantified. Historic engineering scale testing has resulted in empirical models for fuel evolution and subsequent fuel performance. However, these historic tests are on a convoluted system, consisting of dynamic temperatures, evolving thermal gradients, varying irradiation damage and damage rates, evolving compositions via fission and redistribution of primary constituents, and morphological evolution. This system proves exceedingly difficult to describe mechanistically due to the coexistence of various intertwined thermodynamic driving forces (e.g., temperature, composition, fluence, and fission rate which all vary concurrently). The driving forces influence the manifestation of the primary life-limiting phenomena present within the U-Zr system, specifically fuel-cladding mechanical interaction, fuel-cladding chemical interaction, fuel swelling, and fuel constituent redistribution. Although the phenomena present in the U-Zr system are known and qualitatively described, they are lacking in fundamental descriptions due to the historic inability to deconvolve the effects of temperature, composition, and fission rate. This study evaluates the current understanding of U-Zr fuel swelling and constituent redistribution in a uniquely quantified manner using Phenomena Identification and Ranking Tables. </p> <p><br></p> <p>In response to these findings, a novel separate effects irradiation test vehicle, housing uniquely fabricated U-Zr alloys, was proposed, developed, and successfully fabricated to provide the community with a means to decouple temperature, composition, initial microstructure, and fission rate from one-another. Initial out-of-pile characterization was conducted with scanning electron microscopy, transmission electron microscopy, and neutron diffraction with in-situ heating on various U-Zr alloys (U- 6, 10, 20, and 30 wt.% Zr). This work quantifies the initial microstructure throughout the fabrication process and the thermal response of the material. Results include the phase morphology, phase boundaries, absolute lattice parameters, and lattice specific coefficients of thermal expansion. The phase boundaries identified in this study were then used to develop a new U-Zr phase diagram. The isolation of thermal and compositional dependencies furthers the understanding of the fuel system and can be used to increase fuel longevity.</p>

The following dissertation is divided into two parts. Part I deals with the modeling of helium trapping at oxide-iron interfaces in nanostructured ferritic alloys (NFAs) using density functional theory (DFT). The modelling that has been performed serves to increase the knowledge and understanding of the theory underlying the prevention of helium embrittlement in materials. Although the focus is for nuclear reactor materials, the theory can be applied to any material that may be in an environment where helium embrittlement is of concern. In addition to an improved theoretical understanding of helium embrittlement, the following DFT models will provide valuable thermodynamic and kinetic information. This information can be utilized in the development of large-scale models (such as kinetic Monte Carlo simulations) of the microstructural evolution of reactor components. Accurate modelling is an essential tool for the development of new reactor materials, as experiments for components can span decades for the lifetime of the reactor. Part II of this dissertation deals with the development, and use of, kinetic Monte Carlo (KMC) simulations for improved efficiency in investigating catalytic chemical reactions on surfaces. An essential technique for the predictive development and discovery of catalysts relies on modelling of large-scale chemical reactions. This requires multi-scale modelling where a common sequence of techniques would require parameterization obtained from DFT, simulation of the chemical reactions for millions of conditions using KMC (requiring millions of separate simulations), and finally simulation of the large scale reactor environment using computational fluid dynamics. The tools that have been developed will aid in the predictive discovery, development and modelling of catalysts through the use of KMC simulations. The algorithms that have been developed are versatile and thus, they can be applied to nearly any KMC simulation that would seek to overcome similar challenges as those posed by investigating catalysis (such as the need for millions of simulations, long simulation time and large discrepancies in transition probabilities). / Ph. D.

In order to increase the waste loading efficiency of nuclear waste glasses, alternative composite structures are sought that trap molybdenum in a water-durable CaMoO4 phase. In this thesis, the formation and stability of CaMoO4 in a borosilicate glass against the attack of internal radiation was investigated. It is a fundamental study that simplified the composition to known contributors of molybdate speciation, and further splits the com- ponents of α and β-decay into integral parts that replicated both nuclear and electronic interactions. Irradiation experiments using 2.5 MeV β, 7 MeV Au, and 92 MeV Xe ions were enlisted to test the hypotheses of whether 100−1000 years of radiation damage given current waste loading standards would: (i) induce phase separation in homogeneous re- gions, (ii) increase the extent of existing phase separation, (iii) induce local annealing that could cause amorphisation of crystalline phases or increase mixing between amorphous phases, or (iv) cause some combination of the above. Results from XRD, SEM, EPR, and Raman spectroscopy suggest that powellite is stable against replicated radiation damage with only minor modifications observed. The main mechanisms of alteration involved: (i) thermal and defect-assisted diffusion, (ii) relaxation from the added ion’s energy, (iii) localised damage recovery from ion tracks, and (iv) the accumulation of point defects or the formation of voids that created significant strain, and led to longer-range modifications. It can be further concluded that no precip- itation or increased phase separation was observed in single-phased glasses. In isolated cases, radiation-induced precipitation of CaMoO4 occurred, but these crystallites were reamorphised at higher doses. At high SHI fluences, minor amorphisation of powellite was also observed, but this occurred alongside bulk-to-surface reprecipitation of CaMo- species. Overall, the components of internal radiation were often found to have opposing effects on the alteration of Si−O−B mixing in the glass, ion migration, and crystallite size. This led to the prediction that a steady-state damage structure could form from cumulative decay processes. These results suggest that CaMoO4 containing borosilicate GCs are resistant to radiation, and that excess molybdenum from increased waste loading can be successfully incorporated into these structures without altering the overall dura- bility of the wasteform. Furthermore, the identified saturation in modifications occurring around 8 x 10¹⁴ Xe ions/cm² can be used as a benchmark in future investigations on more complex systems where the maximum damage state is required.