Thesis (Ph.D. (Natural and Agricultural Science)) -- University of Pretoria, 2009. / Includes bibliographical references.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1980. / Bibliography: leaf 49. / by Gerald Ellsworth Hammond Jr. / Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1980.

Thesis: M.S., Massachusetts Institute of Technology, Department of Nuclear Engineering, 1979 / Bibliography: leaves 195-198. / M.S. / M.S. Massachusetts Institute of Technology, Department of Nuclear Engineering

The metallic binary-alloy fuel Uranium-Zirconium is important for use in the new generation of advanced fast reactors. Uranium-Zirconium goes through a phase transition at higher temperatures to a (gamma) Body Centered Cubic (BCC) phase. The BCC high temperature phase is particularly important since it corresponds to the temperature range in which the fast reactors will operate. A semi-empirical Modified Embedded Atom Method (MEAM) potential is presented for Uranium-Zirconium. This is the first interatomic potential created for the U-Zr system. The bulk physical properties of the Uranium-Zirconium binary alloy were reproduced using Molecular Dynamics (MD) and Monte Carlo (MC) simulations with the MEAM potential. The simulation of bulk metallic alloy separation and ordering phenomena on the atomic scale using iterative MD and MC simulations with interatomic potentials has never been done before. These simulations will help the fundamental understanding of complex phenomena in the metallic fuels. This is a large step in making a computationally acceptable fuel performance code, able to replicate and predict fuel behavior.

Uranium (U) exhibits a high temperature body-centered cubic (bcc) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U–Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Metallic nuclear fuels swell under fission conditions, creating fission product gases such as helium, xenon and krypton. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. The bulk modulus, the lattice constant, and the Birch–Murnaghan equation of state for the defect free bcc uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr, He, Xe and Kr interstitial and substitutional defects. This work is utilized in the construction of modified Embedded-Atom Method interatomic potentials for the bcc phase of uranium as well as the binary systems of U-Xe, U-Kr and U-He. Using this potential, equilibrium volume and elastic constants are calculated at 0 K and found to be in close agreement with previous first principles calculations. Further, the melting point, heat capacity, enthalpy of fusion, thermal expansion and volume change upon melting are calculated and found to be in reasonable agreement with experiment. Calculations of dilute fission gas defects show reasonable agreement with first principles calculations. Finally, void and xenon bubble energetics are analyzed as a function of temperature.

Made available in DSpace on 2014-10-09T12:34:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:45Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:34:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:45Z (GMT). No. of bitstreams: 0 / O IPEN-CNEN/SP desenvolveu e disponibilizou para produção rotineira a tecnologia de fabricação de elementos combustíveis tipo dispersão, para uso em reatores nucleares de pesquisas. O combustível fabricado no IPEN-CNEN/SP está limitado à concentração de urânio de 3,0gU/cm3, para dispersões a base de U3Si2-Al, e de 2,3gU/cm3, para dispersões a base de U3O8-Al. O aumento da concentração de urânio nas placas combustíveis possibilita aumentar a reatividade do núcleo do reator e a vida útil do combustível. É possível aumentar-se a concentração de urânio no combustível até o limite tecnológico de 4,8gU/cm3 para a dispersão U3Si2-Al, e de 3,2gU/cm3 para a dispersão U3O8-Al, as quais estão bem qualificadas ao redor do mundo. Este trabalho tem como objetivo desenvolver o processo de fabricação de ambos os combustíveis com alta concentração de urânio, redefinindo-se os procedimentos de fabricação atualmente adotados no Centro do Combustível Nuclear do IPEN-CNEN/SP. Com base nos resultados obtidos conclui-se que para atingir a concentração desejada devem ser feitas algumas alterações nos procedimentos já estabelecidos, tais como mudança no tamanho de partícula dos pós e mudança no processo de alimentação da matriz de compactação. Os estudos realizados mostraram que as placas combustíveis com alta concentração de urânio a base da dispersão U3Si2-Al com 4,8 gU/cm3 fabricadas atenderam às especificações vigentes. Contudo, apesar da subjetividade da análise, a aparência da microestrutura obtida no núcleo das placas combustíveis a base da dispersão U3O8-Al com 3,2 gU/cm3 não foi considerada satisfatória devido à aparência da distribuição de vazios. O novo procedimento de fabricação foi aplicado na produção de placas combustíveis de dispersão U3Si2-Al com 4,8 gU/cm3 com urânio enriquecido, as quais foram utilizadas na montagem do elemento combustível parcial IEA-228 para ser testado quanto ao desempenho sob irradiação no reator IEA-R1 do IPEN-CNEN/SP. Esses novos combustíveis têm potencial para serem utilizados no novo Reator Multipropósito Brasileiro RMB. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

No description available.

An accidental criticality may occur in a solution of fissile material. Since the processing of nuclear materials in solution is prevalent throughout the fuel cycle, it would be judicious to have the capability to predict a possible hazard. In view of this concern, a computer simulation was performed of the Los Alamos accident of December 30, 1958, in which the actuation of an electric stirrer produced a sudden criticality. A complete equation of state for a liquid containing gas bubbles was coupled with the equations of energy, momentum, and space-independent point kinetics. Multiplication calculations, implemented with the Monte Carlo Code for Neutron and Photon Transport (MCNP), were performed on thermally expanding solution geometries, to generate a reactivity feedback representation. With the knowledge of the total energy produced in the accident, the maximum reciprocal period on which the power rose was computed.

No description available.