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Fabrication of Nuclear Fuel Pellet With a Mixture of Micro and Nano UO2 Powders And Investigation of Its Physical and Mechanical Properties

Karami, Mahdiyeh | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48296 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Otukesh, Mohammad; Taghizade, Mohammad; Roshanzamir, Manouchehr
  7. Abstract:
  8. Uranium dioxide (UO2) has been widely used as nuclear fuel in water cooled reactors since 1960s. UO¬2 is a refractory oxide with a melting point in excess of 28000C and therefore, requires high sintering temperature around 17000C in a hydrogen atmosphere for several hours. In the conventional method, the sintering is incorporated with indirect heating of green pellets in a refractory-type electrical resistance furnace. This furnace, however, uses a large number of expensive heating elements and refractory materials to achieve and maintain the high temperature for a long time. Moreover, it consumes much electrical energy and time. Since nanocrystalline materials contain a large fraction of atoms in grain boundaries, many of these interfaces provide high density of short diffusion paths. Therefore, it is expected that these materials show increased diffusivity compared to polycrystalline materials of the same composition and conventional particle size (of the order of microns). The consequence of such increased diffusivity is increased sinter ability of nano metric powders, which causes decrease in sintering temperature of these powders when compared to the same material with conventional particle size. Various authors agree that uranium diffusion at grain boundaries probably governs the two first sintering stages, of over stoichiometric uranium dioxide. One the other hand, The sintering rate of uranium dioxide UO2+x increases with deviation from the stoichiometry x. The objective of this research is to decrease the time and temperature of sintering while not significantly affecting the neutronic property of UO2. The concept is to incorporate different percent of tri uranium oxide (U3O8) and nano powder of UO2 into the UO2 pellets. In the first step nano powder of UO2 was produced by two different methods, hydrothermal and Sol-Gel. Secondly due to high demand for nano powder of UO2, the micrometric powder was converted using physical methods. The influence of different parameters was investigated, so that pellets with high density, more than 97% TD were achieved
  9. Keywords:
  10. Sintering ; Nanopowder ; Uranium Dioxide ; Uranium Dioxide Pellet ; Triuranium Octoxide ; Stoichiometry

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