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| 2. |
- Obbard, E. G., et al.
(författare)
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Anisotropy in the thermal expansion of uranium silicide measured by neutron diffraction
- 2018
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 508, s. 516-520
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Tidskriftsartikel (refereegranskat)abstract
- In-situ neutron diffraction patterns were collected for a sample of as-cast U3Si2 during heating to 1600 degrees C. Anomalous changes were observed above 1000 degrees C, including the formation of a new diffraction peak not belonging to P4/mbm U3Si2, unequal changes in the peak intensities and onset of anisotropic lattice expansion. The large data-set enabled derivation of a function-fitted isotropic thermal expansion coefficient to high precision, in close agreement with previous dilatometry results but reducing linearly with temperature over the studied interval. Anisotropy in the instantaneous lattice thermal expansion corresponded to anomalies reported by White et al. (2015) at a similar temperature.
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| 3. |
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| 4. |
- Johnson, Kyle D., et al.
(författare)
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Fabrication and microstructural analysis of UN-U3Si2 composites for accident tolerant fuel applications
- 2016
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 477, s. 18-23
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Tidskriftsartikel (refereegranskat)abstract
- In this study, U3Si2 was synthesized via the use of arc-melting and mixed with UN powders, which together were sintered using the SPS method. The study revealed a number of interesting conclusions regarding the stability of the system - namely the formation of a probable but as yet unidentified ternary phase coupled with the reduction of the stoichiometry in the nitride phase - as well as some insights into the mechanics of the sintering process itself. By milling the silicide powders and reducing its particle size ratio compared to UN, it was possible to form a high density UN-U3Si2 composite, with desirable microstructural characteristics for accident tolerant fuel applications.
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| 5. |
- Johnson, Kyle D., et al.
(författare)
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Grain growth in uranium nitride prepared by spark plasma sintering
- 2018
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 503, s. 75-80
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Tidskriftsartikel (refereegranskat)abstract
- Uranium mononitride (UN) has long been considered a potential high density, high performance fuel candidate for light water reactor (LWR) and fast reactor (FR) applications. However, deployability of this fuel has been limited by the notable resistance to sintering and subsequent difficulty in producing a desirable microstructure, the high costs associated with N-15 enrichment, as well as the known proclivity to oxidation and interaction with steam. In this study, the stimulation of grain growth in UN pellets sintered using SPS has been investigated. The results reveal that by using SPS and controlling temperature, time, and holding pressure, grain growth can be stimulated and controlled to produce a material featuring both a desired porosity and grain size, at least within the range of interest for nuclear fuel candidates. Grain sizes up to 31 mm were obtained using temperatures of 1650 degrees C and hold times of 15 min. Evaluation by EBSD reveal grain rotation and coalescence as the dominant mechanism in grain growth, which is suppressed by the application of higher external pressure. Moreover, complete closure of the porosity of the material was observed at relative densities of 96% TD, resulting in a material with sufficient porosity to accommodate LWR burnup. These results indicate that a method exists for the economic fabrication of an N-15-bearing uranium mononitride fuel with favorable microstructural characteristics compatible with use in a light water-cooled nuclear reactor.
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| 6. |
- Lopes, Denise Adorno, et al.
(författare)
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Spark plasma sintering and microstructural analysis of pure and Mo doped U3Si2 pellets
- 2017
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Ingår i: Journal of Nuclear Materials. - : ELSEVIER SCIENCE BV. - 0022-3115 .- 1873-4820. ; 496, s. 234-241
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Tidskriftsartikel (refereegranskat)abstract
- U3Si2 has been considered as an alternative fuel for Light Water Reactors (LWRs) within the Accident Tolerant Fuels (ATF) initiative, begun after the Fukushima-Daiichi Nuclear accidents. Its main advantages are high thermal conductivity and high heavy metal density. Despite these benefits, U3Si2 presents an anisotropic crystallographic structure and low solubility of fission products, which can result in undesirable effects under irradiation conditions. In this paper, spark plasma sintering (SPS) of U3Si2 pellets is studied, with evaluation of the resulting microstructure. Additionally, exploiting the short sintering time in SPS, a molybdenum doped pellet was produced to investigate the early stages of the Mo-U3Si2 interaction, and analyze how this fission product is accommodated in the fuel matrix. The results show that pellets of U3Si2 with high density (>95% TD) can be obtained with SPS in the temperature range of 1200 degrees C-1300 degrees C. Moreover, the short time employed in this technique was found to generate a unique microstructure for this fuel, composed mainly of closed nano-pores (<1 mu m) and small average grain size (similar to 4.5 mu m). The addition of Mo (1.5 at%) demonstrated no solubility of Mo in the U3Si2 matrix. The interaction of this fission product with the fuel matrix at 1200 degrees C formed, in the early stages, the stoichiometric U2Mo3Si4 ternary as well as precipitates of free uranium with small quantities of dissolved Si and Mo at the front of the reaction.
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| 7. |
- Mishchenko, Yulia, et al.
(författare)
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Design and fabrication of UN composites : From first principles to pellet production
- 2021
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 553, s. 153047-
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Tidskriftsartikel (refereegranskat)abstract
- In this study the composite UN-AlN, UN-Cr, UN-CrN and UN-AlN-CrN pellets were fabricated, and the advanced microstructure with different modes of interaction between the phases was obtained. The dopants for this study were selected based on the results of the ab-initio modeling calculations, that identified the AlN phase as insoluble and CrN and Cr as soluble in the UN matrix. This method allowed to investigate the possibility of improving the corrosion resistance of UN by protecting the grain boundaries with insoluble AlN and by hindering the diffusion of oxygen through the bulk by adding soluble CrN and Cr. The UN powder was produced by hydriding-nitriding method and mixed with the AlN, CrN and Cr powders. High density (>90 %TD) composite pellets were sintered by Spark Plasma Sintering (SPS). The microstructure of the pellets was analysed using SEM coupled with EDS. The phase purity was determined by XRD. For the first time the presence of the ternary U2CrN3 phase was observed in the composite pellets containing Cr and CrN dopants. The results obtained in this study allowed to assess the methodology for fabrication of the UN composites with controlled microstructure.
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| 8. |
- Mishchenko, Yulia, et al.
(författare)
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Uranium nitride advanced fuel : an evaluation of the oxidation resistance of coated and doped grains
- 2021
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 556
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation behaviour of the composite UN-AlN, UN-Cr 2 N/CrN and UN-AlN-Cr 2 N/CrN pellets in air and anoxic steam under thermal transient conditions was investigated and compared with the pure UN pellet. The composite pellets were manufactured to contain the engineered microstructure of coated (the addition of matrix-insoluble AlN) and doped (the addition of matrix-soluble Cr 2 N/CrN) grains. The composite powders were produced by powder metallurgy and sintered into pellets using the SPS method. Sintered composite pellets were subjected to a thermal transient up to 1273 K in an STA-EGA (TGA-DSC-Gas-MS) system, followed by crystallographic characterization by XRD and morphological and elemental analysis by FEG-SEM. Improved oxidation behaviour in air compared to pure UN was demonstrated by the UN-Cr 2 N/CrN composite pellet. The formation of the ternary oxide UCrO 4 from the ternary (U 2 Cr)N 3 phase (doped grain) was observed, consistent with the delayed oxidation onset and slower reaction rates. In an anoxic steam environment UN-Cr 2 N/CrN exhibited a higher onset oxidation temperature relative to UN, although the reaction progressed faster than for UN sample. Composite UN-AlN pellet oxidised at a lower temperature in both air and steam, compared to pure UN, due to internal stresses in the fuel matrix. A mechanism for degradation of the composite materials is proposed and the influence of the individual phases on the oxidation behaviour of the composites is discussed.
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| 9. |
- Patnaik, Sobhan, et al.
(författare)
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Crystallographic characterization of U 2 CrN 3 : A neutron diffraction and transmission electron microscopy approach
- 2023
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 35
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Tidskriftsartikel (refereegranskat)abstract
- In this study, neutron diffraction and transmission electron microscopy (TEM) have been implemented to study the crystallographic structure of the ternary phase U2CrN3 from pellet to nano scale respectively. Recently microstructural evaluation of this ternary phase has been performed for the first time in pellet condition, overcoming the Cr evaporation issue during the conventional sintering process. In this work for the first time, the crystallographic structure of the ordered ternary U2CrN3 phase, stabilized in pellet condition, has been obtained by implementing neutron diffraction. For this study, pellets of the composite material UN with 20 vol% CrN were fabricated by powder metallurgy by mixing UN and CrN powders followed by Spark Plasma Sintering (SPS). TEM was used to investigate the nanoscale structure with a thin lamella of the order of 100–140 nm produced by focused ion beam (FIB). The neutron data revealed the phase composition of the pellet to be primarily 54(8) wt.% U2CrN3, in good agreement with the stoichiometry of starting reagents (UN and CrN powder) and metallographic analysis. Neutron data analysis confirms that all the crystallographic sites in U2CrN3 phase are fully occupied reinforcing the fully stoichiometric composition of this phase, however, the position of the N at the 4i site was found to be closer to the Cr than previously thought. TEM and selected area electron diffraction rendered nano-level information and revealed the presence of nano domains along grain boundaries of UN and U2CrN3, indicating a formation mechanism of the ternary phase, where the phase likely nucleates as nano domains in UN grains from migration of Cr.
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