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- El Jamal, Sawsan, et al.
(författare)
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Uranium nitride stability in aqueous solutions under anoxic and oxidizing conditions – Expected behaviour under repository conditions in comparison to alternative nuclear fuel materials
- 2023
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Ingår i: Journal of Nuclear Materials. - : Elsevier B.V.. - 0022-3115 .- 1873-4820. ; 578
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Tidskriftsartikel (refereegranskat)abstract
- Uranium nitride (UN) has good thermo-physical properties which makes it a promising fuel candidate for generation IV nuclear reactors. In addition to its performance as a nuclear fuel, it is important to elucidate every novel fuel material in terms of its stability in aqueous environments. This can be highly relevant under certain accident scenarios and also for the safety assessment of geological repositories for used nuclear fuel. The fuel matrix contains the fission products and heavier actinides formed under normal reactor operation. Upon dissolution of the fuel matrix, these highly radiotoxic constitiuents can be released. In this work UN has been studied under aqueous conditions similar to a geological repository for spent nuclear fuel. For UN, direct hydrolysis as well as oxidative dissolution induced by water radiolysis can lead to degradation of the fuel matrix. The latter process leads to formation of oxidative radiolysis products of which H2O2 has been shown to be the most important oxidant for other fuel materials. The experiments show that hydrolysis of UN in aqueous solutions and exposure to solutions containing H2O2 resulted in matrix dissolution. However, this oxidative dissolution induced by H2O2 is more prominent than hydrolysis in water with or without added HCO3−. The dissolution of UN was compared with other nuclear fuel materials (UC, UO2 and U3Si2) under the same conditions. The results show that UN is the second most reactive fuel material towards H2O2. However, the so-called dissolution yield is the lowest for UN. The rationale for the observed differences in reactivity are discussed.
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- 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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- Mishchenko, Yulia
(författare)
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Engineered microstructure composites as means of improving the oxidation resistance of uranium nitride
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Owing to its high uranium density and good thermophysical properties,uranium nitride (UN) fuel has been considered as a potential Accident TolerantFuel (ATF) candidate for use in Light Water Reactors (LWRs). However,the main disadvantage of UN is its low oxidation resistance in water/steamcontaining atmospheres at the operating temperatures of LWRs.The main objective of this thesis is to investigate a concept of engineeredmicrostructure composites as means of improving the response of UN to watersidecorrosion. The methodology for incorporating the corrosion resistantadditives in the form of metals, nitrides and oxides into the UN matrix hasbeen developed and tested. The additives were proposed to produce coated(no interaction with UN) or doped (incorporation of the additive into theUN bulk) grains, which will be able to shield the UN from the oxidising environmentand slow down the oxygen diffusion through the bulk. The UNcomposite pellets containing the selected additives were sintered using theSpark Plasma Sintering (SPS) technique. The resulting microstructures ofthe composite pellets were well characterised prior to subjecting some of theengineered microstructure representative samples to oxidation testing in airand steam containing environments.The obtained results indicate that the response to air and steam oxidationof the composite samples differs from that of pure UN. Moreover, a delay inthe oxidation onset was observed for the composite samples UN-20CrNpremixand UN-20ZrNpremix in steam and for UN-20CrNpremix pellet in air. Theimproved response to oxidation was accompanied by the formation of theternary oxides, an observation that could be applied to the screening processof the additive candidates for waterproofing of UN.
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- Mishchenko, Yulia, et al.
(författare)
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Potential accident tolerant fuel candidate : Investigation of physical properties of the ternary phase U2CrN3
- 2022
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 568
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, physical properties of the ternary phase U2CrN3 are evaluated experimentally and by modeling methods. High density pellets containing the ternary phase were prepared by spark plasma sintering (SPS). The microstructural and crystallographic analyses of the composite pellets were performed using scanning electron microscopy (SEM), standardised energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Evaluation of the mechanical properties was performed by nanoindentation test. The impact of temperature on lattice properties was evaluated using high temperature X-ray diffraction (XRD) coupled with modeling. Progressive change in the lattice parameters was obtained from room temperature (RT) to 673 K, and the result was used to calculate average linear thermal expansion coefficients, as well as an input for the density functional theory (DFT) modeling to reassess the degradation of the mechanical properties. The ab-initio calculation provides an initial assessment of electronic configuration of this ternary phase in a direct comparison with one of UN phase. For this goal, modeling was also employed to evaluate point defect formation energies and electronic charge distribution in the ternary phase. Results indicate that the U2CrN3 phase has similar mechanical properties to UN (Young's, bulk, shear moduli, hardness). No preferential crystallographic orientation was observed in the composite pellet. However, charge electron density distribution highlights the significant directionality of chemical bonds, which is in agreement with the anisotropy and non-linear behaviour of the obtained thermal expansion (α¯(aa) = 9.12 × 10−6/K, α¯(ab) = 5.81 × 10−6/K and α¯(ac) = 6.08 × 10−6/K). As a consequence, uranium was found to be more strongly bound in the ternary structure which may delay diffusion and vacancy formation, promising an acceptable performance as nuclear fuel.
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- Mishchenko, Yulia, 1983-, et al.
(författare)
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Thermophysical properties and oxidation behaviour of the U0.8Zr0.2N solid solution
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermophysical properties and oxidation behaviour of the composite pellet UN-20vol%ZrN were investigated experimentally and compared with the behaviour of the pure UN pellet. A compound of one phase, a solid solution of the average composition U0.8Zr0.2N, was obtained by Spark Plasma Sintering (SPS) of the powders UN and ZrN. Crystallographic and microstructural characterisation of the composite was performed using Scanning Electron Microscopy (SEM), standardised Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Nano hardness and Young’s modulus were also measured by the nanoindentation method. High Temperature X-ray diffraction (XRD) was applied to obtain the lattice expansion as a function of temperature (room temperature to 673 K). Thermogravimetric Analysis (TGA) was applied to evaluate oxidation behaviour in air. Results demonstrate that the fabrication method results in a matrix of solid solution with homogeneous composition averaged to U0.8Zr0.2N. The mechanical properties of such solution are uniform, with variation only due to the crystallographic orientation of the grains of the solution phase, similarly to pure UN. The obtained value for the average linear thermal expansion coefficient is = 7.94*10-6/K, which compares well to UN ( = 7.95*10-6/K) for the same temperature range. The degradation behaviour of the composite pellet UN-20vol%ZrN in air shows a lower oxidation onset temperature, compared to pure UN, with the final product of oxidation being mainly U3O8. Smaller crystallites in the product of corrosion of the composite pellet indicate that the mechanism of degradation of the solid solution phase U0.8Zr0.2N is accompanied by the formation of two distinct oxides and their interaction.
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