| 1. |
- Abrahamsson, Johanna, 1990, et al.
(author)
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A Raman spectroscopic approach to investigate the production of biodiesel from soybean oil using 1-alkyl-3-methylimidazolium ionic liquids with intermediate chain length
- 2015
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In: Applied Energy. - : Elsevier BV. - 1872-9118 .- 0306-2619. ; 154, s. 763-770
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Journal article (peer-reviewed)abstract
- We present a Raman spectroscopic study of the conversion of soybean oil into biodiesel by methanol using the lipase B enzyme pseudomonas cepacia as the bio-catalyst and 1-alkyl-3-methylimidazolium ionic liquids (CnC1Im) as co-solvents, where the alkyl chain length is varied from ethyl (n=2) to decyl (n=10). We have limited this study to a low reaction temperature, 40°C, and a low enzyme concentration to investigate the possibility of producing biodiesel with low energy and cost demands. We also demonstrate that Raman spectroscopy is a powerful and straightforward method to estimate the yield of the transesterification reaction, by analysis of the characteristic CO stretching mode found in the range ~1730-1750cm-1. Our results indicate that both reaction yield and reaction rate increase with the chain length, but also that for chains longer than decyl further improvements are marginal. We discuss these results from the viewpoint of local interactions and local structure of the investigated ionic liquids.
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| 2. |
- Bauhn, Lovisa, 1981, et al.
(author)
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The fate of hydroxyl radicals produced during H2O2 decomposition on a SIMFUEL surface in the presence of dissolved hydrogen
- 2018
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 507, s. 38-43
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Journal article (peer-reviewed)abstract
- Over geologic timescales hydrogen peroxide will be one of the most important radiolytic oxidants challenging the spent fuel integrity in a deep repository. Consequently, the reaction between hydrogen peroxide and different kinds of UO 2 based materials has been the subject of several studies over recent decades. Parts of these studies have investigated the effect of dissolved hydrogen on this reaction, as large amounts of hydrogen are expected to be produced by anoxic corrosion of iron in the deep repositories. In some of the studies hydrogen has been shown to offset the radiolysis-driven oxidative dissolution of the fuel despite the expected inertia of hydrogen at repository temperatures. However, the underlying mechanism is primarily based on the effect of the metallic particles contained in the spent fuel. One clue to the mechanistic understanding is whether or not a reaction takes place between dissolved hydrogen and hydroxyl radicals adsorbed to a fuel surface resulting from the decomposition of H 2 O 2 . In the study presented here this reaction could be confirmed in an autoclave system with SIMFUEL, a hydrogen peroxide spiked solution, and deuterium gas. The results show that the studied reaction does not only occur, but accounts for a substantial part of the hydrogen peroxide consumption in the system. Only a very minor part, 0.02%, of the total consumed hydrogen peroxide caused oxidative dissolution of the SIMFUEL. The conclusion is supported by quantitative measurements of HDO, dissolved U in solution and O 2 in the gas phase.
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| 3. |
- Bauhn, Lovisa, 1981, et al.
(author)
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The interaction of molecular hydrogen with α-radiolytic oxidants on a (U,Pu)O2 surface
- 2018
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 505, s. 54-61
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Journal article (peer-reviewed)abstract
- In order to assess the impact of α-radiolysis of water on the oxidative dissolution of spent fuel, an un-irradiated, annealed MOX fuel pellet with high content of Pu (∼24 wt%), and a specific α-activity of 4.96 GBq/g MOX , was leached in carbonate-containing solutions of low ionic strength. The high Pu content in the pellet stabilizes the (U,Pu)O 2 (s) matrix towards oxidative dissolution, whereas the α-decays emitted from the surface are expected to produce ∼3.6 × 10 −7 mol H 2 O 2 /day, contributing to the oxidative dissolution of the pellet. Two sets of leaching tests were conducted under different redox conditions: Ar gas atmosphere and deuterium gas atmosphere. A relatively slow increase of the U and Pu concentrations was observed in the Ar case, with U concentrations increasing from 1·10-6 M after 1 h to ∼7 × 10-5 M after 58 days. Leaching under an atmosphere starting at 1 MPa deuterium gas was undertaken in order to evaluate any effect of dissolved hydrogen on the radiolytic dissolution of the pellet, as well as to investigate any potential recombination of the α-radiolytic products with dissolved deuterium. For the latter purpose, isotopic analysis of the D/H content was carried out on solution samples taken during the leaching. Despite the continuous production of radiolytic oxidants, the concentrations of U and Pu remained quite constant at the level of ∼3 × 10-8 M during the first 30 days, i.e. as long as the deuterium pressure remained higher than 0.8 MPa. These data rule out any oxidative dissolution of the pellet during the first month. The un-irradiated MOX fuel does not contain metallic ε-particles, hence it is mainly the interaction of ra diolytic oxidants and dissolved deuterium with the surface of the mixed actinide oxide that causes the neutralization of the oxidants. This conclusion is supported by the steadily increasing levels of HDO measured in the leachate samples.
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| 4. |
- Hansson, Niklas, 1992, et al.
(author)
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Alpha dose rate calculations for UO2 based materials using stopping power models
- 2020
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In: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 22
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Journal article (peer-reviewed)abstract
- Accurate dose rate models for UO2 based materials in contact with water are important in the modeling of the radiolytically promoted dissolution of spent fuel. Dose rates of α-doped UO2 and un-irradiated MOX fuel were modelled using the ASTAR and SRIM stopping power databases. Dose rates were calculated as a function of distance from the active surface. Comparisons with common dose rate calculation models and the combined Bethe-Bloch and Lindhard–Scharff (LS) equation were performed. It was shown that the ASTAR and SRIM databases could more accurately simulate an α-spectrum compared to the Bethe-Bloch-LS equation. A comparison between the continuous slowing down approximation (CSDA) and the radial projection algorithm in the SRIM program was performed, and it was shown that CSDA overestimates the range of the α-particles by a few percent. This leads to an overestimation of the α-dose rate at distances close to the maximum range of the α-particle in water. A relationship between the average dose rate to specific α-activity ratio as a function of α-energy was obtained from the calculations, which can easily be implemented in alpha dose rate calculations of a UO2 based materials.
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| 5. |
- Hansson, Niklas, 1992, et al.
(author)
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Exploring H2-effects on radiation-induced oxidative dissolution of UO2-based spent nuclear fuel using numerical simulations
- 2023
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In: Radiation Physics and Chemistry. - : Elsevier BV. - 0969-806X .- 1879-0895. ; 210, s. 111055-
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Journal article (peer-reviewed)abstract
- Using a recently developed approach for numerical simulation of radiation-induced oxidative dissolution of spent nuclear fuel, we have explored the impact of three possible contributions to the inhibiting effect of molecular hydrogen. The three contributions are (1) effect on oxidant production in irradiated water, (2) reduction of oxidized uranium catalyzed by noble metal inclusions (fission products) and (3) reaction with surface-bound hydroxyl radicals preventing the oxidation of uranium. The simulations show that the first contribution is of fairly small importance while the second contribution can result in complete inhibition of the oxidative dissolution. This is well in line with previous work. Interestingly, the simulations imply that the third contribution, the reaction between H2 and the surface-bound hydroxyl radical formed upon reaction between the radiolysis product H2O2 and UO2, can account for the inhibition observed in systems where noble metal inclusions are not present. This is discussed in view of previously published experimental data.
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| 6. |
- Hansson, Niklas, 1992, et al.
(author)
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Geometrical aspects of alpha dose rates from UO2 based fuels
- 2022
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In: Radiation Physics and Chemistry. - : Elsevier BV. - 0969-806X .- 1879-0895. ; 199, s. 110336-
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Journal article (peer-reviewed)abstract
- Models for calculating dose rates of spherical particles as well as in fuel cracks are important for radionuclide source term estimations. Dose rates from UO2 based fuels were calculated for planar, spherical, and crack geometries. The escape probability for a-particles in spherical UO2 particles was derived as a continuous equation. The dose rate increased with increasing spherical radius due to the decreasing relative volume of the surrounding water layer. The model produced escape probabilities that were closely predicted by the theoretical derivation. It was shown that the dose rate in water filled fuel cracks with width smaller than the range of an alpha-particle led to dissolution rates that were lower per unit surface area.
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| 7. |
- Hansson, Niklas, 1992, et al.
(author)
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Influence of groundwater composition on the reductive precipitation of U(VI) on corroding iron foil surfaces
- 2023
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 577
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Journal article (peer-reviewed)abstract
- In order to assess the disposal of spent nuclear fuel in a deep geological nuclear waste repository, the interactions between U(VI) and corroded iron present in the canister material are of importance. It is important to correctly model the fate of the oxidatively dissolved uranium in order to correctly estimate radium releases from the canister in the long term. The release of radionuclides into the environment depends on the dissolution of the UO2 matrix which is dependent on the redox conditions at the fuel surface. The effect of metallic iron on the reduction of U(VI) was studied under anoxic conditions using synthetic groundwaters with different compositions, chosen to investigate the influence of calcium-uranyl-carbonato complexes on the thermodynamics and kinetics of U(VI) reduction on anoxically corroding iron. The corrosion products formed on the iron surface were investigated using SEM-EDS and XPS to identify elemental composition and oxidation states of uranium and iron on the surface. The iron foils efficiently reduced U(VI) to U(IV) causing its significant sorption and precipitation on the iron foil surfaces in the form of U(IV).
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| 8. |
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| 9. |
- Hansson, Niklas, 1992
(author)
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Oxidative dissolution of UO2 by α-radiolysis
- 2022
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Doctoral thesis (other academic/artistic)abstract
- To prevent the spread of radiotoxic nuclides in the environment, spent nuclear fuel generated by decades of nuclear power operation must be safely stored for at least 100 000 years. The KBS-3 method is a highly developed deep geological repository concept and is the first final repository design for high-level nuclear waste to be constructed. It contains a number of engineered barriers designed to prevent groundwater from coming into contact with the spent nuclear fuel. However, the consequences of groundwater coming into contact with the fuel must be considered when assessing the safety of this repository concept. After ~1000 years, the initially dominant γ-emitting elements have largely decayed, and the α-emitters dominate the radiation field. At the fuel-water interface, the fuel’s strong α-radiation field causes extensive radiolysis, creating locally oxidizing conditions. The oxidants formed can cause oxidation of the UO2 matrix from the U(IV) state to the U(VI) state, significantly increasing its solubility in the process. The water intrusion also leads to anoxic corrosion of the iron inserts, forming large amounts of H2 in the process. This process has been shown to protect nuclear fuel against oxidative dissolution. The oxidative dissolution of UO2-based materials has been experimentally studied and modelled in this work. Oxidation and dissolution of UO2 pellets were studied under an external irradiation source, in both Ar and H2 atmospheres. In the Ar atmosphere, the oxidation of UO2 was shown to take place through the incorporation of a significant U(V) oxidation state fraction. In the H2 atmosphere, the surface was protected during exposure to the external irradiation source against both surface oxidation and dissolution. Very low dissolution yields were found in the study of SIMFUEL, with H2 catalytically activated on the pellet surface, efficiently causing catalytic decomposition of H2O2 without leading to oxidative dissolution of the UO2 matrix. Highly Pu-doped MOX pellets showed a strong oxidative dissolution in the Ar atmosphere. This was somewhat mitigated in the D2 atmosphere. The modelled data were shown to accurately replicate the experimental results. Dissolved U(VI) was shown to be strongly reductively precipitated on corroding iron foils under anoxic conditions. This decreased the initially dissolved concentrations by three orders of magnitude over relatively short periods. This work furthers the understanding of oxidative dissolution of UO2-based materials under α-radiation fields and the effect of reducing agents present in the canister.
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| 10. |
- Hansson, Niklas, 1992
(author)
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The Influence of Hydrogen on the Radiolytic Oxidation of UO2
- 2020
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Licentiate thesis (other academic/artistic)abstract
- Spent nuclear fuel from the nuclear fuel cycle contains radiotoxic nuclides which must be safely stored for over 100 000 years. The Swedish final repository concept, KBS-3, is based on engineered and geological barriers that prevent the nuclear fuel from coming in contact with groundwater, which is the most credible vector to transport the radionuclides into the biosphere. In the safety assessment of a repository, the water intrusion scenario must therefore be investigated. The UO2 matrix contains the majority of the long-lived radiotoxic elements. As the U(IV) form is highly insoluble, the release of the radiotoxic nuclides is largely governed by oxidation of the UO2 matrix into the much more soluble U(VI) form. Oxidation can occur due to the formation of radiolytic oxidants through the ionization or excitation of water molecules in contact with fuel. Oxidation of UO2 pellets using external Am-241 sources was studied under conditions where the UO2 surface and the source were separated by 30 µm water. H2 was shown to suppress the surface oxidation as well as dissolution. This was shown by direct measurement of the surface oxidation state using XPS, as well as through concentration measurements in solution using mass spectrometry (ICP-MS). Oxidative dissolution of 10 and 24 wt% Pu-doped MOX pellets was also studied under Ar and D2 atmospheres. The D2 atmosphere suppressed the uranium dissolution. However, corrosion of the stainless-steel materials present in the autoclave system was also observed. A calculation model was also developed for calculating dose-rates from α-doped UO2 based material.
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| 11. |
- Hansson, Niklas, 1992, et al.
(author)
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XPS study of external α-radiolytic oxidation of UO2 in the presence of argon or hydrogen
- 2021
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 543
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Journal article (peer-reviewed)abstract
- UO2 pellets exposed to 1.85 MBq and 3.30 MBq Am-241 sources with a 30 μm separation in solution were studied under Ar and H2 atmospheres. The 1.85 MBq Am-241 source was shown to have a too low radiolytic production to cause any measurable change in UO2 surface oxidation state. However, a significant oxidation was observed after exposure to the 3.30 MBq source under Ar atmosphere through the production of U(V), as seen from the U4f7/2-peak deconvolution and valence band peak analysis. The H2 atmosphere was shown to suppress the oxidation of the UO2 pellet surface as well as the dissolution in both Milli-Q and NaHCO3 solutions. The dissolved uranium concentration was reduced by a factor 10 under H2 atmosphere as compared to the Ar atmosphere experiments.
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