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  • 1.
    Barreiro Fidalgo, Alexandre
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Experimental studies of radiation-induced dissolution of UO2: The effect of intrinsic solid phase properties and external factors2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Dissolution of the UO2 matrix is one of the potential routes for radionuclide release in a future deep geological repository for spent nuclear fuel. This doctoral thesis focuses on interfacial reactions of relevance in radiation-induced dissolution of UO2 and is divided in two parts:

    In the first part, we sought to explore the effects of solid phase composition:

    The impact of surface stoichiometry on the reactivity of UO2 towards aqueous radiolytic oxidants was studied. H2O2 reacts substantially faster with stoichiometric UO2 than with hyperstoichiometric UO2. In addition, the release of uranium from stoichiometric UO2 is lower than from hyperstoichiometric UO2. The behavior of stoichiometric powder changes with exposure to H2O2, approaching the behavior of hyperstoichiometric UO2 with the number of consecutive H2O2 additions.

    The impact of Gd-doping on the oxidative dissolution of UO2 in an aqueous system was investigated. A significant decrease in uranium dissolution and higher stability towards H2O2 for (U,Gd)O2 pellets compared to standard UO2 was found.

    In the second part, we sought to look at the effect of external factors:

    The surface reactivity of H2 and O2 was studied to understand the overall oxide surface reactivity of aqueous molecular radiolysis products. The results showed that hydrogen-abstracting radicals and H2O2 are formed in these systems. Identical experiments performed in aqueous systems containing UO2 powder showed that the simultaneous presence of H2 and O2 enhances the oxidative dissolution of UO2 compared to a system not containing H2.

    The effect of groundwater components such as bentonite and sulfide on the oxidative dissolution of UO2 was also explored. The presence of bentonite and sulfide in water could either delay or prevent in part the release of uranium to the environment. The Pd catalyzed H2 effect is more powerful than the sulfide effect. The poisoning of Pd catalyst is not observed under the conditions studied.

  • 2.
    Barreiro Fidalgo, Alexandre
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Radiation induced dissolution of (U, Gd)O-2 pellets in aqueous solution - A comparison to standard UO2 pellets2019In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 514, p. 216-223Article in journal (Refereed)
    Abstract [en]

    The behavior of spent nuclear fuel exposed to groundwater is crucial in the safety assessment of a deep geological repository for spent nuclear fuel. For this reason, leaching experiments on spent nuclear fuel as well as non-radioactive analogues have been conducted for several decades. Although the processes involved can be considered to be fairly well understood, there is a need for further experimental studies whenever new fuel types are introduced. Fuels with burnable absorbers are now in use but very little is known about their behavior under repository conditions. In this work, the impact of burnable absorbers doping (Gd, 3-8%wt.) on the oxidative dissolution of UO2 in an aqueous system was studied in H2O2 and gamma-irradiation induced dissolution experiments. The results showed a significant decrease in uranium dissolution and lower reactivity towards H2O2 for (U,Gd)O-2 pellets compared to standard UO2. The resulting decrease in the final oxidative dissolution yield was mainly attributed to decreased redox reactivity of the UO2-matrix upon doping. The results of the gamma radiation exposures display an even larger effect of Gd-doping. These findings indicate that other processes are involved in the radiation-induced dissolution of Gd-doped UO2 compared to pure UO2. 

  • 3.
    Barreiro Fidalgo, Alexandre
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Radiation induced dissolution of (U, Gd)O2 pellets in aqueous solution: A comparison to standard UO2 pelletsManuscript (preprint) (Other academic)
    Abstract [en]

    In this work, the impact of burnable absorbers doping (Gd, 3-8%wt.) on the oxidative dissolution of UO2 in an aqueous system was studied by H2O2 and γ-irradiation induced dissolution experiments. The results showed a significant decrease in uranium dissolution and higher stability towards H2O2 for (U,Gd)O2 pellets compared to standard UO2. The resulting decrease in the final oxidative dissolution yield was mainly attributed to decreased redox reactivity of the UO2-matrix upon doping. During gamma radiation induced experiments, the difference in uranium release was even more pronounced compared to H2O2 induced dissolution experiments.

  • 4.
    Barreiro Fidalgo, Alexandre
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Kumagai, Yuta
    Japan Atomic Energy Agency.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Impact of stoichiometry on the reactivity of UO2 towards radiolytic oxidantsManuscript (preprint) (Other academic)
  • 5.
    Barreiro Fidalgo, Alexandre
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    Kumagai, Yuta
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    Jonsson, Mats
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).
    The role of surface-bound hydroxyl radicals in the reaction between H2O2 and UO22018In: Journal of coordination chemistry (Print), ISSN 0095-8972, E-ISSN 1029-0389, Vol. 71, no 11-13, p. 1799-1807Article in journal (Refereed)
    Abstract [en]

    In this work, we have studied the reaction between H2O2 and UO2 with particular focus on the nature of the hydroxyl radical formed as an intermediate. Experiments were performed to study the kinetics of H2O2 consumption and uranium dissolution at different initial H2O2 concentrations. The results show that the consumption rates at a given H2O2 concentration are different depending on the initial H2O2 concentration. This is attributed to an alteration of the reactive interface, likely caused by blocking of surface sites by oxidized U/surface-bound hydroxyl radicals. The dissolution yield given by the amount of dissolved uranium divided by the amount of consumed hydrogen peroxide was used to compare the different cases. For all initial H2O2 concentrations, the dissolution yield increases with reaction time. The final dissolution yield decreases with increasing initial H2O2 concentration. This is expected from the mechanism of catalytic decomposition of H2O2 on oxide surfaces. As the experiments were performed in solutions containing 10mM H2O2 and a strong concentration dependence was observed in the 0.2-2.0mM H2O2 concentration range, we conclude that the intermediate hydroxyl radical is surface bound rather than free.

  • 6.
    Barreiro Fidalgo, Alexandre
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Sundin, Sara
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Effect of bentonite on radiation induced dissolution of UO2 in an aqueous system2014In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 447, no 1-3, p. 73-76Article in journal (Refereed)
    Abstract [en]

    In order to elucidate the impact of bentonite on the process of radiation induced oxidative dissolution of UO2 in an aqueous system, the dissolution of U(VI) and consumption of H2O2 over time has been studied. In addition, γ-irradiation experiments were performed to study a more relevant and complex system, serving as a comparison with the previously stated system. In both cases, the experiments revealed that the presence of bentonite in water could either delay or prevent in part the release of uranium to the environment. The cause is mainly attributed to the scavenging of radiolytic oxidants rather than to the adsorption of uranium onto bentonite.

  • 7.
    Fidalgo, Alexandre Barreiro
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Dahlgren, Björn
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Brinck, Tore
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Surface Reactions of H2O2, H-2, and O-2 in Aqueous Systems Containing ZrO22016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 3, p. 1609-1614Article in journal (Refereed)
    Abstract [en]

    In radiolysis of water, three molecular products are formed (H2O2, O-2, and H-2). It has previously been shown that aqueous hydrogen peroxide is catalytically decomposed on many oxide surfaces and that the decomposition proceeds via the formation of surface-bound hydroxyl radicals. In this work, we have investigated the behavior of aqueous H-2 and O-2 in contact with ZrO2. Experiments were carried out in an autoclave with high H2 pressure and low O-2 pressure (40 and 0.2 bar, respectively). In the experiments the concentration of H-abstracting radicals was monitored as a function of time using tris(hydroxymethyl)aminomethane (Tris) as scavenger and the subsequent formation of formaldehyde to probe radical formation. The plausible formation of H2O2 was also monitored in the experiments. In addition, density functional theory (employing the hybrid PBE0 functional) was used to search for reaction pathways. The results from the,experiments show that hydrogen-abstracting radicals: are formed in the aqueous H2O2-system in contact with solid ZrO2. Formation of H2O2 is also detected, and the time dependent production of hydrogen-abstracting radicals follows the time-dependent H2O2 concentration, strongly:indicating that the radicals are produced upon catalytic decomposition of H2O2. The DFT study implies that H2O2 formation proceeds via a pathway where HO2 is a key intermediate. It is interesting to note that all the stable molecular products from aqueous radiolysis are precursors of quite intriguing radical reactions at water/oxide interfaces.

  • 8.
    Fidalgo, Alexandre Barreiro
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Can H-2 enhance the oxidative dissolution of UO2?2016In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 477, p. 85-87Article in journal (Refereed)
  • 9. Kumagai, Y.
    et al.
    Barreiro Fidalgo, Alexandre
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Impact of Stoichiometry on the Mechanism and Kinetics of Oxidative Dissolution of UO 2 Induced by H 2 O 2 and γ-Irradiation2019In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 15, p. 9919-9925Article in journal (Refereed)
    Abstract [en]

    Radiation-induced oxidative dissolution of uranium dioxide (UO 2 ) is one of the most important chemical processes of U driven by redox reactions. We have examined the effect of UO 2 stoichiometry on the oxidative dissolution of UO 2 in aqueous sodium bicarbonate solution induced by hydrogen peroxide (H 2 O 2 ) and γ-ray irradiation. By comparing the reaction kinetics of H 2 O 2 between stoichiometric UO 2.0 and hyper-stoichiometric UO 2.3 , we observed a significant difference in reaction speed and U dissolution kinetics. The stoichiometric UO 2.0 reacted with H 2 O 2 much faster than the hyper-stoichiometric UO 2.3 . The U dissolution from UO 2.0 was initially much lower than that from UO 2.3 but gradually increased as the oxidation by H 2 O 2 proceeded. Increase in the initial H 2 O 2 concentration caused decrease in the U dissolution yield with respect to the H 2 O 2 consumption both for UO 2.0 and UO 2.3 . This decrease in the U dissolution yield is attributed to the catalytic decomposition of H 2 O 2 on the surface of UO 2 . The γ-ray irradiation induced the U dissolution that is analogous to the kinetics by the exposure to a low concentration (2 × 10 -4 mol dm -3 ) of H 2 O 2 . The exposure to higher H 2 O 2 concentrations caused lower U dissolution and resulted in deviation from the U dissolution behavior by γ-ray irradiation.

  • 10.
    Yang, Miao
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Barreiro Fidalgo, Alexandre
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Nilsson, Sara
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Inhibition of radiation induced dissolution of UO2 by sulfide: a comparision with the hydrogen effect2011Manuscript (preprint) (Other academic)
  • 11.
    Yang, Miao
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Barreiro Fidalgo, Alexandre
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Sundin, Sara
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Inhibition of radiation induced dissolution of UO2 by sulfide-A comparison with the hydrogen effect2013In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 434, no 1-3, p. 38-42Article in journal (Refereed)
    Abstract [en]

    In this work we have studied the influence of H2S on radiation induced dissolution of spent nuclear fuel using simple model systems. The reaction between H2O2 and H2S/HS- has been studied experimentally as well as the effect of H2S/HS - on γ-radiation induced dissolution of a UO2 pellet. The experiments clearly show that the reaction of H2O 2 and H2S/HS- is fairly rapid and that H 2O2 and H2S/HS- stoichiometry is favorable for inhibition. Radiolysis experiments show that H2S/ HS- can effectively protect UO2 from oxidative dissolution. The effect depends on sulfide concentration in combination with dose rate. Autoclave experiments were also conducted to study the role of H 2S/HS- in the reduction of U(VI) in the presence and absence of H2 and Pd particles in anoxic aqueous solution. The aqueous solutions were pressurized with H2 or N2 and two different concentrations of H2S/HS- were used in the presence and absence of Pd. No catalytic effect of Pd on the U(VI) reduction by H2S/HS- could be found in N2 atmosphere. U(VI) reduction was found to be proportional to H2S/HS- concentration in H2 and N2 atmosphere. It is clearly shown the Pd catalyzed H2 effect is more powerful than the effect of H2S/HS-. H2S/HS- poisoning of the Pd catalyst is not observed under the present conditions.

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