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Reactivity of H2O2 towards different UO2-based materials: The relative impact of radiolysis products revisited
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0002-0086-5536
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-0663-0751
2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 434, no 1/3, 434-439 p.Article in journal (Refereed) Published
Abstract [en]

The reactivity of doped UO2 such as SIMFUEL towards H2O2 has been shown to be fairly similar to that of pure UO2. However, the oxidative dissolution yield, i.e. the ratio between the amount of dissolved uranium and the amount of consumed H2O2 is significantly lower for doped UO2. In this work we have studied the mechanistic difference between SIMFUEL and pure UO2. H2O2 can be catalytically decomposed on UO2 in competition with the redox process in which U(IV) is oxidized. The latter process leads to the dissolution of oxidized uranium. The first step in the catalytic decomposition is the formation of hydroxyl radicals. The presence of hydroxyl radicals was verified using Tris buffer as a radical scavenger. For both UO2 and SIMFUEL pellets, significant amounts of hydroxyl radicals were formed. The results also show that the difference in dissolution yield between the two materials can mainly be attributed to differences in the redox reactivity. Based on this, the rate constants for electron transfer were revised and the relative impact of the radiolytic oxidants in oxidative dissolution of UO2 and SIMFUEL pellets were calculated. The impact of H2O2 is shown to be slightly reduced.

Place, publisher, year, edition, pages
2013. Vol. 434, no 1/3, 434-439 p.
Keyword [en]
Radiation-Induced Dissolution, Waste-Disposal Conditions, Spent Nuclear-Fuel, Oxidative Dissolution, Uranium-Dioxide, Uo2, Hydrogen, Corrosion, Kinetics, Decomposition
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-32918DOI: 10.1016/j.jnucmat.2011.06.003ISI: 000315752000056Scopus ID: 2-s2.0-84882753951OAI: oai:DiVA.org:kth-32918DiVA: diva2:412884
Funder
EU, FP7, Seventh Framework Programme, FP7-212287
Note

 Updated from manuscript to article in journal. QC 20130322

Available from: 2011-04-27 Created: 2011-04-26 Last updated: 2017-12-11Bibliographically approved
In thesis
1. The effect of solid state inclusions on the reactivity of UO2: A kinetic and mechanistic study
Open this publication in new window or tab >>The effect of solid state inclusions on the reactivity of UO2: A kinetic and mechanistic study
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The release of radionuclides is a key process in the safety assessment of a deep geological repository for spent nuclear fuel. A large fraction of the release is assumed to be a consequence of dissolution of the fuel matrix, UO2. In this doctoral thesis, the kinetics and the mechanisms behind oxidative U(IV) dissolution were studied. The eects of solid phase inclusions mimicking the presence of fission products, and solutes mimicking expected groundwater components were also evaluated.

Palladium, as a model substance for noble metal particle (fission products) inclusions, was shown to catalyze surface oxidation of U(IV), as well as reduction of U(VI). The second order rate constant for the surface reduction of U(VI) by H2was found to be on the order of 10-6 m s-1 (diusion controlled). Under 40 bar H2, 1 wt.% Pd was sufficient to suppress oxidative U(IV) dissolution in 2mM H2O2 aqueous solution. During g γirradiation under 1 bar H2, 0.1 wt.% Pd were sufficient to completely suppress oxidative dissolution. Under inert conditions, where H2 is only produced radiolytically, complete inhibition is observed for 3 wt.% Pd.

The presence of Y2O3 as a model substance for trivalent fission products was found to decrease U(VI) dissolution significantly under inert, as well as reducing conditions. Based on kinetic data, it was shown that pure competition kinetics cannot explain the observed decrease. From experiments using pure oxidants it was shown that Y2O3 doping of UO2 decreases the redox reactivity. In addition, from experiments where hydroxyl radical formation from the catalytic decomposition of H2O2 was monitored, it could be concluded that doping has a minor influence on this process.

On the basis of numerical simulations, the H2 concentration necessary to suppressradiolytic H2O2 production was found to increase with an increase in dose rate or HCO-3 concentration. Furthermore, the steady state concentration of H2O2 was found to be inversely proportional to the H2 pressure, and proportional to the square root of the dose rate. Fe2 diers strongly from the total reaction volume, the actual dose rate should not be converted into a homogeneous dose rate in numerical simulations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xiv, 67 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:32
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-33070 (URN)978-91-7415-960-8 (ISBN)
Public defence
2011-05-27, K2, Teknikringen 28, KTH, Stockholm, 18:02 (English)
Opponent
Supervisors
Funder
EU, FP7, Seventh Framework Programme, 212287
Note
QC 20110511Available from: 2011-05-11 Created: 2011-04-27 Last updated: 2011-05-11Bibliographically approved
2. Reactions of aqueous radiolysis products with oxide surfaces: An experimental and DFT study
Open this publication in new window or tab >>Reactions of aqueous radiolysis products with oxide surfaces: An experimental and DFT study
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The reactions between aqueous radiolysis products and oxide surfaces are important in nuclear technology in many ways. In solid-liquid systems, they affect (and at the same time are dependent on) both the solution chemistry and the stability of materials under the influence of ionizing radiation. The stability of surface oxides is a factor that determines the longevity of the materials where such oxides are formed. Additionally, the aqueous radiolysis products are responsible for corrosion and erosion of the materials.

  In this study, the reactions between radiolysis products of water – mainly H2O2 and HO radicals – with metal, lanthanide and actinide oxides are investigated. For this, experimental and computational chemistry methods are employed. For the experimental study of these systems it was necessary to implement new methodologies especially for the study of the reactive species – the HO radicals. Similarly, the computational study also required the development of models and benchmarking of methods. The experiments combined with the computational chemistry studies produced valuable kinetic, energetic and mechanistic data.

  It is demonstrated here that the HO radicals are a primary product of the decomposition of H2O2. For all the materials, the catalytic decomposition of H2O2 consists first of molecular adsorption onto the surfaces of the oxides. This step is followed by the cleavage of the O-O bond in H2O2 to form HO radicals. The HO radicals are able to react further with the hydroxylated surfaces of the oxides to form water and a surface bound HO center. The dynamics of formation of HO vary widely for the different materials studied. These differences are also observed in the activation energies and kinetics for decomposition of H2O2. It is found further that the removal of HO from the system where H2O2 undergoes decomposition, by means of a scavenger, leads to the spontaneous formation of H2.

  The combined theoretical-experimental methodology led to mechanistic understanding of the reactivity of the oxide materials towards H2O2 and HO radicals. This reactivity can be expressed in terms of fundamental properties of the cations present in the oxides. Correlations were found between several properties of the metal cations present in the oxides and adsorption energies of H2O, adsorption energies of HO radicals and energy barriers for H2O2 decomposition. This knowledge can aid in improving materials and processes important for nuclear technological systems, catalysis, and energy storage, and also help to better understand geochemical processes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 121 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:12
Keyword
hydrogen peroxide, hydrogen production, metal, oxides, lanthanide, catalysis, density functional theory, surface, reactions, chemistry
National Category
Physical Chemistry Materials Chemistry Theoretical Chemistry
Research subject
SRA - Energy; SRA - Production
Identifiers
urn:nbn:se:kth:diva-119780 (URN)978-91-7501-683-2 (ISBN)
Public defence
2013-04-12, K2, Teknikringen 28, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
StandUpXPRES - Initiative for excellence in production research
Note

QC 20130322

Available from: 2013-03-22 Created: 2013-03-21 Last updated: 2013-03-22Bibliographically approved

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Lousada, Cláudio M.Jonsson, Mats

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