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On the redox reactivity of doped UO2 pellets: Influence of dopants on the H2O2 decomposition mechanism.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0002-0086-5536
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The reactivity of doped UO2 such as SIMFUEL, Y2O3 doped UO2 and Y2O3/Pd doped UO2 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. The rationale for the observed differences in dissolution yield is a difference in the ratio between the rates of the two possible reactions between H2O2 and the doped UO2. In this work we have studied the effect of doping on the two possible reactions, electron transfer and catalytic decomposition. The catalytic decomposition was studied by monitoring the hydroxyl radical production (the primary product) as a function of time. The redox reactivity of the doped pellets was studied by using MnO4- and IrCl62- as model oxidants, only capable of electron transfer reaction with the pellets. In addition, the activation energies for oxidation of UO2 and SIMFUEL by MnO4- were determined experimentally. The experiments show that the rate of catalytic decomposition of H2O2 varies by 30 % between the fastest and the slowest material. This is a negligible difference. The redox reactivity study shows that doping of UO2 influences the redox reactivity of the pellet. This is further illustrated by the observed activation energy difference for oxidation of UO2 and SIMFUEL by MnO4-. The redox reactivity study also shows that the sensitivity to dopants increases with decreasing reduction potential of the oxidant. These findings imply that the relative impact of radiolytic oxidants in oxidative dissolution of spent nuclear fuel must be reassessed taking the actual fuel composition into account.

National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-32920OAI: oai:DiVA.org:kth-32920DiVA: diva2:412902
Funder
EU, FP7, Seventh Framework Programme
Note
QS 2011Available from: 2011-04-27 Created: 2011-04-26 Last updated: 2011-05-11Bibliographically 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

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