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Resolving the H-2 effect on radiation induced dissolution of UO2-based spent nuclear fuel
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
2010 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 396, no 2-3, 163-169 p.Article in journal (Refereed) Published
Abstract [en]

In recent years, the impact of H-2 on alpha-radiation induced dissolution Of UO2-based spent nuclear fuel has been studied and debated extensively. Experimental results on the effect of H-2 on the concentration of H2O2 during alpha-radiolysis have been shown to disagree with numerical simulations. For this reason, the reaction scheme used in simulations of aqueous radiation chemistry has sometimes been questioned.

In this work, we have studied the impact of H-2 on the H2O2 concentration in alpha-irradiated aqueous solution using numerical simulations. The effects of H-2 pressure, alpha-dose rate and HCO3- concentration were investigated by performing systematic variations in these parameters. The simulations show that the discrepancy between the previously published experimental result and numerical simulations is due to the use of a homogeneous dose rate (the energy is assumed to be equally distributed in the whole volume). Taking the actual dose rate of the alpha-irradiated volume into account, the simulation is in perfect agreement with the experimental results. This shows that the H-2 effect is strongly alpha-dose rate dependent, and proves the reliability of the reaction scheme used in the simulations.

The simulations also show that H-2 influences the H2O2 concentration under alpha-radiolysis. The magnitude of the effect depends on the dose rate and the H-2 pressure as well as on the concentration of HCO3-. The impact of the radiolytic H-2 effect on the rate of alpha-radiation induced dissolution of spent nuclear fuel is discussed along with other (alpha- and gamma-) radiation induced processes capable of reducing the concentration of uranium in solution. The radiolytic H-2 effect is quantitatively compared to the previously presented noble metal catalyzed H-2 effect. This comparison shows that the noble metal catalyzed H-2 effect is far more efficient than the radiolytic H-2 effect. Reduction of U(VI) in solution due to low dose rate gamma-radiolysis in the presence of H-2 is proposed to be the cause of the H-2 effect observed in leaching experiments on alpha-doped UO2.

Place, publisher, year, edition, pages
2010. Vol. 396, no 2-3, 163-169 p.
Keyword [en]
Spent nuclear fuel, Dissolution, Radiolysis, H-2 effect, Simulation
National Category
Materials Engineering Subatomic Physics
URN: urn:nbn:se:kth:diva-19214DOI: 10.1016/j.jnucmat.2009.10.067ISI: 000274557100003ScopusID: 2-s2.0-73449094492OAI: diva2:337261
QC 20110210Available from: 2010-08-05 Created: 2010-08-05 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.
Trita-CHE-Report, ISSN 1654-1081 ; 2011:32
National Category
Inorganic Chemistry
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)
EU, FP7, Seventh Framework Programme, 212287
QC 20110511Available from: 2011-05-11 Created: 2011-04-27 Last updated: 2011-05-11Bibliographically approved

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