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The effect of solid state inclusions on the reactivity of UO2: A kinetic and mechanistic study
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
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: urn:nbn:se:kth:diva-33070ISBN: 978-91-7415-960-8OAI: oai:DiVA.org:kth-33070DiVA: diva2:413181
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
List of papers
1. On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel
Open this publication in new window or tab >>On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 378, no 1, 55-59 p.Article in journal (Refereed) Published
Abstract [en]

Radiation induced oxidative dissolution of UO2 is a key process for the safety assessment of future geological repositories for spent nuclear fuel. This process is expected to govern the rate of radionuclide release to the biosphere. In this work, we have studied the catalytic effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. The experimental studies were performed using UO2 pellets containing 0%, 0.1%, 1% and 3% Pd as a model for spent nuclear fuel. H2O2 was used as a model for radiolytical oxidants (previous studies have shown that H2O2 is the most important oxidant in such systems). The pellets were immersed in aqueous solution containing H2O2 and HCO3- and the consumption of H2O2 and the dissolution of uranium were analyzed as a function of H2 pressure (0-40 bar). The noble metal inclusions were found to catalyze oxidation of UO2 as well as reduction of surface bound oxidized UO2 by H2. In both cases the rate of the process increases with increasing Pd content. The reduction process was found to be close to diffusion controlled. This process can fully account for the inhibiting effect of H2 observed in several studies on spent nuclear fuel dissolution.

Keyword
Fission products, Metals, Nuclear fuels, Precious metals, Radiation, Radiation effects
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8391 (URN)10.1016/j.jnucmat.2008.04.018 (DOI)000258553500010 ()2-s2.0-47549110547 (ScopusID)
Note
QC 20100924. Uppdaterad från Submitted till Published (20100924).Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved
2. Hinhibition of radiation induced dissolution of spent nuclear fuel
Open this publication in new window or tab >>Hinhibition of radiation induced dissolution of spent nuclear fuel
2009 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 383, no 3, 226-230 p.Article in journal (Refereed) Published
Abstract [en]

In order to elucidate the effect of noble metal clusters in spent nuclear fuel on the kinetics of radiation induced spent fuel dissolution we have used I'd particle doped UO2 pellets. The catalytic effect of Pd particles on the kinetics of radiation induced dissolution Of UO2 during gamma-irradiation in HCO3- containing solutions purged with N-2 and H-2 was studied in this work. Four pellets with Pd concentrations of 0%, 0.1%, 1% and 3% were produced to mimic spent nuclear fuel. The pellets were placed in 10 mM HCO3- aqueous solutions and gamma-irradiated, and the dissolution of UO22+ was measured spectrophotometrically as a function of time. Under N-2 atmosphere, 3% I'd prevent the dissolution of uranium by reduction with the radiolytically produced H-2, while the other pellets show a rate of dissolution of around 1.6 x 10(-9) mol m(-2) s(-1). Under H-2 atmosphere already 0.1% Pd effectively prevents the dissolution of uranium, while the rate of dissolution for the pellet without Pd is 1.4 x 10(-9) mol m(-2) s(-1). It is also shown in experiments without radiation in aqueous solutions containing H2O2 and O-2 that epsilon-particles catalyze the oxidation of the UO2 matrix by these molecular oxidants, and that the kinetics of the catalyzed reactions is close to diffusion controlled.

Keyword
WASTE-DISPOSAL CONDITIONS, OXIDATIVE DISSOLUTION, AQUEOUS-SOLUTION, URANIUM-DIOXIDE, UO2, KINETICS, H2O2, REDUCTION, CORROSION, MECHANISM
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-9369 (URN)10.1016/j.jnucmat.2008.09.021 (DOI)000261905600004 ()2-s2.0-56349130830 (ScopusID)
Note
QC 20100908. Uppdaterad från submitted till published (20100908).Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2011-05-11Bibliographically approved
3. Factors influencing the rate of radiation-induced dissolution of spent nuclear fuel
Open this publication in new window or tab >>Factors influencing the rate of radiation-induced dissolution of spent nuclear fuel
2009 (English)In: Research on chemical intermediates (Print), ISSN 0922-6168, E-ISSN 1568-5675, Vol. 35, no 4, 465-478 p.Article in journal (Refereed) Published
Abstract [en]

Several countries plan to store spent nuclear fuel in deep geological repositories. Accurate prediction of the spent fuel dissolution rate is a key issue in the safety assessment of a future deep repository. A reliable quantitative model for radiation-induced spent fuel dissolution must be based on an accurate description of the dose distribution around the spent fuel and fundamental knowledge about the elementary processes involved. In this paper, we discuss factors influencing the rate of radiation-induced dissolution of spent nuclear fuel, focusing on solutes (H-2, HCO3 (-), Fe(II) and organic substances affecting the H2O2 concentration and factors influencing the reactivity of the fuel surface towards H2O2. Taking these factors into account, we have also simulated dissolution of spent nuclear fuel under realistic deep repository conditions.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-18579 (URN)10.1007/s11164-009-0048-2 (DOI)000267709500013 ()2-s2.0-77952554243 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-05-11Bibliographically approved
4. Can redox sensitive radionuclides be immobilized on the surface of spent nuclear fuel? - A model study on the reduction of Se(IV)(aq) on Pd-doped UO2 under H-2 atmosphere
Open this publication in new window or tab >>Can redox sensitive radionuclides be immobilized on the surface of spent nuclear fuel? - A model study on the reduction of Se(IV)(aq) on Pd-doped UO2 under H-2 atmosphere
2009 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 392, no 3, 505-509 p.Article in journal (Refereed) Published
Abstract [en]

Spent nuclear fuel contains noble metal particles composed of fission products (Pd, Mo, Ru, Tc, Rh and Te, often referred to as ε-particles). Studies have shown that these particles play a major role in catalyzing oxidative dissolution as well as H2 reduction of the oxidized UO2 fuel matrix, depending on the conditions. Thus it is possible that these particles also could have a major impact on the state of other redox sensitive radionuclides (such as the long lived fission product 79Se) present in spent nuclear fuel. In this study, Pd-doped UO2 pellets are used to simulate noble metal particles inclusions in spent nuclear fuel and the effect on dissolved selenium in the form of selenite (250 μM selenite) in simulated ground water solution (10 mM NaCl, 10 mM NaHCO3) at 1 and 10 bar hydrogen pressure. The selenite was found to be reduced to elemental Se, forming colloidal particles. At hydrogen pressures of 10 bar, the rate of selenite reduction was found to be linearly correlated to the fraction of Pd in the UO2 pellets. No selenium was detected on the surface of the pellets. For the lowest Pd loading (0.1% Pd) the selenite reduction does not appear to proceed to completion indicating that the surface becomes less active.

Identifiers
urn:nbn:se:kth:diva-18772 (URN)10.1016/j.jnucmat.2009.04.019 (DOI)000269963500019 ()2-s2.0-67651005007 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-05-11Bibliographically approved
5. Resolving the H-2 effect on radiation induced dissolution of UO2-based spent nuclear fuel
Open this publication in new window or tab >>Resolving the H-2 effect on radiation induced dissolution of UO2-based spent nuclear fuel
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.

Keyword
Spent nuclear fuel, Dissolution, Radiolysis, H-2 effect, Simulation
National Category
Materials Engineering Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-19214 (URN)10.1016/j.jnucmat.2009.10.067 (DOI)000274557100003 ()2-s2.0-73449094492 (ScopusID)
Note
QC 20110210Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-05-11Bibliographically approved
6. The effect of Y2O3 on the dynamics of oxidative dissolution of UO2
Open this publication in new window or tab >>The effect of Y2O3 on the dynamics of oxidative dissolution of UO2
2010 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 407, no 3, 195-199 p.Article in journal (Refereed) Published
Abstract [en]

In this work, we have studied the impact of Y2O3 on the kinetics of oxidative dissolution of UO2 and the consumption of H2O2. The second order kinetics of catalytic consumption of H2O2 on Y2O3 was investigated in aqueous Y2O3 powder suspensions by varying the solid surface area to solution volume ratio. The resulting second order rate constant is 10(-8) m s(-1), which is of the same magnitude as for the reaction between H2O2 and UO2. Powder experiments with mixtures of UO2 and Y2O3 show that Y2O3 has no effect on the oxidative dissolution of UO2, whereas the consumption of H2O2 seems to be slightly slower in the presence of Y2O3 and H-2 respectively. UO2 pellets with solid inclusions of Y2O3 show a decrease in oxidative dissolution by a factor of 3.3 and 5.3 under inert and hydrogen atmosphere, respectively. The rate of H2O2 consumption is similar for all cases and is well in line with kinetic data from powder experiments. The effects of H-2 and Y2O3 on the oxidative dissolution of UO2 under gamma irradiation are similar to those found in experiments with H2O2. No significant difference in dissolution between inert and reducing atmosphere can be observed for pure UO2.

Keyword
SPENT NUCLEAR-FUEL, RADIATION-INDUCED DISSOLUTION, WASTE-DISPOSAL CONDITIONS, FISSION-PRODUCTS, URANIUM-DIOXIDE, RADIOLYSIS PRODUCTS, AQUEOUS-SOLUTION, HYDROGEN, KINETICS, SOLUBILITY
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-29530 (URN)10.1016/j.jnucmat.2010.10.014 (DOI)000285859200009 ()2-s2.0-78649484630 (ScopusID)
Note
QC 20110207Available from: 2011-02-07 Created: 2011-02-07 Last updated: 2011-05-11Bibliographically approved
7. Reactivity of H2O2 towards different UO2-based materials: The relative impact of radiolysis products revisited
Open this publication in new window or tab >>Reactivity of H2O2 towards different UO2-based materials: The relative impact of radiolysis products revisited
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.

Keyword
Radiation-Induced Dissolution, Waste-Disposal Conditions, Spent Nuclear-Fuel, Oxidative Dissolution, Uranium-Dioxide, Uo2, Hydrogen, Corrosion, Kinetics, Decomposition
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-32918 (URN)10.1016/j.jnucmat.2011.06.003 (DOI)000315752000056 ()2-s2.0-84882753951 (ScopusID)
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: 2013-04-04Bibliographically approved
8. On the redox reactivity of doped UO2 pellets: Influence of dopants on the H2O2 decomposition mechanism.
Open this publication in new window or tab >>On the redox reactivity of doped UO2 pellets: Influence of dopants on the H2O2 decomposition mechanism.
(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:nbn:se:kth:diva-32920 (URN)
Funder
EU, FP7, Seventh Framework Programme
Note
QS 2011Available from: 2011-04-27 Created: 2011-04-26 Last updated: 2011-05-11Bibliographically approved
9. An electrochemical and kinetic study of oxidative dissolution of Y2O3-doped UO2
Open this publication in new window or tab >>An electrochemical and kinetic study of oxidative dissolution of Y2O3-doped UO2
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-32874 (URN)
Note
QS 2011Available from: 2011-04-27 Created: 2011-04-26 Last updated: 2012-04-02Bibliographically approved

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