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Effects of HCO3- and ionic strength on the oxidation and dissolution of UO2
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

The kinetics for radiation induced dissolution of spent nuclear fuel is a key issue in the safety assessment of a future deep repository. Spent nuclear fuel mainly consists of UO2 and therefore the release of radionuclides (fission products and actinides) is assumed to be governed by the oxidation and subsequent dissolution of the UO2 matrix. The process is influenced by the dose rate in the surrounding groundwater (a function of fuel age and burn up) and on the groundwater composition. In this licentiate thesis the effects of HCO3- (a strong complexing agent for UO22+) and ionic strength on the kinetics of UO2 oxidation and dissolution of oxidized UO2 have been studied experimentally.

The experiments were performed using aqueous UO2 particle suspensions where the oxidant concentration was monitored as a function of reaction time. These reaction systems frequently display first order kinetics. Second order rate constants were obtained by varying the solid UO2 surface area to solution volume ratio and plotting the resulting pseudo first order rate constants against the surface area to solution volume ratio. The oxidants used were H2O2 (the most important oxidant under deep repository conditions), MnO4- and IrCl62-. The kinetics was studied as a function of HCO3- concentration and ionic strength (using NaCl and Na2SO4 as electrolytes).

The rate constant for the reaction between H2O2 and UO2 was found to increase linearly with the HCO3- concentration in the range 0-1 mM. Above 1 mM the rate constant is independent of the HCO3- concentration. The HCO3- concentration independent rate constant is interpreted as being the true rate constant for oxidation of UO2 by H2O2 [(4.4 ± 0.3) x 10-6 m min-1] while the HCO3- concentration dependent rate constant is used to estimate the rate constant for HCO3- facilitated dissolution of UO22+ (oxidized UO2) [(8.8 ± 0.5) x 10-3 m min-1]. From experiments performed in suspensions free from HCO3- the rate constant for dissolution of UO22+ was also determined [(7 ± 1) x 10-8 mol m-2 s-1]. These rate constants are of significant importance for simulation of spent nuclear fuel dissolution.

The rate constant for the oxidation of UO2 by H2O2 (the HCO3- concentration independent rate constant) was found to be independent of ionic strength. However, the rate constant for dissolution of oxidized UO2 displayed ionic strength dependence, namely it increases with increasing ionic strength.

The HCO3- concentration and ionic strength dependence for the anionic oxidants is more complex since also the electron transfer process is expected to be ionic strength dependent. Furthermore, the kinetics for the anionic oxidants is more pH sensitive. For both MnO4- and IrCl62- the rate constant for the reaction with UO2 was found to be diffusion controlled at higher HCO3- concentrations (~0.2 M). Both oxidants also displayed ionic strength dependence even though the HCO3- independent reaction could not be studied exclusively.

Based on changes in reaction order from first to zeroth order kinetics (which occurs when the UO2 surface is completely oxidized) in HCO3- deficient systems the oxidation site density of the UO2 powder was determined. H2O2 and IrCl62- were used in these experiments giving similar results [(2.1 ± 0.1) x 10-4 and (2.7 ± 0.5) x 10-4 mol m-2, respectively].

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 32 p.
Series
Trita-KKE, ISSN 0349-6465 ; 0603
Keyword [en]
UO2, H2O2, spent nuclear fuel, HCO3-, ionic strength, oxidation, dissolution, surface site density.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4172ISBN: 91-7178-484-5 (print)OAI: oai:DiVA.org:kth-4172DiVA: diva2:11055
Presentation
2006-11-24, Sal H1, KTH, Teknikringen 33, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101116Available from: 2006-11-17 Created: 2006-11-17 Last updated: 2010-11-16Bibliographically approved
List of papers
1. Effects of HCO3- on the kinetics of UO2 oxidation by H2O2
Open this publication in new window or tab >>Effects of HCO3- on the kinetics of UO2 oxidation by H2O2
2006 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 358, no 2-3, 202-208 p.Article in journal (Refereed) Published
Abstract [en]

The effect of HCO3- on the kinetics of UO2 oxidation by H2O2 in aqueous solution has been studied using powder suspensions where the concentration of H2O2 was monitored as a function of time. By varying the UO2 surface to solution volume ratio second order rate constants were obtained for HCO3- concentrations ranging from 0 to 100 mM. The second order rate constant increases linearly with HCO3- concentration from 0 to approximately 1 mM. Above 1 mM HCO3- the rate constant is 4.4 × 10-6 m min-1 independent of [HCO3-]. This indicates that the kinetics of the reaction depends on both oxidation and dissolution below 1 mM HCO3- while at higher concentrations it is solely governed by oxidation. Hence, the rate constant obtained at HCO3- concentrations above 1 mM is the true rate constant for oxidation of UO2 by H2O2. The results also imply that the reaction between HCO3- and oxidized UO2 on the UO2 surface (i.e. HCO3- facilitated dissolution) is limited by diffusion (ca 10-3 m min-1 in the present system). Furthermore, the experimental results were used to estimate the oxidation site density of the powder used (126 sites nm-1) and the rate constant for dissolution of UO22 + from the UO2 surface (7 × 10-8 mol m-2 s-1).

Keyword
F0700, U0200, W0200
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-6360 (URN)10.1016/j.jnucmat.2006.07.008 (DOI)000242254200012 ()2-s2.0-33750371503 (Scopus ID)
Note
QC 20100910Available from: 2006-11-17 Created: 2006-11-17 Last updated: 2017-12-14Bibliographically approved
2. Effects of ionic strength on the kinetics for UO2 oxidation
Open this publication in new window or tab >>Effects of ionic strength on the kinetics for UO2 oxidation
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 373, no 1-3, 190-193 p.Article in journal (Refereed) Published
Abstract [en]

The effect of ionic strength on the kinetics of UO2 oxidation by H2O2 in aqueous solution has been studied using powder suspensions, where the concentration of H2O2 was monitored as a function of time. Experiments were performed at 0 and 10 mM HCO3-. NaCl and Na2SO4 were used to adjust the ionic strength. At 0 mM HCO3- (where the kinetics is influenced by both oxidation and dissolution) the rate constant for the reaction increases with increasing ionic strength while at 10 mM HCO3- (where the kinetics is independent of the dissolution of oxidized UO2) the rate constant is virtually independent of ionic strength. This implies that dissolution of oxidized UO2 in the absence of HCO3- is ionic strength dependent. As expected, the reaction between H2O2 and UO2 is not affected by ionic strength since H2O2 has no charge. This finding also implies that peroxymonocarbonate (HCO4-) cannot be an important oxidant under the present conditions.

Keyword
Dissolution, Ionic strength, Oxidation, Rate constants, Reaction kinetics, Suspensions (fluids)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-6361 (URN)10.1016/j.jnucmat.2007.05.043 (DOI)000253059100027 ()2-s2.0-37549065200 (Scopus ID)
Note
QC 20101116. Uppdaterad från Submitted till Published (20101116)Available from: 2006-11-17 Created: 2006-11-17 Last updated: 2017-12-14Bibliographically approved
3. UO2 oxidation site densities determined by one- and two-electron oxidants
Open this publication in new window or tab >>UO2 oxidation site densities determined by one- and two-electron oxidants
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 373, no 1-3, 186-189 p.Article in journal (Refereed) Published
Abstract [en]

The oxidation site density (number of oxidation sites per m2) for UO2 powder was determined by measuring the amount of oxidant needed to fully oxidize the surface (denoted the critical oxidant conversion). The point where the surface becomes fully oxidized is identified by a change in reaction order from first to zeroth order in HC O3- free systems. At the critical oxidant conversion the kinetics of the reaction becomes completely governed by dissolution of oxidized UO2. The oxidants used in this study are H2O2 (two-electron oxidant) and IrC l62 - (one-electron oxidant). The oxidation site densities determined using the two different oxidants are (2.1 ± 0.1) × 10-4 and (2.7 ± 0.5) × 10-4 mol m-2, respectively, expressed in two electron equivalents. The fairly good agreement between the two oxidants implies that the methodology used indeed gives a reasonable measure of the oxidation site density. In addition, oxidation site densities for different size fractions of UO2 powder were determined. The results are discussed in terms of surface roughness.

Keyword
Density of gases, Dissolution, Oxidants, Powder metals, Surface chemistry, Surface roughness
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-6362 (URN)10.1016/j.jnucmat.2007.05.042 (DOI)000253059100026 ()2-s2.0-37549015664 (Scopus ID)
Note
QC 20101005. Uppdaterad från Submitted till Published (20101005).Available from: 2006-11-17 Created: 2006-11-17 Last updated: 2017-12-14Bibliographically approved
4. Effects of HCO3- and ionic strength on the kinetics of UO2 oxidation by anionic oxidants
Open this publication in new window or tab >>Effects of HCO3- and ionic strength on the kinetics of UO2 oxidation by anionic oxidants
(English)Manuscript (Other academic)
Identifiers
urn:nbn:se:kth:diva-6363 (URN)
Note
QC 20101116Available from: 2006-11-17 Created: 2006-11-17 Last updated: 2010-11-16Bibliographically approved

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