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Radiation Induced Oxidative Dissolution of UO2
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [sv]

Sammanfattning

Denna doktorsavhandling berör oxidativ upplösning av UO2 (som modellsubstans för utbränt kärnbränsle) orsakad av radiolys av vatten samt effekten av H2 på denna denna process.

Hastighetskonstanter för oxidation av UO2-pulver med olika oxidanter såsom H2O2 har bestämts experimentellt. Logaritmen för andra ordningens hastighetskonstant, ln k, för oxidation av UO2 uppvisar ett linjärt beroende av oxidantens en-elektronreduktionspotential, E0. Detta indikerar att det hastighetsbestämmande steget för oxidation av UO2 är en en-elektronöverföring. Oxidationshastigheten för reaktionen mellan UO2 och OH• samt CO3•- är diffusionskontrollerad, k = 10-3 m/min.

Effekten av karbonat på oxidationskinetiken har studerats där H2O2 har adderats till suspensioner innehållande UO2-pulver. Andra ordningens hastighetskonstant för den rena oxidationsreaktionen mellan UO2 och H2O2 har bestämts till 4.4 x 10 -6 m/min. Den karbonatberoende upplösningshastigheten har visat sig vara diffusionskontrollerad. Hastighetskonstanten för upplösning av UO22+ i vatten fritt från karbonat är (7 +- 1) x 10-8 mol m-2 s-1.

Den relativa effektiviteten (per elektron) för en- respektive två-elektronoxidanter som orsakar upplösning av UO2 har studerats. Utbytena för upplösning med en-elektronoxidanter har ett starkt koncentrationsberoende m.a.p. oxidanten (särskilt vid låga konc.).

Reduktion av UO22+ i karbonatlösningar med H2 (p = 40 bar) vid olika temperaturer har studerats som en funktion av reaktionstiden. Hastighetskonstanten och aktiveringsenergin har bestämts, k298 = 3.6 x 10-9 M-1 s-1 och Ea = 130 +- 24 kJ/mol. Reduktionen av UO22+ till UO2 sker i frånvaro av en katalysator.

Den relativa inverkan av radiolysprodukter på oxidativ upplösning av UO2 har klargjorts. Mängden upplöst UO22+ mättes som en funktion av tiden i gasmättade gamma-bestrålade karbonatlösningar innehållande en UO2-kuts. Den simulerade upplösningshastigheten beräknades utifrån ytarea, oxidantkoncentrationer och hastighetskonstanter erhållna från det linjära sambandet (se ovan). De simulerade hastigheterna jämfördes med de experimentellt bestämda upplösningshastigeherna och korrelation är förhållandevis bra. Därmed kan inverkan av varje enskild oxidant i det undersökta systemet uppskattas. Endast de molekylära oxidanterna är av vikt i system som är relevanta för säkerhetsanalyser av ett djupförvar.

Abstract [en]

This doctoral thesis is focused on oxidative dissolution of UO2 (as a model for spent nuclear fuel) induced by radiolysis of water and the effect of H2 on this process.

Rate constants for oxidation of UO2 powder by different oxidants such as H2O2 in aqueous suspensions have been experimentally determined. The logarithm of the second order rate constant, ln k, for UO2 oxidation appears to be linearly dependent on the one-electron reduction potential, E0, of the oxidant. This indicates that the rate limiting step in oxidation of UO2 is one-electron transfer. The rate of UO2 oxidation by OH and CO3•- is estimated to be diffusion controlled, k = 10-3 m/min.

The effect of carbonate on the oxidation kinetics using H2O2 in UO2 powder suspensions has been studied. The second order rate constant for the pure oxidation reaction between UO2 and H2O2 was determined to 4.4 x 10-6 m/min. The carbonate facilitated dissolution rate is found to be limited by diffusion and the rate constant for non-carbonate mediated UO22+ dissolution was determined to (7 ± 1) x 10-8 mol m-2 s-1.

Furthermore, the relative efficiency (per electron) of one- and two-electron oxidants in causing dissolution of UO2 has been studied. The dissolution yields of one-electron oxidants are strongly dependent on the amount of oxidant (especially at low amounts).

The reduction of UO22+ in carbonate solutions by H2 (p = 40 bar) at different temperatures has been studied as a function of time. The rate constant and the activation energy were determined, k298 =3.6 x 10 -9 M-1 s-1 and Ea = 130 ± 24 kJ/mol, respectively. The reduction of UO22+ to UO2 occurs in the absence of a catalyst.

The relative impact of radiolysis products on oxidative dissolution of UO2 has been elucidated. The amount of dissolved UO22+ was measured as a function of time in γ -irradiated aqueous carbonate solutions saturated with various gases containing a UO2-pellet. The simulated dissolution rates were calculated from the surface area, oxidant concentrations and rate constants obtained from the linear relationship mentioned above. The simulated dissolution rates were compared with the experimental dissolution rates and the correlation was fairly good. Thus, the impact of each oxidant in the system studied can be estimated from the simulations. Only the molecular oxidants are of importance in systems of relevance for safety analysis of a deep repository.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , v, 66 p.
Series
Trita-KKE, ISSN 0349-6465 ; 0349-6465
Keyword [en]
radiation, radiolysis, UO2, spent nuclear fuel, radicals, oxidants, reductants, rate constants, dissolution, H2O2, H2
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-3891ISBN: 91-7178-298-2 (print)OAI: oai:DiVA.org:kth-3891DiVA: diva2:9893
Public defence
2006-04-07, Sydvästra Galleriet, KTHB, Osquars Backe 31, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20110121Available from: 2006-03-21 Created: 2006-03-21 Last updated: 2012-04-02Bibliographically approved
List of papers
1. Oxidation of UO2 by radiolytic oxidants
Open this publication in new window or tab >>Oxidation of UO2 by radiolytic oxidants
2003 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 322, no 03-feb, 242-248 p.Article in journal (Refereed) Published
Abstract [en]

The kinetics of UO2 oxidation by H2O2 has been studied using aqueous suspensions Of UO2-powder. The second order,rate constant for the reaction between H2O2 and UO2 has been determined to 8 x 10(-7) m/min (based on the surface to solution volume ratio) in the absence of carbonate. By studying the reaction between UO2 and other oxidants, it is possible to draw conclusions concerning the mechanism. The logarithm of the second order rate constant, ln k, for UO2 oxidation appears to be linearly related to the one-electron reduction potential, E-0, of the oxidant. This indicates that the rate limiting step in the oxidation of UO2 is one-electron transfer. A Fenton like mechanism is plausible for the reaction between UO2 and H2O2. The diffusion controlled rate constant in this particular system is approximately 10(3) m/min, and therefore the reactions with OH- and CO3- are estimated to be diffusion controlled.

Keyword
waste-disposal conditions, hydrogen-peroxide, equilibria, radicals, rates
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-22893 (URN)10.1016/j.jnucmat.2003.07.001 (DOI)000186005600016 ()
Note
QC 20100525Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
2. 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: 2011-01-21Bibliographically approved
3. Dissolution of UO2 by one- and two-electron oxidants
Open this publication in new window or tab >>Dissolution of UO2 by one- and two-electron oxidants
2004 (English)In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, Vol. 807, 77-82 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the efficiency of one- and two-electron oxidants in oxidative dissolution Of UO2 has been investigated. This was accomplished by measuring the U(VI)-concentration in solution after exposing UO2-powder to controlled amounts of oxidants in aqueous solution. The oxidants used in this study are H2O2, IrCl62-, CO3.- and OH..H2O2 acts as a two-electron oxidant while the remaining three oxidants are one-electron oxidants. CO3.- and OH. were generated using gamma-radiolysis. The results clearly show that the dissolution yields for one-electron oxidants (per electron pair) are lower than the yields for two-electron oxidants. Furthermore, the yields for one-electron oxidants increase with increasing amount of oxidant (especially at low amounts of oxidant). The rationale for this is that U(VI) is the main soluble species which is formed directly upon two-electron oxidation. For one-electron oxidants the primary oxidation product is U(V) which can form U(VI) upon reaction with a second oxidant. The probability for a second oxidation is however low at low concentrations of oxidant.

Keyword
Carbonates, Concentration (process), Dissolution, Negative ions, Nuclear fuels, Probability, Radiolysis, Reaction kinetics, Solutions, Uranium dioxide, Carbonate radical anion, Magnetic stirrer, Oxidants, Oxidation products
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-9087 (URN)000225038400013 ()2-s2.0-12844256411 (Scopus ID)
Note
QC 20100907Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2011-10-31Bibliographically approved
4. Reduction of UO22+ by H-2
Open this publication in new window or tab >>Reduction of UO22+ by H-2
Show others...
2004 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 334, no 1, 35-39 p.Article in journal (Refereed) Published
Abstract [en]

The reactivity of H, towards UO22+ has been studied experimentally using a PEEK coated autoclave where the UO22+ concentration in aqueous solution containing 2 mM carbonate was measured as a function of time at p(H2) similar to 40 bar. The experiments were performed in the temperature interval 74-100 degreesC. In addition, the suggested catalytic activity of UO2 on the reduction of UO22+ by H-2 was investigated. The results clearly show that H-2 is capable of reducing UO22+ to UO2 without the presence of a catalyst. The reaction is of first order with respect to UO22+. The activation energy for the process is 130 +/- 24 U mol(-1) and the rate constant is k(298K) = 3.6 x 10(-9) l mol(-1) s(-1). The activation enthalpy and entropy for the process was determined to 126 kJ mol(-1) and 16.5 J mol(-1) K-1, respectively. Traces of oxygen were shown to inhibit the reduction process. Hence, the suggested catalytic activity of freshly precipitated U02 on the reduction of UO22+ by H-2 could not be confirmed.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-23680 (URN)10.1016/j.jnucmat.2004.04.324 (DOI)000223548400005 ()2-s2.0-3343018403 (Scopus ID)
Note

QC 20100525 QC 20110923

Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
5. The relative impact of radiolysis products in radiation induced oxidative dissolution of UO2
Open this publication in new window or tab >>The relative impact of radiolysis products in radiation induced oxidative dissolution of UO2
2006 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 355, no 1-3, 38-46 p.Article in journal (Refereed) Published
Abstract [en]

The relative impact of radiolysis products in radiation induced oxidative dissolution Of UO2 has been studied experimentally. The experiments were performed by y-irradiating an aqueous solution containing HCO3 and a UO2-pellet. The U(VI) concentration in the solution was measured as a function of irradiation time. The aqueous solution was saturated with Ar, N2O, N2O/O-2 (80/20), air and O-2 in order to vary the conditions and the initial oxidant yields. The measured rate of oxidation was significantly higher for the O-2- and air saturated systems compared to the other systems. Using oxidant concentrations derived from numerical simulations of the corresponding homogeneous systems and previously determined rate constants for oxidation Of UO2, the relative trend in rate of oxidation in the different systems was reproduced. The results from the simulations were also used to estimate the relative impact of the oxidative radiolysis products as a function of irradiation time, both for gamma- and alpha-irradiated systems. For 7-irradiated systems saturated with Ar, air or 02, the most important oxidant is H2O2 while for N2O- and N2O/O-2-saturated systems the most important oxidant is CO3.-. For a-irradiated systems the most important oxidant was found to be H2O2.

Keyword
fuel corrosion, one-electron, carbonate, irradiation, oxidants, uranium, media, water
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-9088 (URN)10.1016/j.jnucmat.2006.04.001 (DOI)000239868800006 ()2-s2.0-33746368836 (Scopus ID)
Note
QC 20100907Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2011-01-21Bibliographically approved

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