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Reduction of UO22+ by H-2
KTH, Superseded Departments, Chemistry.
KTH, Superseded Departments, Chemistry.ORCID iD: 0000-0003-0663-0751
KTH, Superseded Departments, Chemistry.
KTH, Superseded Departments, Chemistry.
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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.

Place, publisher, year, edition, pages
2004. Vol. 334, no 1, 35-39 p.
National Category
Inorganic Chemistry
URN: urn:nbn:se:kth:diva-23680DOI: 10.1016/j.jnucmat.2004.04.324ISI: 000223548400005ScopusID: 2-s2.0-3343018403OAI: diva2:342379
QC 20100525 QC 20110923Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2011-09-23Bibliographically approved
In thesis
1. Radiation Induced Oxidative Dissolution of UO2
Open this publication in new window or tab >>Radiation Induced Oxidative Dissolution of UO2
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [sv]


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.
Trita-KKE, ISSN 0349-6465 ; 0349-6465
radiation, radiolysis, UO2, spent nuclear fuel, radicals, oxidants, reductants, rate constants, dissolution, H2O2, H2
National Category
Inorganic Chemistry
urn:nbn:se:kth:diva-3891 (URN)91-7178-298-2 (ISBN)
Public defence
2006-04-07, Sydvästra Galleriet, KTHB, Osquars Backe 31, Stockholm, 10:00
QC 20110121Available from: 2006-03-21 Created: 2006-03-21 Last updated: 2012-04-02Bibliographically approved

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