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

Denna licensiatavhandling behandlar oxidativ upplösning av UO2. Upplösning av UO2 studeras huvudsakligen då UO2-matrisen hos använt kärnbränsle förväntas fungera som en barriär mot frigörande av radionuklider i ett framtida djupförvar. Lösligheten av U(IV) är mycket låg under i djupförvaret rådande förhållanden emedan U(VI) har betydligt högre löslighet. Oxidation av UO2-matrisen kommer därför att påverka dess löslighet och därmed dess funktion som barriär. I denna avhandling studeras den relativa effektiviteten av en- och två-elektronoxidanter för upplösning av UO2. Vid låga oxidantkoncentrationer är utbytet för upplösningen för en-elektronoxidanter signifikant lägre än för två-elektronoxidanter. För en-elektronoxidanter ökar dock utbytet med ökande oxidanthalt, vilket kan förklaras av den ökade sannolikheten för två konsekutiva en-elektronoxidationer av samma reaktionssite och den ökade möjligheten till disproportionering.

Radikaler och molekylära radiolysprodukters relativa inverkan på oxidativ upplösning av UO2 studeras också i denna avhandling genom mätning av mängden upplöst U(VI) i γ-bestrålade system som dominerades av olika oxidanter. Dessa studier visade att upplösningshastigheten av UO2 kan uppskattas från oxidantkoncentrationer framtagna genom simuleringar av radiolys i motsvarande homogena system och hastighetskonstanterna för ytreaktionerna. Simuleringarna visar att de molekylära oxidanterna kommer vara de viktigaste oxidanterna i alla system i denna studie vid långa bestrålningstider (>10 timmar). Vid liknande simuleringar av α-bestrålade system fanns att vid förhållanden relevanta för ett djupförvar för använt kärnbränsle, är det endast de molekylära oxidanterna (i huvudsak H2O2) som är av betydelse för upplösningen av bränslematrisen.

Då använt kärnbränsle innehåller en mängd radionuklider som utsätter UO2-matrisen för kontinuerlig bestrålning, är det av vikt att undersöka hur bestrålning påverkar reaktiviteten av UO2. Bestrålningseffekten på reaktionen mellan UO2 och MnO4- studerades. Dessa försök visade att bestrålning av UO2 vid doser >40 kGy leder till att reaktiviteten ökar upp till 1.3 gånger reaktiviteten av obestrålad UO2. Den ökade reaktiviteten kvarstår efter bestrålningen och effekten kan därför möjligen tillskrivas permanenta förändringar i materialet. Vid uppskattning av reaktiviteten hos använt kärnbränsle måste hänsyn tas till denna effekt då bränslet redan efter ett par dagar i reaktor blivit utsatt för doser >40 kGy.

Det har tidigare föreslagits att hastigheten för en heterogen västka/fast-fas reaktion är beroende av partikelstorleken hos det fasta materialet, vilket har studerats för UO2-partiklar i denna avhandling. Experimentellt bestämda kinetiska parametrar jämförs med de föreslagna ekvationerna för fyra storleksfraktioner av UO2-pulver och en UO2-pellet. Studien visade partikelstorleksberoendet av andra ordningens hastighetskonstant och aktiveringsenergin för oxidation av UO2 med MnO4- beskrivs relativt väl av de föreslagna ekvationerna.

Abstract [en]

The general subject of this thesis is oxidative dissolution of UO2. The dissolution of UO2 is mainly investigated because of the importance of the UO2 matrix of spent nuclear fuel as a barrier against radionuclide release in a future deep repository. U(IV) is extremely insoluble under the reducing conditions prevalent in a deep repository, whereas U(VI) is more soluble. Hence, oxidation of the UO2-matrix will affect its solubility and thereby its function as a barrier. In this thesis the relative efficiency of one- and two electron oxidants in dissolving UO2 is studied. The oxidative dissolution yield of UO2 was found to differ between one- and two-electron oxidants. At low oxidant concentrations the dissolution yields for one-electron oxidants are significantly lower than for two-electron oxidants. However, the dissolution yield for one-electron oxidants increases with increasing oxidant concentration, which could be rationalized by the increased probability for two consecutive one-electron oxidations at the same site and the increased possibility for disproportionation.

Furthermore, the relative impact of radical and molecular radiolysis products on oxidative dissolution of UO2 is investigated. Experiments were performed where the amount of dissolved U(VI) was measured in γ-irradiated systems dominated by different oxidants. We have found that the UO2 dissolution rate in systems exposed to γ-irradiation can be estimated from oxidant concentrations derived from simulations of radiolysis in the corresponding homogeneous systems and rate constants for the surface reactions. These simulations show that for all systems studied in this work, the molecular oxidants will be the most important oxidants for long irradiation times (>10 hours). Similar simulations of α-irradiated systems show that in systems relevant for a deep repository for spent nuclear fuel, only the molecular oxidants (mainly H2O2) are of importance for the dissolution of the fuel matrix.

The effect on UO2 reactivity by irradiation of the material is of importance when predicting the spent fuel dissolution rate since the fuel, due to its content of radionuclides, is exposed to continuous self-irradiation. The effect of irradiation on the reaction between solid UO2 and MnO4- in aqueous solutions was studied. It was found that irradiation of UO2 at doses >40 kGy increases the reactivity of the material up to ~1.3 times the reactivity of unirradiated UO2. The increased reactivity remains after the irradiation and can possibly be attributed to permanent changes in the material. This issue must be taken into account when predicting the reactivity of spent nuclear fuel since the fuel is exposed to doses >40 kGy after only a few days in the reactor.

It has earlier been suggested that the rate of a heterogeneous liquid-solid reaction depends on the size of the solid particles. This was investigated for UO2 particles in this thesis. Experimental kinetic parameters are compared to the previously proposed equations for UO2 powder of four size fractions and a UO2 pellet. We have found that the particle size dependence of the second order rate constant and activation energy for oxidation of UO2 by MnO4- is described quite well by the proposed equations.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 29 p.
Series
Trita-KKE, ISSN 0349-6465 ; 0601
Keyword [en]
UO2, spent nuclear fuel, dissolution, oxidation, radiolysis, irradiation, particle size, modeling
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-622ISBN: 91-7178-248-6 (print)OAI: oai:DiVA.org:kth-622DiVA: diva2:14652
Presentation
2006-02-10, E3, Osquarsbacke 14, Stockholm, 14:00
Supervisors
Note
QC 20101123Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2010-11-23Bibliographically approved
List of papers
1. 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, E-ISSN 1946-4274, 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: 2017-12-14Bibliographically approved
2. 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: 2017-12-14Bibliographically approved
3. Radiation enhanced reactivity of UO2
Open this publication in new window or tab >>Radiation enhanced reactivity of UO2
2006 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 354, no 1-3, 131-136 p.Article in journal (Refereed) Published
Abstract [en]

Pure UO2 is often used as a model compound when studying reactions of importance in a future deep repository for spent nuclear fuel. The reactivity of pure UO2 is not expected to be identical to the reactivity of the UO2-matrix of spent nuclear fuel for several reasons. One reason is that the spent fuel, due to the content of radionuclides, is continuously being self-irradiated. The aim of this study is to investigate how irradiation of solid UO2 surfaces affects their reactivity towards oxidants. The effect of irradiation (gamma or electrons) on the reaction between solid UO2 and MnO4- in aqueous solutions containing carbonate has been studied. It was found that irradiation with high doses (> 40 kGy) increased the reactivity of the UO2 to about 1.3 times the reactivity of unirradiated UO2.

Keyword
Carbonates, Electron irradiation, Manganese compounds, Mathematical models, Nuclear fuels, Radioisotopes, F0700, U0200, W0200
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8387 (URN)10.1016/j.jnucmat.2006.03.011 (DOI)000239109300011 ()2-s2.0-33745422542 (Scopus ID)
Note
QC 20100908Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2017-12-14Bibliographically approved
4. The influence of particle size on the kinetics of UO2 oxidation in aqueous powder suspensions
Open this publication in new window or tab >>The influence of particle size on the kinetics of UO2 oxidation in aqueous powder suspensions
2006 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 353, no 1-2, 75-79 p.Article in journal (Refereed) Published
Abstract [en]

Previous studies have indicated that the rate of a heterogeneous liquid-solid reaction depends on the size of the solid particles. It has been suggested that both the pre-exponential factor and the activation energy depend on the particle size. The processes involved in dissolution of UO2 have been extensively studied because of their importance for the safety analysis of a future deep repository for spent nuclear fuel and in many of these studies powder suspensions of UO2 are used as a model system. Therefore, it is of importance to investigate and quantify the particle size effect on the kinetics of UO2 oxidation in order to enable comparison of data from studies oil different solid substrates. In this work the influence of particle size on the second order rate constant and on the activation energy of the reaction between MnO4- and UO2 was studied using aqueousUO(2)-particle suspensions of four different size distributions. A comparative study of the activation energy for the reaction using a UO2 pellet was also performed.

Keyword
dissolution mechanism, uranium-dioxide, one-electron, reduction, oxidants
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-15821 (URN)10.1016/j.jnucmat.2006.03.005 (DOI)000238770300009 ()2-s2.0-33744994983 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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