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Hinhibition of radiation induced dissolution of spent nuclear fuel
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
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.

Place, publisher, year, edition, pages
2009. Vol. 383, no 3, 226-230 p.
Keyword [en]
WASTE-DISPOSAL CONDITIONS, OXIDATIVE DISSOLUTION, AQUEOUS-SOLUTION, URANIUM-DIOXIDE, UO2, KINETICS, H2O2, REDUCTION, CORROSION, MECHANISM
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-9369DOI: 10.1016/j.jnucmat.2008.09.021ISI: 000261905600004Scopus ID: 2-s2.0-56349130830OAI: oai:DiVA.org:kth-9369DiVA: diva2:113656
Note
QC 20100908. Uppdaterad från submitted till published (20100908).Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Redox Chemistry in Radiation Induced Dissolution of Spent Nuclear Fuel: from Elementary Reactions to Predictive Modeling
Open this publication in new window or tab >>Redox Chemistry in Radiation Induced Dissolution of Spent Nuclear Fuel: from Elementary Reactions to Predictive Modeling
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The focus of this doctoral thesis is the redox chemistry involved in radiation induced oxidative dissolution of spent nuclear fuel and UO2 (as a model substance for spent nuclear fuel). It is shown that two electron oxidants are more efficient than one electron oxidants in oxidative dissolution of UO2 at low oxidant concentrations. Furthermore, it is shown that H2O2 is the only oxidant that has to be taken into account in radiation induced dissolution of UO2 under deep repository conditions (granite groundwater dominated by α-radiolysis). Previously determined rate constants for oxidation of UO2 by H2O2 and O2, and rate constants for dissolution of U(VI) from the surface are successfully used to reproduce numerous UO2 dissolution rates reported in the literature. The impact of reactive solutes (Fe(II)(aq), 2-propanol and chloride) and Pd-inclusions (as a model for ε-particles) in combination with H2, on radiation induced oxidative dissolution of UO2 is investigated. It is shown that both the studied reactive solutes (under oxygen free conditions) and the combination of Pd inclusions and H2 inhibit the dissolution. Calculations (based on the fuel inventory) show that 1 µM Fe(II)(aq) decreases the dissolution rate by a factor of ~50 and that 1 ppm surface coverage of ε-particles is sufficient to completely stop the dissolution of 100 year old fuel (assuming 40 bar H2).The dissolution behavior of NpO2 and PuO2 in H2O2 containing aqueous solution without complexing agent is studied and compared to UO2. Based on the measured dissolution rates, we would not expect the dissolution of the actinides to be congruent. Instead, in a system without complexing agent, the rates Np and Pu are expected to be lower than the U release rate. The effect of ionizing irradiation on the UO2 reactivity is studied in order to elucidate the effect of self-irradiation on the reactivity of the spent fuel matrix. It is shown that a threshold dose must be achieved before any effect of irradiation can be seen. Beyond the threshold the reactivity seems to increase with increasing dose. Furthermore, the effect appears to be permanent. The effect of particle size on the reactivity of UO2 powder is studied in view of proposed theories suggesting a particle size dependence of both the pre-exponential factor and the activation energy for redox reactions. The rate constant and activation energy for oxidation of UO2 by MnO4- seems to agree with the proposed equations. The radiation chemical synthesis of UO2 nanoparticles is studied. It is shown that U(VI) released by dissolution of spent nuclear fuel could be reduced to UO2 nanoparticles.These particles could, due to their high reactivity towards H2O2, act as oxidant scavenger in a future deep repository for spent nuclear fuel.

Abstract [sv]

Denna doktorsavhandling behandlar redoxprocesser involverade i strålnings­inducerad oxidativ upplösning av använt kärnbränsle och UO2 (som modellsubstans för använt kärn­bränsle).Detta arbete visar att två-elektron oxidanter är mer effektiva än en-elektron oxidanter i oxidativ upplösning av UO2 vid låga oxidantkoncentrationer. Dessutom visas, på kinetiska grunder,att H2O2 är den enda oxidant som måste tas hänsyn till vid stålningsinducerad oxidativ upplösning av UO2 under djupförvarsförhållanden (granitiskt grundvatten dominerat av α-radiolys). Tidigare bestämda hastighets­konstanter för oxidation av UO2 med H2O2 och O2, samt hastighets­konstanter för upplösning av U(VI) från ytan har framgångrikt använts för att återskapa UO2 upplösningshastigheter rapporterade i litteraturen. Inverkan av reaktiva ämnen i vattenfas (Fe(II)(aq), 2-propanol och klorid) samt av Pd-inneslutningar (som modell av ε-partiklar) i UO2 matrisen i kombination med H2, på strålningsinducerad upplösning av UO2 har studerats. Studien visar att både de reaktiva ämnena i vattenfasen (under syrefria förhållanden) och Pd- inneslutningar i kombination med H2 hämmar upplösningen. Beräkningar (baserade på ett bränsle­inventarie) visar att 1 µM Fe(II)(aq) minskar upplösningshastigheten med en faktor ~50 samt att 1 ppm ytbeläggning av ε-partiklar är tillräckligt för att effektivt stoppa uppslöningen av 100 år gammalt bränsle (vid 40 bar H2).Upplösning av NpO2 och PuO2, i jämförelse med UO2,har studerats i vatten­lösning innehållande H2O2 utan komplexbildare. Baserat på de uppmätta upp­lösnings­hastigheterna förväntas upplösningen av dessa aktinider från UO2-bränsle vara inkongruent. I ett system utan komplexbildare kan NpO2 odh PuO2 upplösningshastighetern förvantas vara lägre än UO2 upplösningshastigheten. Effekten av joniserande strålning på reaktiviteten hos UO2 har studerats för att klargöra effekten av egen-bestrålning. Studien visar att dosen måste nå ett visst tröskelvärde innan någon effekt på reaktiviteten kan observeras. Vid doser över tröskelvärdet ökar reaktiviteten med ökande dos. Effekten verkar vara permanent. Partikelstorlekens inverkan på reaktiviteten hos UO2 pulver har studerats med utgångspunkt i föreslagna samband mellan partikelstorlek och pre-exponentiell faktor och mellan partikelstorlek och aktiveringsenergi. Studien visar att hastighets­konstanten och aktiveringsenergin för reaktionen mellan UO2 och MnO4- överenstämmer med de föreslagna sambanden. Stålningskemisk syntes av UO2 nanopartiklar har studerats. Studien visar att U(VI) frigjort genom upplösning av använt kärnbränsle i ett djupförvar kan reduceras till UO2 nanopartiklar. Dessa partiklar kan, på grund av sin höga reaktivitet med H2O2, fungera som infångare av oxidanter i ett framtida djupförvar för använt kärnbränsle.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. 73 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:60
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-4901 (URN)978-91-7415-100-8 (ISBN)
Public defence
2008-09-19, F3, KTH, Lindstedtsvägen 26, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20100908Available from: 2008-09-18 Created: 2008-09-18 Last updated: 2010-09-08Bibliographically approved
2. The effect of solid state inclusions on the reactivity of UO2: A kinetic and mechanistic study
Open this publication in new window or tab >>The effect of solid state inclusions on the reactivity of UO2: A kinetic and mechanistic study
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:nbn:se:kth:diva-33070 (URN)978-91-7415-960-8 (ISBN)
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

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