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On the catalytic effects of UO2(s) and Pd(s) on the reaction between H2O2 and H-2 in aqueous solution
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
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 372, no 2-3, 160-163 p.Article in journal (Refereed) Published
Abstract [en]

The possible catalytic effects of UO2 and Pd (as a model for noble metal particles) on the reaction between H2O2 and H2 have been studied experimentally. The experiments were performed in aqueous solution using an autoclave. The aqueous solutions were pressurized with H2 or N2 and the H2O2 concentration was measured as a function of time. The experiments clearly showed that Pd catalyzes the reaction between H2O2 and H2 while UO2 has no catalytic effect. The rate constant of the reaction between H2O2 and H2 catalyzed by Pd was found to be close to diffusion controlled and independent of the H2 pressure in the range 1-40 bar. The impact of the catalytic effect on the reaction between H2O2 and H2 on spent nuclear fuel dissolution is, however, fairly small. Other possible effects of noble metal particles are also discussed, e.g. reduction of U(VI) to U(IV) in the liquid and solid phase.

Place, publisher, year, edition, pages
2008. Vol. 372, no 2-3, 160-163 p.
Keyword [en]
Catalyst activity, Concentration (process), Hydrogen peroxide, Palladium, Pressurization, Reaction kinetics, Solutions
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-8388DOI: 10.1016/j.jnucmat.2007.03.040ISI: 000253180400003Scopus ID: 2-s2.0-37249074552OAI: oai:DiVA.org:kth-8388DiVA: diva2:13695
Note
QC 20100924Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved
In thesis
1. Influence of metallic fission products and self irradiation on the rate of spent nuclear fuel-matrix dissolution
Open this publication in new window or tab >>Influence of metallic fission products and self irradiation on the rate of spent nuclear fuel-matrix dissolution
2008 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [sv]

Denna licentiatavhandling behandlar effekten av två inneboende egenskaper (fissions produkter och egenbestrålning) hos utbränt kärnbränsle på hastigheten för strålningsinducerad upplösning av bränslematris (UO2). I ett framtida djupförvar kommer det utbrända kärnbränslet att deponeras 500 meter ner i berggrunden i en reducerande miljö. Under dessa förhållanden är UO2-matrisen själv en av de skyddande barriärerna mot frigörande av radionuklider, på grund av dess låga löslighet. När bränslet kommer i kontakt med vatten kommer U(IV) att oxideras till U(VI) av radiolysprodukter från vattnet och lösligheten för bränslematrisen kommer därmed att öka betydligt.

De flesta tidigare studier har utförts på obestrålad UO2 som skiljer sig signifikant från utbränt kärnbränsle. I utbränt kärnbränsle är de flesta fissionsprodukterna och neutronaktivieringsprodukterna radioaktiva och bränslet kommer därför bli bestrålat av sig självt. Effekten av joniserande strålning på reaktiviteten för UO2(s) har undersökts här. UO2 (pulver och fragment av en kuts) bestrålades i en 60Co γ-källa eller framför en elektronaccelerator varpå reaktiviteten för UO2 studerades genom oxidation av UO2 med MnO4 -. Det visade sig att reaktiviteten för UO2 ökar när det blir bestrålat för första gången (<20 kGy). Effekten ökar med ökande dos tills den når ett maxvärde ~1.3 gånger reaktiviteten för obestrålad UO2 vid torrbestrålning. Vid våtbestrålning ökar en dos på 140 kGy reaktiviteten 2.5 gånger. Effekten verkar vara permanent.

Tidigare studier har visat att H2O2 är den viktigaste oxidanten för upplösning av utbränt kärnbränsle under djupförvarsförhållanden. I vätgasatmosfär, som förväntas i ett djupförvar, har det visat sig att upplösningshastigheten är långsammare. Det har delvis förklarats med reaktionen mellan H2O2 och H2, som är väldigt långsam utan katalysator. Den katalytiska effekten av UO2 på den reaktionen har undersökts och det visades att den inte katalyseras av UO2.

En annan möjlig katalysator för reaktionen är ε-partiklar (ädelmetallpartiklar bestående av Mo, Ru, Tc, Pd och Rh) som bildats av fissionsprodukterna. Pd är en välkänd katalysator för reduktion med H2. Den eventuella katalytiska effekten av Pd har undersökts här. Även en eventuell katalytisk effekt av Pd på reduktionen av U(VI) med H2 undersöktes, både i vattenfas och i UO2-kutsar innehållande olika mängder Pd (som en modell för ε-partiklar).

Vi fann att Pd har en katalytisk effekt på reaktionen mellan H2O2 och H2 och andra ordningens hastighetskonstant är bestämd till (2.1±0.1)x10-5 m s-1. Pd har också en katalytisk effekt på reduktionen av U(VI) med H2 både i vattenlösning, hastighetskonstant (1.5±0.1)x10-5 m s-1, och i den fasta fasen. Hastighetskonstanten för processen i fast fas är 4x10-7 m s-1 och 7x10-6 m s-1 för kutsar med 1 respektive 3 % Pd. Dessa värden är väldigt nära diffusionsgränsen för den här typen av system. Den katalytiska effekten i den fasta fasen visar att upplösningen för 100 år gammalt bränsle kan stoppas helt. Vid 40 bar H2 krävs 10-20 ppm ädelmetallpartiklar och med 1 % ädelmetallpartiklar räcker det med 0.1 bar H2 för att stoppa upplösningen.

Abstract [en]

This licentiate thesis deals with the influence of two inherent properties (fission products and self irradiation) of spent nuclear fuel on the rate of radiation induced fuel matrix (UO2) dissolution. In a future deep repository the spent nuclear fuel will be deposited 500 meters down in the bedrock in a reducing environment. Under these conditions the UO2-matrix itself is one of the protective barriers against release of radionuclides due to its very low solubility. When the fuel comes in contact with water, U(IV) will be oxidized to U(VI) by products from radiolysis of water and the solubility of the fuel matrix will increase significantly.

Most previous studies have been performed on unirradiated UO2 which differ significantly from spent nuclear fuel. In spent nuclear fuel most of the fission products and neutron activation products are radioactive and therefore the fuel will be irradiated by itself. The effect of ionizing radiation on the reactivity of UO2 has been investigated here. UO2 (powder and fragment of a pellet) has been exposed to irradiation in a 60Co γ-source or in an electron accelerator and then the redox reactivity was studied. The kinetics for oxidation of UO2 by MnO4 - was used as a monitoring reaction. It was shown that the reactivity of UO2 increases when being irradiated for the first time (<20kGy). The effect increases with increasing dose until reaching a maximum value ~1.3 times the reactivity of unirradiated UO2 for dry irradiation. For wet irradiation a dose of 140 kGy increases the reactivity ~2.5 times. This effect appears to be permanent.

Previous studies have shown that H2O2 is the most important oxidant for spent nuclear fuel dissolution under deep repository conditions. Under H2 atmosphere, as expected in a deep repository, it has been shown that the dissolution rate is slower. This has partly been attributed to the reaction between H2O2 and H2 which is very slow without a catalyst. The catalytic effect of UO2 on this reaction was examined showing that UO2 does not catalyze this reaction.

Another possible catalyst for this reaction is the ε-particles (noble metal particles containing Mo, Ru, Tc, Pd and Rh) formed by the fission products. Pd is a well known catalyst for reduction by H2. The possible catalytic effect of Pd on the reaction between H2O2 and H2 is examined here. The possible catalytic effect of Pd on the reduction of U(VI) by H2 is also examined, both in aqueous phase and in UO2 pellets containing different amounts of Pd (as a model for spent fuel containing ε-particles).

It was found that Pd has a catalytic effect on the reaction between H2O2 and H2, the second order rate constant is determined to (2.1±0.1)x10-5 m s-1. Pd also has a catalytic effect on the reduction of U(VI) by H2 both in aqueous solution, rate constant (1.5±0.1)x10-5 m s-1, and in the solid phase, rate constants 4x10-7 m s-1 and 7x10-6 m s-1 for pellets with 1 and 3 % Pd respectively. These values are very close to the diffusion limit for these systems. The catalytic effect in the solid phase shows that the dissolution for 100 year old fuel can be completely inhibited, at 40 bar H2 a noble metal particle content of 10-20 ppm is needed and with 1 % noble metal particle content 0.1 bar H2 is enough to stop the dissolution.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. viii, 31 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:35
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-4736 (URN)978-91-7178-967-9 (ISBN)
Presentation
2008-05-10, K1, KTH, Teknikringen 56, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101119Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2010-11-19Bibliographically approved
2. Radiation induced dissolution of model compounds for spent nuclear fuel: mechanistic understanding of oxidative dissolution and its inhibition
Open this publication in new window or tab >>Radiation induced dissolution of model compounds for spent nuclear fuel: mechanistic understanding of oxidative dissolution and its inhibition
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This doctoral thesis is focused on radiation induced oxidative dissolution of UO2, Pd-doped UO2, SIMFUEL (as model substances for spent nuclear fuel) and UN (a possible future fuel) and inhibition of the oxidative dissolution.

H2O2 is assumed to be the most important oxidant for spent nuclear fuel dissolution under deep repository conditions. The dissolution of uranium has been studied by oxidation by added H2O2 and by gamma irradiation in the presence and absence of carbonate.

In carbonate free solutions very low amounts of uranium are dissolved from UO2 due to formation of metastudtite, UO4·2H2O on the UO2 surface which blocks the surface from further oxidation. Metastudite formation was confirmed with Raman spectroscopy.

In the presence of carbonate, the concentration of dissolved uranium increases linearly over time for UO2 and UN, due to the complex formation between carbonate and oxidized uranium.

For SIMFUEL a large fraction of H2O2 is consumed by catalytic decomposition under all conditions examined. This results in very low amounts of uranium released. Metastudtite formation was not observed on SIMFUEL.

The oxidation during gamma radiolysis shows a larger difference in dissolution rates between UO2 and UN in carbonate solutions compared to upon oxidation by added H2O2. UN was found to have a lower dissolution rate, most probably because 50 % more oxidant is needed to reach the soluble U(VI).

It was shown that the redox reactivity of UO2 appears to increase ~1.3 times, after being irradiated at doses > 40 kGy. The effect is permanent and delayed.

The presence of sulfide shows an inhibiting effect on radiation induced dissolution due to scavenging of radiolytic oxidants and reduction of U(VI).

The catalytic properties of Pd (as a model for the noble metal particles containing Mo, Ru, Tc, Pd and Rh, formed by the fission products) are examined. It was found that Pd has a catalytic effect on the reaction between H2O2 and H2 and the second order rate constant is determined to (2.1±0.1)x10-5 m s-1. The reaction between UO2 and H2O2 is catalyzed by Pd. Pd also has a catalytic effect on the reduction of U(VI) by H2 both in aqueous solution, rate constant (1.5±0.1)x10-5, and in the solid phase, rate constants 4x10-7 m s-1 and 7x10-6 m s-1 for pellets with 1 and 3 % Pd respectively. These values are very close to the diffusion limit for these systems. The catalytic effect was not influenced by the presence of sulfide. The catalytic effect in the solid phase reduction shows that the expected conditions in a deep repository, 40 bar H2 and 1 % noble metal particle content, is sufficient to stop the dissolution.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 67 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:30
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-95433 (URN)978-91-7501-381-1 (ISBN)
Public defence
2012-06-15, K2, Teknikringen 28, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120530Available from: 2012-05-30 Created: 2012-05-24 Last updated: 2012-07-06Bibliographically approved

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