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Influence of metallic fission products and self irradiation on the rate of spent nuclear fuel-matrix dissolution
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
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: urn:nbn:se:kth:diva-4736ISBN: 978-91-7178-967-9 (print)OAI: oai:DiVA.org:kth-4736DiVA: diva2:13699
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
List of papers
1. 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: 2012-05-30Bibliographically approved
2. On the catalytic effects of UO2(s) and Pd(s) on the reaction between H2O2 and H-2 in aqueous solution
Open this publication in new window or tab >>On the catalytic effects of UO2(s) and Pd(s) on the reaction between H2O2 and H-2 in aqueous solution
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.

Keyword
Catalyst activity, Concentration (process), Hydrogen peroxide, Palladium, Pressurization, Reaction kinetics, Solutions
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8388 (URN)10.1016/j.jnucmat.2007.03.040 (DOI)000253180400003 ()2-s2.0-37249074552 (Scopus ID)
Note
QC 20100924Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved
3. On the catalytic effect of Pd(s) on the reduction of UO22+ with H-2 in aqueous solution
Open this publication in new window or tab >>On the catalytic effect of Pd(s) on the reduction of UO22+ with H-2 in aqueous solution
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 374, no 1-2, 290-292 p.Article in journal (Refereed) Published
Abstract [en]

The catalytic effect of Pd(s) (as a model for noble metal particles) on the reduction of UO22+ by H-2 has been studied experimentally. The experiments were performed in aqueous solution in an autoclave. The aqueous solutions were pressurized with H-2 or N-2 and the UO22+ concentration was measured as a function of time. The experiments clearly show that Pd catalyzes the reaction between UO22+ and H-2. The rate constant of the reaction was found to be close to diffusion controlled and independent of the H-2 pressure in the range 1.5-40 bar. The effect of a catalyzed reduction of U(VI) to U(IV) in the solid phase is also discussed.

Keyword
OXIDATION; URANIUM; FUEL
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8389 (URN)10.1016/j.jnucmat.2007.08.010 (DOI)000254413900035 ()2-s2.0-38749145101 (Scopus ID)
Note
QC 20100621Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved
4. Radiation induced spent nuclear fuel dissolution under deep repository conditions
Open this publication in new window or tab >>Radiation induced spent nuclear fuel dissolution under deep repository conditions
Show others...
2007 (English)In: Environmental Science and Technology, ISSN 0013-936X, Vol. 41, no 20, 7087-7093 p.Article in journal (Refereed) Published
Abstract [en]

The dynamics of spent nuclear fuel dissolution in groundwater is an important part of the safety assessment of a deep geological repository for high level nuclear waste. In this paper we discuss the most important elementary processes and parameters involved in radiation induced oxidative dissolution of spent nuclear fuel. Based on these processes, we also present a new approach for simulation of spent nuclear fuel dissolution under deep repository conditions. This approach accounts for the effects of fuel age, burn up, noble metal nanoparticle contents, aqueous H-2 and HCO3- concentration, water chemistry, and combinations thereof. The results clearly indicate that solutes consuming H2O2 and combined effects of noble metal nanoparticles and H-2 have significant impact on the rate of spent nuclear fuel dissolution. Using data from the two possible repository sites in Sweden, we have employed the new approach to estimate the maximum rate of spent nuclear fuel dissolution. This estimate indicates that H-2 produced from radiolysis of groundwater alone will be sufficient to inhibit the dissolution, completely for spent nuclear fuel older than 100 years.

Keyword
Computer simulation, Concentration (process), Dissolution, Groundwater, Nanoparticles, Nuclear fuels, Radiation, Radioactive wastes, Radiolysis, Noble metal, Nuclear fuel dissolution, Water pollution, bicarbonate, ground water, hydrogen peroxide, metal, nanoparticle, nuclear fuel, water, Computer simulation, Concentration (process), Dissolution, Groundwater, Nanoparticles, Nuclear fuels, Radiation, Radioactive wastes, Radiolysis, Water pollution, assessment method, concentration (composition), dissolution, fuel, groundwater, oxidation, radioactive waste, repository, safety, water chemistry, article, concentration response, dissolution, oxidation, radiation, radioactive waste processing, radiolysis, Sweden
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8390 (URN)10.1021/es070832y (DOI)000250110800036 ()2-s2.0-35348828672 (Scopus ID)
Note
QC 20100811Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved
5. On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel
Open this publication in new window or tab >>On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel
2008 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 378, no 1, 55-59 p.Article in journal (Refereed) Published
Abstract [en]

Radiation induced oxidative dissolution of UO2 is a key process for the safety assessment of future geological repositories for spent nuclear fuel. This process is expected to govern the rate of radionuclide release to the biosphere. In this work, we have studied the catalytic effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. The experimental studies were performed using UO2 pellets containing 0%, 0.1%, 1% and 3% Pd as a model for spent nuclear fuel. H2O2 was used as a model for radiolytical oxidants (previous studies have shown that H2O2 is the most important oxidant in such systems). The pellets were immersed in aqueous solution containing H2O2 and HCO3- and the consumption of H2O2 and the dissolution of uranium were analyzed as a function of H2 pressure (0-40 bar). The noble metal inclusions were found to catalyze oxidation of UO2 as well as reduction of surface bound oxidized UO2 by H2. In both cases the rate of the process increases with increasing Pd content. The reduction process was found to be close to diffusion controlled. This process can fully account for the inhibiting effect of H2 observed in several studies on spent nuclear fuel dissolution.

Keyword
Fission products, Metals, Nuclear fuels, Precious metals, Radiation, Radiation effects
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8391 (URN)10.1016/j.jnucmat.2008.04.018 (DOI)000258553500010 ()2-s2.0-47549110547 (Scopus ID)
Note
QC 20100924. Uppdaterad från Submitted till Published (20100924).Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2012-05-30Bibliographically approved

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