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Dynamics of Spent Nuclear Fuel Dissolution: Impact of Catalysis, Matrix Composition and Time Evolution
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0002-4505-0920
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Worldwide, nuclear power produces a large portion of the electricity that we consume every day. While nuclear energy comes with certain advantages, waste originating from its use is of particular concern. As of today, most countries are planning to store spent nuclear fuel in deep geological repositories to protect humans and the environment from this highly radiotoxic waste. Through a number of natural and engineered barriers, a repository is designed to remain intact and keep the radionuclides contained for millennia to come. To assess the safety of a repository, long-term predictions based on model systems are required. Given that one day the barriers of a repository fail, groundwater intrusion is inevitable. Once spent fuel is in contact with groundwater, the mobility of radionuclides in the environment is significantly enhanced. Spent nuclear fuel is a complex material which consists to around 95 % of UO2. The remainders are fission products and heavier actinides. In this thesis a bottom up approach is used to study dissolution of UO2 based model systems with a particular focus on dissolution induced by H2O2. H2O2 forms upon water radiolysis and can enhance UO2 dissolution. The mechanism for H2O2 consumption on metals and metal oxides is therefore revisited. It was found that the mechanism for catalytic decomposition of H2O2 on Pd differs from that on metal oxides. In addition, coumarin was demonstrated to be an efficient scavenger for reaction intermediates i.e. HO . To simulate longterm dissolution under repository conditions, UO2 and Gd-doped UO2 pellets were leached to reach high H2O2 exposures. Surface passivation reducing the dissolution of UO2 pellets was found to be accompanied by the formation of an oxidized layer. Studtite, a urnayl peroxide mineral can passivate the UO2 surface under certain conditions. Upon exposure to g-radiation studtite was found to dissolve readily, inhibiting passivation of real spent fuel by this surface precipitate.

Abstract [sv]

Globalt sett står kärnkraften för en stor del av vår dagliga elproduktion. Kärnkraften har stora fördelar men produktionen av radiotoxiskt avfall utgör ett stort problem. De flesta länder som använder kärnkraft idag planerar att förvara det använda kärnbränslet i geologiska djupförvar för att skydda människor och miljö från radioaktiva ämnen. I ett geologiskt djupförvar används ett antal skyddsbarriärer för att isolera det utbrända kärnbränslet från biosfären. För att göra en säkerhetsanalys för ett geologiskt djupförvar krävs modeller som beskriver tänkbara processer i systemet. Om samtliga skyddsbarriärer brister är intrång av grundvatten oundvikligt. När det utbrända kärnbränslet kommer i kontakt med grundvatten kan bränslet lösas upp vilket leder till att radioaktivitet sprids. Utbränt kärnbränsle är ett mycket komplext material som till 95 % består av UO2. Resten är fissionsprodukter och tyngre aktinider som bildas i kärnreaktorn. Vissa av fissionsprodukterna bildar metalliska partiklar i bränslet. I detta arbete har UO2 använts som modellsystem för utbränt kärnbränsle. H2O2 som produceras under radiolys av vatten oxiderar UO2 vilket leder till ökad löslighet av bränslet. Reaktionsmekanismen för reaktionen mellan H2O2 och metalloxider är därför av central betydelse i detta arbete. Även de reaktioner som kan ske på de metalliska partiklarna har studerats. Experimentella resultat visar att reaktionsmekanismen för katalytisk sönderdelning av H2O2 på Pd (som modell för de metalliska partiklarna) skiljer sig från mekanismen för katalytisk sönderdelning på metalloxider. Detta arbete har också visat att kumarin fungerar som radikalfångare för ytbunden HO . Hydroxylradikaler är en viktig mellanprodukt vid den katalytiska sönderdelningen av H2O2 på metalloxider. För att simulera den långsiktiga lösligheten för utbränt kärnbränsle under slutförvarförhållanden genomfördes lakningstester på både UO2 och Gd-dopade UO2 kutsar med upprepad tillsats av H2O2. Detta säkerställer en hög exponeringsnivå för kutsarna. Som en konsekvens av den höga exponeringen kan man detektera en passivering av ytan. För UO2 kutsar innefattar denna passivering bildandet av ett oxiderat skikt på kutsarnas ytor. Studtit, ett mineral bestående av uranylperoxid som bildas under vissa förhållanden, skulle kunna passivera ytan på liknande sätt. I samband med detta arbete har det visat sig att studtit är oväntat lösligt under påverkan av g-strålning. Ingen passiverande effekt av studtit kan därför förväntas på ytan av verkligt använt kärnbränsle.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 95
Series
TRITA-CBH-FOU ; 2019:59
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-263163ISBN: 978-91-7873-345-3 (print)OAI: oai:DiVA.org:kth-263163DiVA, id: diva2:1367092
Public defence
2019-12-05, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-11-06

Available from: 2019-11-06 Created: 2019-10-31 Last updated: 2019-11-06Bibliographically approved
List of papers
1. Confirming the formation of hydroxyl radicals in the catalytic decomposition of H2O2 on metal oxides using coumarin as a probe
Open this publication in new window or tab >>Confirming the formation of hydroxyl radicals in the catalytic decomposition of H2O2 on metal oxides using coumarin as a probe
2019 (English)In: ChemCatChem, ISSN 1867-3880, E-ISSN 1867-3899, Vol. 11, no 22, p. 5435-5438Article in journal (Refereed) Published
Abstract [en]

Hydrogen peroxide can be catalytically decomposed to O2 and H2O on metal oxide surfaces in contact with aqueous solutions containing H2O2. The initial step in this process has been proposed to be the formation of surface‐bound hydroxyl radicals which has recently been verified using tris as a radical scavenger. Here, we make use of the unique fluorescent product 7‐hydroxycoumarin formed in the reaction between hydroxyl radicals and coumarin to probe the formation of surface‐bound hydroxyl radicals. The experiments clearly show that 7‐hydroxycoumarin is formed upon catalytic decomposition of H2O2 in aqueous suspensions containing ZrO2‐particles and coumarin, thereby confirming the formation of surface‐bound hydroxyl radicals in this process. The results are quantitatively compared to results on the same system using tris as a probe for hydroxyl radicals. The effects of the two probes on the system under study are compared and it is concluded that coumarin has a significantly lower impact on the system.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2019
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-261217 (URN)10.1002/cctc.201901316 (DOI)000497857800007 ()2-s2.0-85074344246 (Scopus ID)
Note

QC 20191212

Available from: 2019-10-03 Created: 2019-10-03 Last updated: 2019-12-12Bibliographically approved
2. Radiation induced dissolution of U3Si2 - A potential accident tolerant fuel
Open this publication in new window or tab >>Radiation induced dissolution of U3Si2 - A potential accident tolerant fuel
2019 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 517, p. 263-267Article in journal (Refereed) Published
Abstract [en]

To assess the integrity of the accident tolerant fuel candidate U 3 Si 2 under geological repository conditions, the kinetics of γ-radiation- and H 2 O 2 - induced oxidative dissolution was studied. The experiments were performed in aqueous solutions containing 10 mM HCO 3 − and in solutions without added HCO 3 − . The same experiments were also performed on UO 2 for comparison. All experiments were performed using powder suspensions. The experiments show that U 3 Si 2 is less than one order of magnitude more reactive towards H 2 O 2 than is UO 2 . The dissolution yield of U 3 Si 2 slightly exceeds the theoretical yield (23%). In experiments with consecutive additions of H 2 O 2 in HCO 3 − solutions, the reactivity remains constant implying that no significant amount of a secondary phase is formed on the U 3 Si 2 surface. The dissolution of Si closely follows that of U in HCO 3 − solution. In solutions without added HCO 3 − the reactivity towards H 2 O 2 is reduced by a factor less than 2. The dissolution is slightly slower than in HCO 3 − containing solutions but precipitation of U is observed after some time. The results of consecutive additions of H 2 O 2 to the HCO 3 − free system shows that the reactivity is decreasing for every addition. This indicates that a secondary phase is formed. XRD shows that the secondary phase is studtite. The irradiation experiments show that the surface area normalized radiation chemical yields for uranium dissolution for U 3 Si 2 and UO 2 in HCO 3 − solution differ by a factor 5–10 in favour of UO 2 . This difference can largely be attributed to the difference in dissolution yield.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-246447 (URN)10.1016/j.jnucmat.2019.01.023 (DOI)000461048800029 ()2-s2.0-85062038198 (Scopus ID)
Note

QC 20190319

Available from: 2019-03-19 Created: 2019-03-19 Last updated: 2019-11-15Bibliographically approved
3. High Surface Conversion UO2 - Leaching Experiments as Model for Long-term Leaching of Spent Nuclear Fuel Under Deep Repository Conditions
Open this publication in new window or tab >>High Surface Conversion UO2 - Leaching Experiments as Model for Long-term Leaching of Spent Nuclear Fuel Under Deep Repository Conditions
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-263302 (URN)
Note

QC 20191209

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2019-12-09Bibliographically approved
4. H2O2 Induced Dissolution of (U1–xGdx)O2 Pellets in Aqueous Solution – Impact of H2 and Consecutive Exposures
Open this publication in new window or tab >>H2O2 Induced Dissolution of (U1–xGdx)O2 Pellets in Aqueous Solution – Impact of H2 and Consecutive Exposures
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-263303 (URN)
Note

QC 20191210

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2019-12-10Bibliographically approved
5. Stability of Studtite in Aqueous Suspension - Impact of HCO3- and Ionizing Radiation on the Dynamics of Dissolution
Open this publication in new window or tab >>Stability of Studtite in Aqueous Suspension - Impact of HCO3- and Ionizing Radiation on the Dynamics of Dissolution
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-263304 (URN)
Note

QC 20191209

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2019-12-09Bibliographically approved
6. Pd‐Catalyzed Surface Reactions of Importance in Radiation Induced Dissolution of Spent Nuclear Fuel Involving H2
Open this publication in new window or tab >>Pd‐Catalyzed Surface Reactions of Importance in Radiation Induced Dissolution of Spent Nuclear Fuel Involving H2
2019 (English)In: ChemCatChem, ISSN 1867-3880, E-ISSN 1867-3899Article in journal (Refereed) Published
Abstract [en]

To assess the influence of metallic inclusions (ϵ‐particles) on the dissolution of spent nuclear fuel under deep repository conditions, Pd‐catalyzed reactions of H2O2, O2 and UO22+ with H2 were studied using Pd‐powder suspensions. U(VI) can efficiently be reduced to less soluble U(IV) on Pd‐particles in the presence of H2. The kinetics of the reaction was found to depend on the H2 partial pressure at pH2≤5.1×10−2 bar. In comparison, the H2 pressure dependence for the reduction of H2O2 on Pd also becomes evident below 5.1×10−2 bar. Surface bound hydroxyl radicals are formed as intermediate species produced during the catalytic decomposition of H2O2 on oxide surfaces. While a significant amount of surface bound hydroxyl radicals were scavenged during the catalytic decomposition of H2O2 on ZrO2, no scavenging was observed in the same reaction on Pd. This indicates a different reaction mechanism for H2O2 decomposition on Pd compared to metal oxides and is in contrast to current literature. While Pd is an excellent catalyst for the synthesis of H2O2 from H2 and O2, a similar catalytic activity that was previously proposed for ZrO2 could not be confirmed.

Place, publisher, year, edition, pages
John Wiley & Sons, 2019
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-261213 (URN)10.1002/cctc.201901128 (DOI)000494251600016 ()2-s2.0-85073687840 (Scopus ID)
Note

QC 20191004

Available from: 2019-10-03 Created: 2019-10-03 Last updated: 2019-12-04Bibliographically approved

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Maier, Annika Carolin

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