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Experimental studies of radiation-induced dissolution of UO2: The effect of intrinsic solid phase properties and external factors
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry. (Nuclear Chemistry)ORCID iD: 0000-0003-0552-6282
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dissolution of the UO2 matrix is one of the potential routes for radionuclide release in a future deep geological repository for spent nuclear fuel. This doctoral thesis focuses on interfacial reactions of relevance in radiation-induced dissolution of UO2 and is divided in two parts:

In the first part, we sought to explore the effects of solid phase composition:

The impact of surface stoichiometry on the reactivity of UO2 towards aqueous radiolytic oxidants was studied. H2O2 reacts substantially faster with stoichiometric UO2 than with hyperstoichiometric UO2. In addition, the release of uranium from stoichiometric UO2 is lower than from hyperstoichiometric UO2. The behavior of stoichiometric powder changes with exposure to H2O2, approaching the behavior of hyperstoichiometric UO2 with the number of consecutive H2O2 additions.

The impact of Gd-doping on the oxidative dissolution of UO2 in an aqueous system was investigated. A significant decrease in uranium dissolution and higher stability towards H2O2 for (U,Gd)O2 pellets compared to standard UO2 was found.

In the second part, we sought to look at the effect of external factors:

The surface reactivity of H2 and O2 was studied to understand the overall oxide surface reactivity of aqueous molecular radiolysis products. The results showed that hydrogen-abstracting radicals and H2O2 are formed in these systems. Identical experiments performed in aqueous systems containing UO2 powder showed that the simultaneous presence of H2 and O2 enhances the oxidative dissolution of UO2 compared to a system not containing H2.

The effect of groundwater components such as bentonite and sulfide on the oxidative dissolution of UO2 was also explored. The presence of bentonite and sulfide in water could either delay or prevent in part the release of uranium to the environment. The Pd catalyzed H2 effect is more powerful than the sulfide effect. The poisoning of Pd catalyst is not observed under the conditions studied.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , 77 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2017:23
Keyword [en]
Oxidation, dissolution, uranium dioxide, gadolinium, bentonite, sulfide, hydrogen, gamma radiation, radiolysis, hydrogen peroxide, hydroxyl radical, repository
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-205605ISBN: 978-91-7729-343-9 (print)OAI: oai:DiVA.org:kth-205605DiVA: diva2:1089678
Public defence
2017-05-12, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170421

Available from: 2017-04-21 Created: 2017-04-20 Last updated: 2017-04-21Bibliographically approved
List of papers
1. Inhibition of radiation induced dissolution of UO2 by sulfide-A comparison with the hydrogen effect
Open this publication in new window or tab >>Inhibition of radiation induced dissolution of UO2 by sulfide-A comparison with the hydrogen effect
2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 434, no 1-3, 38-42 p.Article in journal (Refereed) Published
Abstract [en]

In this work we have studied the influence of H2S on radiation induced dissolution of spent nuclear fuel using simple model systems. The reaction between H2O2 and H2S/HS- has been studied experimentally as well as the effect of H2S/HS - on γ-radiation induced dissolution of a UO2 pellet. The experiments clearly show that the reaction of H2O 2 and H2S/HS- is fairly rapid and that H 2O2 and H2S/HS- stoichiometry is favorable for inhibition. Radiolysis experiments show that H2S/ HS- can effectively protect UO2 from oxidative dissolution. The effect depends on sulfide concentration in combination with dose rate. Autoclave experiments were also conducted to study the role of H 2S/HS- in the reduction of U(VI) in the presence and absence of H2 and Pd particles in anoxic aqueous solution. The aqueous solutions were pressurized with H2 or N2 and two different concentrations of H2S/HS- were used in the presence and absence of Pd. No catalytic effect of Pd on the U(VI) reduction by H2S/HS- could be found in N2 atmosphere. U(VI) reduction was found to be proportional to H2S/HS- concentration in H2 and N2 atmosphere. It is clearly shown the Pd catalyzed H2 effect is more powerful than the effect of H2S/HS-. H2S/HS- poisoning of the Pd catalyst is not observed under the present conditions.

Keyword
Anoxic aqueous solutions, Autoclave experiments, Catalytic effects, Dose rate, Hydrogen effect, Model system, Oxidative dissolution, Pd catalyst, Pd particle, Radiation-induced, Spent nuclear fuels, Sulfide concentration, Experiments, Hydrogen, Radiation chemistry, Radioactive wastes, Solutions, Stoichiometry, Dissolution
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-118121 (URN)10.1016/j.jnucmat.2012.10.050 (DOI)000315752000006 ()2-s2.0-84871742513 (Scopus ID)
Note

QC 20130212

Available from: 2013-02-12 Created: 2013-02-12 Last updated: 2017-04-21Bibliographically approved
2. Effect of bentonite on radiation induced dissolution of UO2 in an aqueous system
Open this publication in new window or tab >>Effect of bentonite on radiation induced dissolution of UO2 in an aqueous system
2014 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 447, no 1-3, 73-76 p.Article in journal (Refereed) Published
Abstract [en]

In order to elucidate the impact of bentonite on the process of radiation induced oxidative dissolution of UO2 in an aqueous system, the dissolution of U(VI) and consumption of H2O2 over time has been studied. In addition, γ-irradiation experiments were performed to study a more relevant and complex system, serving as a comparison with the previously stated system. In both cases, the experiments revealed that the presence of bentonite in water could either delay or prevent in part the release of uranium to the environment. The cause is mainly attributed to the scavenging of radiolytic oxidants rather than to the adsorption of uranium onto bentonite.

Keyword
Spent Nuclear-Fuel, Induced Oxidative Dissolution, Radiolysis Products, Hydrogen-Peroxide, Relative Impact, H2o2, Montmorillonite, Water, Reactivity, Kinetics
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-142319 (URN)10.1016/j.jnucmat.2013.12.012 (DOI)000334134600010 ()2-s2.0-84893123145 (Scopus ID)
Note

QC 20140305

Available from: 2014-03-05 Created: 2014-02-28 Last updated: 2017-04-21Bibliographically approved
3. Surface Reactions of H2O2, H-2, and O-2 in Aqueous Systems Containing ZrO2
Open this publication in new window or tab >>Surface Reactions of H2O2, H-2, and O-2 in Aqueous Systems Containing ZrO2
2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 3, 1609-1614 p.Article in journal (Refereed) Published
Abstract [en]

In radiolysis of water, three molecular products are formed (H2O2, O-2, and H-2). It has previously been shown that aqueous hydrogen peroxide is catalytically decomposed on many oxide surfaces and that the decomposition proceeds via the formation of surface-bound hydroxyl radicals. In this work, we have investigated the behavior of aqueous H-2 and O-2 in contact with ZrO2. Experiments were carried out in an autoclave with high H2 pressure and low O-2 pressure (40 and 0.2 bar, respectively). In the experiments the concentration of H-abstracting radicals was monitored as a function of time using tris(hydroxymethyl)aminomethane (Tris) as scavenger and the subsequent formation of formaldehyde to probe radical formation. The plausible formation of H2O2 was also monitored in the experiments. In addition, density functional theory (employing the hybrid PBE0 functional) was used to search for reaction pathways. The results from the,experiments show that hydrogen-abstracting radicals: are formed in the aqueous H2O2-system in contact with solid ZrO2. Formation of H2O2 is also detected, and the time dependent production of hydrogen-abstracting radicals follows the time-dependent H2O2 concentration, strongly:indicating that the radicals are produced upon catalytic decomposition of H2O2. The DFT study implies that H2O2 formation proceeds via a pathway where HO2 is a key intermediate. It is interesting to note that all the stable molecular products from aqueous radiolysis are precursors of quite intriguing radical reactions at water/oxide interfaces.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
National Category
Engineering and Technology Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-182780 (URN)10.1021/acs.jpcc.5b10491 (DOI)000369116100026 ()2-s2.0-84956691050 (Scopus ID)
Note

QC 20160223

Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2017-04-21Bibliographically approved
4. Can H-2 enhance the oxidative dissolution of UO2?
Open this publication in new window or tab >>Can H-2 enhance the oxidative dissolution of UO2?
2016 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 477, 85-87 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Mineral and Mine Engineering
Identifiers
urn:nbn:se:kth:diva-189349 (URN)10.1016/j.jnucmat.2016.04.059 (DOI)000377327000010 ()
Note

QC 20160707

Available from: 2016-07-07 Created: 2016-07-04 Last updated: 2017-04-21Bibliographically approved
5. Radiation induced dissolution of (U, Gd)O2 pellets in aqueous solution: A comparison to standard UO2 pellets
Open this publication in new window or tab >>Radiation induced dissolution of (U, Gd)O2 pellets in aqueous solution: A comparison to standard UO2 pellets
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this work, the impact of burnable absorbers doping (Gd, 3-8%wt.) on the oxidative dissolution of UO2 in an aqueous system was studied by H2O2 and γ-irradiation induced dissolution experiments. The results showed a significant decrease in uranium dissolution and higher stability towards H2O2 for (U,Gd)O2 pellets compared to standard UO2. The resulting decrease in the final oxidative dissolution yield was mainly attributed to decreased redox reactivity of the UO2-matrix upon doping. During gamma radiation induced experiments, the difference in uranium release was even more pronounced compared to H2O2 induced dissolution experiments.

Keyword
Uranium oxide, dissolution, oxidation, gadolinium, radiolysis, H2O2, gamma radiation, reactivity
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-205629 (URN)
Note

QC 20170421

Available from: 2017-04-20 Created: 2017-04-20 Last updated: 2017-04-21Bibliographically approved
6. Impact of stoichiometry on the reactivity of UO2 towards radiolytic oxidants
Open this publication in new window or tab >>Impact of stoichiometry on the reactivity of UO2 towards radiolytic oxidants
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-205630 (URN)
Note

QC 20170421

Available from: 2017-04-20 Created: 2017-04-20 Last updated: 2017-04-21Bibliographically approved

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