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H2O2 and radiation induced dissolution of UO2 and SIMFUEL in HCO3- deficient aqueous solution
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-0596-0222
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-0663-0751
2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 443, no 1-3, 291-297 p.Article in journal (Refereed) Published
Abstract [en]

Understanding UO2 matrix dissolution is of major importance for the safety assessment of a future deep geological repository. Oxidative dissolution of UO2 and SIMFUEL pellets have been extensively studied in HCO3- solutions, while less is known about systems with no or very low HCO3- concentrations. The aim of this work is to elucidate the oxidative dissolution of UO2 and SIMFUEL pellets in HCO3- free solutions by studying the dissolution of U (VI) and consumption of H2O2 over time. The results are compared with previous experiments performed in HCO3- solutions. The oxidative dissolution rate is higher for the UO2 pellet in HCO3- compared to the other systems. It is evident that the kinetics of the reaction with H2O2 is qualitatively different for SIMFUEL in comparison with pure UO2. For the UO2 pellet in pure water, the presence of a secondary phase (meta) studtite, on the surface of the pellet is confirmed by Raman spectroscopy. The kinetic impact of the secondary phase is evaluated in separate UO2 powder experiments. The (meta) studtite (surface) precipitation leads to a slower release of uranium into the solution. Numerical simulations using experimentally determined rate constants are used to evaluate a simple mechanism of surface precipitation. The numerical results are in fair agreement with the experimental observations given certain criteria. In addition, the γ-radiation induced dissolution of UO 2 and SIMFUEL pellets were investigated in pure water, and compared with HCO3- systems. Also here the dissolution rate of uranium is higher for UO2 in HCO3- compared to pure water, while for SIMFUEL longer irradiation times are needed to observe any difference between pure and HCO3- containing water.

Place, publisher, year, edition, pages
Elsevier, 2013. Vol. 443, no 1-3, 291-297 p.
Keyword [en]
Deep geological repository, Matrix dissolutions, Numerical results, Oxidative dissolution, Powder experiment, Radiation-induced, Safety assessments, Surface precipitation
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-95825DOI: 10.1016/j.jnucmat.2013.07.025ISI: 000327905800040Scopus ID: 2-s2.0-84882789875OAI: oai:DiVA.org:kth-95825DiVA: diva2:529352
Funder
Swedish Radiation Safety Authority
Note

QC 20131115. Updated from manuscript to article in journal.

Available from: 2012-05-30 Created: 2012-05-30 Last updated: 2017-12-07Bibliographically approved
In thesis
1. 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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Dahlgren, BjörnJonsson, Mats

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