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Immobilization of selenate by iron in aqueous solution under anoxic conditions and the influence of uranyl
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
2009 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 392, no 3, 519-524 p.Article in journal (Refereed) Published
Abstract [en]

In proposed high level radioactive waste repositories a large part of the spent nuclear fuel (SNF) canisters are commonly composed of iron. Selenium is present in spent nuclear fuel as a long lived fission product. This study investigates the influence of iron on the uptake of dissolved selenium in the form of selenate and the effect of the presence of dissolved uranyl on the above interaction of selenate. The iron oxide, and selenium speciation on the surfaces was investigated by Raman spectroscopy. X-ray Absorption Spectroscopy was used to determine the oxidation state of the selenium and uranium on the surfaces. Under the simulated groundwater conditions (10 mM NaCl, 2 mM NaHCO3, <0.1 ppm O2) the immobilized selenate was found to be reduced to oxidation states close to zero or lower and uranyl was found to be largely reduced to U(IV). The near simultaneous reduction of uranyl was found to greatly enhance the rate of selenate reduction. These findings suggest that the presence of uranyl being reduced by an iron surface could substantially enhance the rate of reduction of selenate under anoxic conditions relevant for a repository.

Place, publisher, year, edition, pages
2009. Vol. 392, no 3, 519-524 p.
URN: urn:nbn:se:kth:diva-18773DOI: 10.1016/j.jnucmat.2009.04.016ISI: 000269963500021ScopusID: 2-s2.0-67651100645OAI: diva2:336820
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-01-18Bibliographically approved
In thesis
1. Near field immobilization of selenium oxyanions
Open this publication in new window or tab >>Near field immobilization of selenium oxyanions
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The topic of this doctoral thesis is the potential near field immobilization of the radionuclide 79Se after intrusion of groundwater into a spent nuclear fuel canister in a repository. 79Se is a non naturally occurring long lived selenium isotope formed as a result of fission in nuclear fuel. Given the long half life (~3 x 105 y) and that the oxyanions of selenium are expected to be highly mobile and potentially difficult toimmobilize the isotope is of interest for the long term safety assessment of high level waste repositories. In this work the near field has been limited to the study of processes at or near the UO2 surface of (simulated) spent nuclear fuel and to processes occurring at or near the surface of iron (canister material) corroding under anoxic conditions. Selenite (HSeO32-) was found to adsorb onto palladium (simulated noble metal inclusion in spent nuclear fuel). Under hydrogen atmosphere selenite was reduced to elemental selenium with a rate constant of ~2 x 10-9 m s-1 (with respect to the Pd surface, 24 bar H2) forming colloidal particles. The rate constant of selenite reduction was increased by about two orders of magnitude to ~2.5 x 10-7 m s-1 (with respect to the Pd surface, 10 bar H2) for a UO2 surface doped with Pd particles, indicating that UO2 is an efficient co-catalyst to Pd. Selenate (SeO42-) was neither adsorbed nor reduced in the presence of Pd, UO2 and hydrogen. In the iron corrosion studies selenate was found to become reduced to predominantly elemental Se in the presence of a pristine iron surface. Iron covered by a corrosion layer of magnetite did however appear inert with respect to selenate whereas selenite was reduced. The reduction of dissolved uranyl into UO2 by the corroding iron surfaces was found to significantly increase the removal rate of selenite as well as selenate. The uranyl was found to transiently transform the outer iron oxide layers on the iron, forming a reactive mixed Fe(II)/Fe(III) oxyhydroxide (Green rust). Exchanging the solution and increasing the carbonate content (from 2 mM to 20 mM NaHCO3) only resulted in a minor, transient remobilization of uranium. Addition of H2O2 did however result in a significant release of uranium as well as selenium from the iron oxide surfaces. An irradiation experiment was also performed confirming the one electron reduction barrier of selenate as an important factor in systems where selenate reduction would be thermodynamically favorable.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 68 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2010:51
Se-79, Selenium, Immobilization, Redox, Iron, Corrosion, Palladium, Catalyst, Spent Nuclear Fuel, Repository
National Category
Inorganic Chemistry
urn:nbn:se:kth:diva-26994 (URN)978-91-7415-819-9 (ISBN)
Public defence
2010-12-17, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
QC 20101208Available from: 2010-12-08 Created: 2010-12-01 Last updated: 2010-12-08Bibliographically approved

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