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Near field immobilization of selenium oxyanions
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
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.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:51
Keyword [en]
Se-79, Selenium, Immobilization, Redox, Iron, Corrosion, Palladium, Catalyst, Spent Nuclear Fuel, Repository
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-26994ISBN: 978-91-7415-819-9 (print)OAI: oai:DiVA.org:kth-26994DiVA: diva2:375080
Public defence
2010-12-17, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20101208Available from: 2010-12-08 Created: 2010-12-01 Last updated: 2010-12-08Bibliographically approved
List of papers
1. Can redox sensitive radionuclides be immobilized on the surface of spent nuclear fuel? - A model study on the reduction of Se(IV)(aq) on Pd-doped UO2 under H-2 atmosphere
Open this publication in new window or tab >>Can redox sensitive radionuclides be immobilized on the surface of spent nuclear fuel? - A model study on the reduction of Se(IV)(aq) on Pd-doped UO2 under H-2 atmosphere
2009 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 392, no 3, 505-509 p.Article in journal (Refereed) Published
Abstract [en]

Spent nuclear fuel contains noble metal particles composed of fission products (Pd, Mo, Ru, Tc, Rh and Te, often referred to as ε-particles). Studies have shown that these particles play a major role in catalyzing oxidative dissolution as well as H2 reduction of the oxidized UO2 fuel matrix, depending on the conditions. Thus it is possible that these particles also could have a major impact on the state of other redox sensitive radionuclides (such as the long lived fission product 79Se) present in spent nuclear fuel. In this study, Pd-doped UO2 pellets are used to simulate noble metal particles inclusions in spent nuclear fuel and the effect on dissolved selenium in the form of selenite (250 μM selenite) in simulated ground water solution (10 mM NaCl, 10 mM NaHCO3) at 1 and 10 bar hydrogen pressure. The selenite was found to be reduced to elemental Se, forming colloidal particles. At hydrogen pressures of 10 bar, the rate of selenite reduction was found to be linearly correlated to the fraction of Pd in the UO2 pellets. No selenium was detected on the surface of the pellets. For the lowest Pd loading (0.1% Pd) the selenite reduction does not appear to proceed to completion indicating that the surface becomes less active.

Identifiers
urn:nbn:se:kth:diva-18772 (URN)10.1016/j.jnucmat.2009.04.019 (DOI)000269963500019 ()2-s2.0-67651005007 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. A study on the immobilization of selenium oxyanions by H2/Pd(s) in aqueous solution: Confirmation of the one-electron reduction barrier of selenate
Open this publication in new window or tab >>A study on the immobilization of selenium oxyanions by H2/Pd(s) in aqueous solution: Confirmation of the one-electron reduction barrier of selenate
2010 (English)In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 116, no 1-4, 16-23 p.Article in journal (Refereed) Published
Abstract [en]

Selenium is a trace element of concern in several geochemical contexts, due to the potentially high mobility of the selenium oxyanions and the narrow range between deficiency and toxicity of the element. For high level nuclear waste repositories the long-lived fission product 79Se has been identified as a potential key dose contributor for the long-term safety. This paper deals with the catalytic effect of Pd(s) on the H2 reduction of selenium oxyanions which was studied experimentally in aqueous solutions containing bicarbonate and chloride. Pd-catalysts and hydrogen have been proposed for the remediation of various groundwater contaminants and can also serve as a model substance for catalytic noble metal inclusions present in spent nuclear fuel. In this study selenite (SeO32) was found to adsorb on Pd. In the presence of hydrogen the rate of selenite removal increased yielding elemental Se. However, no adsorption or reduction of selenate (SeO42) was observed. A simple radiation chemical experiment revealed a notable barrier towards stepwise one-electron reduction of selenate to selenite. This provides an explanation for the lower reactivity of selenate in systems where reductive immobilization of selenite as well as selenate is thermodynamically favorable.

Keyword
Selenium, Immobilization, Redox, Hydrogen, Palladium, Catalyst
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-26990 (URN)10.1016/j.jconhyd.2010.04.007 (DOI)000280532600002 ()2-s2.0-77955661068 (Scopus ID)
Note
QC 20101208Available from: 2010-12-01 Created: 2010-12-01 Last updated: 2017-12-12Bibliographically approved
3. Immobilization of selenate by iron in aqueous solution under anoxic conditions and the influence of uranyl
Open this publication in new window or tab >>Immobilization of selenate by iron in aqueous solution under anoxic conditions and the influence of uranyl
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.

Identifiers
urn:nbn:se:kth:diva-18773 (URN)10.1016/j.jnucmat.2009.04.016 (DOI)000269963500021 ()2-s2.0-67651100645 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Reduction of selenite and selenate on anoxically corroded iron and the synergistic effect of uranyl reduction
Open this publication in new window or tab >>Reduction of selenite and selenate on anoxically corroded iron and the synergistic effect of uranyl reduction
2010 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 406, no 2, 230-237 p.Article in journal (Refereed) Published
Abstract [en]

In this work the uptake of dissolved selenium in the form of selenite and selenate on iron covered by amagnetite corrosion layer and the effect of dissolved uranyl on this process is studied. Under simulatedanoxic groundwater conditions (bicarbonate and chloride solutions) no immobilization of selenate was found in the absence of uranyl whereas selenite was immobilized, albeit slowly. When uranyl was present, it was found to become reduced and greatly enhanced the rate of reductive immobilization of the dissolved selenite as well as selenate. The increased rate of immobilization is proposed to be due to the oxidative transformation of the passifying magnetite layer as the uranyl is reduced with the observed formation of a transient mixed Fe(II)/Fe(III) oxyhydroxide phase. The iron oxide, and selenium speciation on the surfaces were investigated by Raman spectroscopy. X-ray Absorption Spectroscopy was used to determine the oxidation state of the selenium and uranium on the surfaces.

Keyword
Anoxic groundwater, Chloride solutions, Corrosion layers, Magnetite layers, Oxidation state, Oxidative transformations, Oxyhydroxides, Reductive immobilization, Selenium speciation, Synergistic effect, Chlorine compounds, Dissolution, Groundwater, Iron oxides, Magnetite, Raman spectroscopy, Selenium, Selenium compounds, Uranium, Uranium alloys
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-26993 (URN)10.1016/j.jnucmat.2010.08.031 (DOI)000284303100005 ()2-s2.0-77957864218 (Scopus ID)
Note
QC 20101208Available from: 2010-12-01 Created: 2010-12-01 Last updated: 2017-12-12Bibliographically approved
5. Remobilization of U and Se reductively immobilized on iron oxide surfaces
Open this publication in new window or tab >>Remobilization of U and Se reductively immobilized on iron oxide surfaces
(English)Manuscript (preprint) (Other academic)
Abstract [en]

 

 

 

 

 

 

 

In this work we have studied the possible remobilization of U and Se reductively immobilized on iron oxide surfaces experimentally. Samples consisting of reductively immobilized U and Se (carefully characterized in previous studies) we exposed to aqueous solutions containing HCO3- and HCO3- and H2O2. HCO3- increases the solubility of U(VI) and H2O2 will oxidize U(IV) to U(VI). Exposure to a HCO3- solution revealed a transient increase in the uranium concentration which can probably attributed to detachment of loosely bound UO2 particles upon addition of the new solution. Upon addition of H2O2, the concentrations of U and Se increased fairly rapidly indicating that oxidative dissolution is taking place. The rate of uranium release was well in line with what could be expected from the kinetics of oxidative dissolution of UO2 by H2O2. The Se/U ratio increases with leaching time approaching the overall original composition of the immobilized surface. Hence, oxidative dissolution of the immobilized U and Se is not congruent. Furthermore, the iron oxide surface exposed to the solution after oxidative removal of the immobilized Se and U, appears to be less reactive towards Se and U in solution compared to the original oxide surface. Consequently, the oxidative remobilization process has also affected the iron oxide. These findings suggest that oxidative re-mobilization of reductively immobilized redox sensitive radionuclides is aprocess of potential impact in a deep geological repository for spent nuclear fuel.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-27179 (URN)
Note
QC 20101208Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2010-12-08Bibliographically approved

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