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The mobility of natural uranium at Forsmark, Sweden, through geologic time
Stockholm University, Faculty of Science, Department of Geological Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the response of uranium minerals and poorly crystalline phases to changes in geochemical conditions through geological time has been assessed in order to understand the mobility of natural uranium in the fracture network of a proposed site for a spent nuclear fuel repository in Forsmark, Sweden. Identification and characterization of solid phase uranium have been performed through electron microprobe analysis and optical petrography (Article I). The identified uraninite, haiweeite, and uranophane crystals have been dated using U-Pb and Pb-Pb isotope ratios obtained from secondary ion mass spectrometry and laser ablation-inductively coupled plasma-mass spectrometry (Article II). The mobility of uranium in current Forsmark groundwaters and fracture system has been modelled using the PHREEQC geochemical program and Ra and Rn isotope systematics (Article IV). The rate of submarine groundwater discharge (SGD) from the Forsmark coast to Öregrundsgrepen has also been modelled using Ra isotopes (Article V). Results from these studies support a geologically early (~1200 Ma) oxidation of U(IV) to U(VI). It is further suggested that the old U(VI) minerals present in the bedrock are soluble at the pe values and alkalinities observed in the Forsmark groundwaters. At pe < −4.6 and alkalinity < 60 mg/L, U(VI) can be reduced to U(IV) and deposited in the fracture network. Although a non-negligible rate of SGD has been observed, this cannot be attributed to the discharge of deep (>200 m.b.s.l.) Forsmark groundwaters on the basis of current data.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2016. , p. 28
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 365
Keyword [en]
uranium, radium, mineralogy, geochronology, submarine groundwater discharge, spent nuclear fuel, geochemical modelling
National Category
Geology
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-135735ISBN: 978-91-7649-595-7 (print)ISBN: 978-91-7649-596-4 (print)OAI: oai:DiVA.org:su-135735DiVA, id: diva2:1048463
Public defence
2017-01-10, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: In press. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2016-12-16 Created: 2016-11-21 Last updated: 2016-12-05Bibliographically approved
List of papers
1. Natural uranium in Forsmark, Sweden: The solid phase
Open this publication in new window or tab >>Natural uranium in Forsmark, Sweden: The solid phase
2015 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 59, p. 178-188Article in journal (Refereed) Published
Abstract [en]

U-bearing solid phases from Forsmark, Sweden, a proposed host for radioactive waste repositories, havebeen identified and characterized. Elevated dissolved U was found in some groundwater samples duringthe site investigations, prompting a need to study the local U geochemistry. Previous hydrochemical andwhole-rock geochemical studies indicated that U was derived from local pegmatites, and mobilized andre-deposited during several geological events. In this study, down-hole gamma logs guided sampling oflocal pegmatites, cataclasites, and fracture fillings. Back-scattered electron-imaging, petrographic microscopy,and electron microprobe analyses were used to find and analyze U phases in thin sections. Theresults show that the principal U sources at Forsmark include pegmatitic uraninite (PbO up to22 wt%) and metamict uranothorite. These primary minerals show variable degrees of alteration suchas enrichment in Ca and Al and/or replacement by secondary Ca–U(VI)-silicates, haiweeite and uranophane.The haiweeite contains up to 5 wt% Al2O3, a chemical signature reflecting early (Proterozoic)events of hydrothermal fluid migration. Coffinitized, secondary uraninite is found in association withFeAl-silicates or Palaeozoic sulfide/sulfate minerals, indicating remobilization-precipitation and/or a secondary,sedimentary source of U. It is inferred that U was oxidized during geologically early periods.Later, U(IV) phases formed in fractures open to fluid circulation during the Palaeozoic. This study establishesthe phases available as local U sources and/or sinks, and which will be considered in future isotopicand hydrochemical studies aimed to constrain the mechanisms and timing of water–U phase interaction.

National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-135736 (URN)10.1016/j.apgeochem.2015.04.020 (DOI)
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2017-11-29Bibliographically approved
2. Geochronology of uranium minerals from Forsmark, Sweden
Open this publication in new window or tab >>Geochronology of uranium minerals from Forsmark, Sweden
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(English)Manuscript (preprint) (Other academic)
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-135737 (URN)
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2016-11-24Bibliographically approved
3. The occurrences of Ca2UO2(CO3)3 complex in Fe(II) containing deep groundwater at Forsmark, eastern Sweden
Open this publication in new window or tab >>The occurrences of Ca2UO2(CO3)3 complex in Fe(II) containing deep groundwater at Forsmark, eastern Sweden
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2017 (English)In: Procedia Earth and Planetary Science, ISSN 1878-5220, Vol. 17, p. 440-443Article in journal (Refereed) Published
Abstract [en]

Elevated U concentrations, most evident in a section ~500 mbsl, have been measured in deep Fe(II)-containing groundwater at Forsmark, eastern Sweden and have prompted detailed geochemical and isotopic investigations. The highest U contents (up to 175μg/L) are associated with HCO3 of 120-135 mg/L and Ca2+ of 900-1050 mg/L. Geochemical modelling shows that elevated dissolved U can be stabilized by Ca-uranyl-carbonate complexes. Indeed, time resolved luminescence spectrometry confirmed the Ca2UO2(CO3)30, which is identified in deep reducing groundwater for the first time. U isotopes have been monitored in several sections with high U, and show stable but fracture specific activity ratios (ARs) around 1.5 to 2, although the U concentration varies. This is explained by mobilization of a solid phase with the same AR present in the fracture system close to the sampled sections. The AR >1 in this solid phase indicates a Quaternary age.

Keyword
Ca-uranyl-bicarbonate complex, groundwater, uranium, luminescence spectrometry, 234U/238 1. Introduction U AR, reducing conditions
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-135738 (URN)10.1016/j.proeps.2016.12.111 (DOI)
Conference
15th Water-Rock Interaction International Symposium, Évora, Portugal, 16 - 21 October 2016
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2017-05-18Bibliographically approved
4. Tracing uranium mobility in groundwater of a fractured crystalline bedrock using radium isotopes and geochemical modelling
Open this publication in new window or tab >>Tracing uranium mobility in groundwater of a fractured crystalline bedrock using radium isotopes and geochemical modelling
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(English)Manuscript (preprint) (Other academic)
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-135739 (URN)
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2016-11-24Bibliographically approved
5. Submarine groundwater discharge at Forsmark, Gulf of Bothnia, provided by Ra isotopes
Open this publication in new window or tab >>Submarine groundwater discharge at Forsmark, Gulf of Bothnia, provided by Ra isotopes
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(English)Manuscript (preprint) (Other academic)
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-135740 (URN)
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2016-11-24Bibliographically approved

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