January 10 | Dissertation by Lindsay Krall

The mobility of natural uranium at Forsmark
Sweden through geologic time

Tuesday, 10th January 2017
10h00, Nordenskiöldsalen, GeoBuilding, Stockholm University

Main supervisor:
Lena Z. Evins, Svensk Kärnbränslehantering
Carl-Magnus Mörth, Department of Geological Sciences, Stockholm University
Per Andersson, Naturhistoriska Riksmuseet

David Read, University of Surrey, UK
Abigail Barker, Uppsala University
Iain Pitcairn, Department of Geological Sciences, Stockholm University
Lena Alankangas, Luleå Tekniska Högskola

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

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