Ulla Olofsson

Sorption of Actinides in Well-Defined Oxidation States on Geologic Media

The long-lived actinides and their daughter products largely dominate the biological hazards from spent nuclear fuel already from some 300 years after the discharge from the reactor and onwards . Therefore it is essential to make reliable assessments of the geochemistry of these elements in any concept for long-term storage of spent fuel or reprocessing waste, etc.

Humic Substances in Ground Waters

The presence of naturally occurring complexing agents that may enhance the migration of disposed radionuclikes and thus facilitate their uptake by plantsis a problem associated with the underground disposal of radioactive wastes in bedrock. The main purpose of this work is to characterized humic substances from ground water and compare them with humic substances from surface water. The humic materials isolated from ground waters of a borehole in Fjaellveden (Sweden) were characterized by elemental and functional group analyses. Spectroscopic properties, molecular weight distributions as well as acid-base properties of the fulvic and humic fractions were also studied. The ground water humic substances were found to be quite similar in many respects (but not identical) to the Swedish surface water humics concentrated from the Goeta River but appeared to be quite different from the American ground water humics from Biscayne Florida Aquifer or Laramie Fox-Hills in Colorado. The physico-chemical properties of the isolated humic materials are discussed.

Transport of Actinides through a Bentonite Backfill

Compacted bentonite has been proposed as a suitable backfill material in the Swedish concept for underground storage of high-level waste. The backfill barrier will serve both as a physical barrier, preventing convective water flow and allowing radionuclide penetration only by diffusion, and as a chemical barrier capable of chemically interacting with the radionuclides.

Properties and mobilities of Actinide Colloids in Geologic Systems

The interaction between radionuclides in solution and exposed geologic media, e.g. in connection with underground storage of radioactive waste, is largely determined by the chemical properties of the system. The mobility of radionuclides in contact with low-capacity minerals would depend on the chemical state of the element (e.g. degree of hydrolysis, formation of organic and inorganic complexes, etc.). Particularly for the actinides in their lower oxidation states (III, IV), however, would a formation of colloid particles be feasible in the groundwater environment. These colloids could be either true radiocolloids, i.e. aggregates of the radionuclide itself, or pseudocolloids, i.e. colloid material already present in the groundwater, onto which the radionuclide has sorbed. These colloidal particles may be very poorly sorbed on water exposed geologic media in comparison with radionuclides in true solution. Studies on the formation and properties of some actinide colloids (Am, Np, Pu) are discussed in this paper.

Formation and transport of americium pseudocolloids in aqueous systems

The sorption of americium on colloidal quartz and montmorillonite has been studied using a batch technique. Americium appeared to be distributed between all available surfaces in the system (particles and vessel walls). Distribution coefficients, defined as the ratio of the amount of americium per mass of colloidal matter and the concentration in the solution phase, were estimated to be of the same order of magnitude as obtained in measurements on crushed material of much larger particle sizes. In the presence of sorbents (alumina or granite) the removal of americium from the solution was enhanced, either due to the desorption of the americium from the particle phase and resorption on all available surfaces (sorbent plus vessel wall) or the sorption of the whole colloidal aggregates on the sorbent. No break-through of americium was observed in column experiments (with alumina or granite packing) after the injection of americium pseudocolloids (quartz or montmorillonite) followed by elution with large volumes of aqueous phase (up to 880 column volumes). However, a minor fraction of the americium passed through the column without significant retention, especially under conditions with high flow rate (short hold-up time in the column).

Formation and Properties of Americium Colloids in Aqueous Systems

The formation and sorption properties of colloidal americium in aqueous solutions have been studied with variations of pH (2–12), ionic strength (0.01–1.0 M NaClO 4 ), americium concentration (10 −7 −10 −11 M) and storage time (6 h–6 months). A large fraction of the americium is sorbed on the container walls or on Al 2 O 3 in the pH-range 7–11. Around pH 5–8 and at pH above 12 centrifugable fractions (particle sizes greater than 100 nm) are obtained. The fraction formed at pH above 12, which seems to be a true hydroxide colloid, migrates through an A1 2 O 3 -filled column with very little retention. The average charge of americium species in solution changes from positive values at pH below 7–8 to negative values at pH above 10–11, as indicated from electromigration studies.

Radionuclide Migration into Natural Fracture Surfaces of Granitic Rock

The update of cesium on natural fracture surfaces taken from granitic bedrock has been studied as well as the depth penetration into the underlying rock. The fracture minerals usually exhibit good sorptive properties with respect to cesium (with the exception of calcite). The surface coatings do not act as physical diffusion barriers and do not prevent a migration into the underlying rock. A considerable depth penetration into the rock (several mm over a period of three months) was observed. The dominating pathways into the rock matrix were the high-capacity minerals in grains or in microfissures. The calculated diffusivity in the rock matrix was of the order of 10/sup -13/ m/sup 2//s.

Generation and transport properties of colloidal tri- and tetravalent actinide species in geologic environments

The formation of colloidal particle fractions of tri- and tetravalent actinides (Am(III), Th(IV), Pu) has been studied in aqueous solutions under various conditions. (Variations of nuclide concentration 10/sup -7/-10/sup -11/ M; pH 3-12; salt medium 0.01-1.0 M NaClO/sub 4/; storage time 6 h-6 months; temperature 5/sup 0/C, 25/sup 0/C, 70/sup 0/C). A large fraction of the nuclides is sorbed on the vessel walls, but there is also a formation of centrifugable particles. For americium this fraction has two maxima, at pH 5-8 and at pH above 12. For thorium and plutonium this fraction is very small except at pH about 12. In the pH-range 7-9, which is expected in underground waste repositories, the particle fractions do not seem to be highly mobile. Plutonium behaves very similar to thorium in all studied systems, so it is probably tetravalent to a large extent. 8 references, 5 figures, 1 table.