Börje Torstenfelt

Sorption of strontium and cesium on rocks and minerals

Sorption, rock mineral, The sorption of strontium and cesium on some common Swedish igneous rocks (granite, gneiss and diabase) and eleven rock-forming and accessory minerals (quartz, orthoclase, biotite, muscovite, hornblende, magnetite, hematite, fluorite, calcite, apatite and serpentine) has been studied using batch techniques. The solid sorbents were characterized with respect to mineralogy and cation-exchange capacity. The water phase used in the experiments was a synthetic groundwater with a composition representative of a groundwater from granitic rock. The influence on the sorption of parameters such as pH of, and presence of anions in the water, contact time, nuclide concentration, particle size and liquid/solid ratio was studied. The most important parameters influencing the sorption were the nuclide concentration, the groundwater composition and the mineral composition (e.g., presence of minerals with high cation-exchange capacity and/or high surface/mass ratio).

Possible Retention of Iodine in the Ground

At the eventual release of radionuclides from a deep underground repository for high level waste, the only anionic fission product would be iodine, assuming reducing conditions (1–3). The major species would be I− although leaching experiments on tuff from the Nevada Test Site have indicated that other species may be present (4). The retention of these anionic species would be low in most bedrocks (5–7). In this paper, the sorption of iodide on various minerals and compounds is discussed as well as the prospect of using common geologic media as iodide retarding backfill materials in an underground waste repository.

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.

Actinide Sorption on Granites and Minerals as a Function of pH and Colloids/Pseudocolloids

The sorption of uranium, neptunium and plutonium was studied on three granites Finnsjö granite, Stripa granite and Westerly granite, and on six minerals albite, anorthite, bentonite, hornblende, illite and microcline. The solids were crushed and sieved, and the size fraction 0.106-0.250 mm was used. Prior to use, the particles were washed with distilled water. The aqueous phase was a natural groundwater collected at the Nevada Test Site (NTS). This water is used as reference water for the U. S. Nevada Nuclear Waste Storage Investigations (NNWSI). The phase separation was done by centrifugation only. An ultracentrifuge with a maximum speed of 65,000 rpm was used for the separation, and three samplings were performed at each pH studied. One sampling with no centrifugation at all, one sampling after an 1 hour centrifugation at 20,000 rpm, and a third sampling where aliquots were withdrawn after centrifugation at 60,000 rpm for ca. 2.3 hours. At 60,000 rpm, the centrifugal force was sufficient for the removal of granite particles larger than ca. 1 nm. At neutral pH there was only a minor difference in sorption between the different methods of separation. Below a pH resulting in hydrolysis of the nuclide, the sorption was low. Uranium and especially neptunium sorbed significantly more at high pH, as compared to the sorption at neutral pH. High pH had only little influence on the sorption of plutonium on the minerals with a high cation exchange capacity (CEC); hornblende, bentonite and illite, but on the feldspars albite, anorthite and microcline, with low CEC, there was a pronounced difference between neutral and high pH. For uranium at high pH, the sorption increased significantly with increasing centrifugal force, indicating that part of the uranium either exists as a colloid or as a polymer. This effect was only in a few cases observed for neptunium and plutonium.

Sorption of Some Fission Products and Actinides in Concrete Systems

The sorption of some actinides (Th, U, Np, Pu and Am) and fission products (I, Cs) was measured on two types of Standard Portland cements as well as on samples from old (70 years) hydro power dam constructions using a batch technique. Pore water compositions were analysed, and artificial pore water solutions were used as aqueous phases in the experiments. Measurements were also performed on five other concrete types (not reported in detail in this paper) to illustrate the effects of the cement matrix composition on the sorption behaviour of the radionuclides. The sorption of actinides in the trivalent (americium), tetravalent (thorium) pentavalent (neptunium) and hexavalent (uranium) states was high in all the studied concrete systems. Generally, the sorption of cesium was low due to the low exchange capacity of the cement and the high concentration of competing cations in the pore waters. The sorption of iodine was much higher than in most silicate minerals of geologic origin. The differences between the various concrete systems were generally minor in terms of their sorbing capacities.

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.

Migration of trace elements into water-exposed natural fissure surfaces of granitic rock

Abstract The migration of Cs, Am and Tc at trace concentration levels (10−6 or 10−8M) into granitic or granodioritic rock through water-exposed natural fissure surfaces has been studied. The rock samples were taken from drilling cores (depths from 72 to 526 m) obtained from three locations in Sweden (Stripa, Studsvik and Finnsjon). The element concentration vs. depth in the rock samples was measured radiometrically, after exposing the rock samples to spiked groundwaters for a period of 3–12 months and then slicing the samples using a grinding procedure. The sorption on individual mineral grains as well as migration pathways into the rock were determined from autoradiographic studies and compared with sorption data from batch-wise studies. Cesium (as Cs+) exhibited a high apparent diffusivity (∼ 10−13 m2 s−1) in all the studied samples. Veins or large grains of high-capacity minerals appeared to be the major transport pathways into the rock. Am (as a mixture of hydroxy and carbonate complexes) was largely sorbed on the exposed rock surface with only minor migration into the rock during the exposure time (3 months). The mobility of technetium (as TcO−4) was similar to that of Cs. However, the migration mechanism was different, as indicated by large differences in sorption properties and effective diffusivities.

The Retention of Redox Sensitive Waste Elements in Compacted Bentonite

The diffusion of technetium, iodine, uranium and neptunium in compacted bentonite has been studied. The possible reduction of the transport rate of these elements (i.e. redoxsensitive elements) by mixing the clay with metallic iron (for technetium, uranium and neptunium) or by adding a chemisorbent (for iodine) to the clay is reported. Technetium has an apparent diffusivity about 5 times higher in the heptavalent state (TcO/sub 4//sup -/) than in the tetravalent state (TcO(OH)/sub 2/ or TcO/sub 2/), uranium and neptunium in their higher oxidation state (VI and V) have apparent diffusivities about 6 and 50 times higher, respectively, than in the tetravalent state. Iodine, as I/sup -/ (or IO/sub 3//sup -/), has a transport rate more than one order of magnitude lower than TcO/sub 4//sup -/. 10 references, 5 figures, 3 tables.

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.