IAEA COORDINATED RESEARCH PROJECT (CRP): The use of selected safety indicators (concentrations; fluxes) in the assessment of radioactive waste disposal. Report 6: Site-specific natural geochemical concentrations and fluxes at the Palmottu U-Th mineralisation (Finland) for use as indicators of nuc... (original) (raw)
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Uranium Mineral – Groundwater Equilibration at the Palmottu Natural Analogue Study Site, Finland
MRS Proceedings, 1992
The redox-potential, pH, chemical composition of fracture waters, and uraninite alteration associated with the Palmottu uranium mineralization, have been studied. The data have been interpreted by means of thermodynamic calculations. The results indicate equilibrium between uraninite, ferric hydroxide and groundwater in the bedrock of the study site. Partially oxidized uraninite (UO2 33) and ferric hydroxide are in equilibrium with the fresh, slightly acidic and oxidized water type, while primary uraninite is stable with deeper waters that have a higher pH and lower Eh. Measured Eh-pH values of groundwater cluster within a relatively narrow range indicating buffering by heterogenous redox-processes. A good consistency between measured Eh and analyzed uranium oxidation states was observed.
Chemical Geology, 2021
Unusually high uranium (U) concentrations (up to 175 μg/L) have been measured in groundwater at depths between 400 and 650 m at the Forsmark site, eastern Sweden. Since it is unlikely that such high concentrations formed under the stagnant and low redox groundwater conditions that currently prevail, this study employs Useries isotopes to understand how the recent evolution (<1 Ma) of the flow system has influenced the observed U distribution. Material from fractures as deep as 700 m along the assumed flow route was subject to U-series disequilibrium (USD) measurements, as well as sequential extractions (SE) and U redox-state analyses that revealed the U-series activity ratios in the bulk and soluble fraction of the fracture precipitates. Uranium isotope data collected over several years of annual groundwater monitoring were scrutinized to evaluate the U sources and U exchange in fractures located in high-U groundwater sections. Numerical simulations with the experimental data were used to study evolution of U-series isotope composition in a fracture in the highest U section at ~500 m depth under various U mobility scenarios. The results show that U redistribution in fractures with certain dissolution/deposition flux ratios during periodic water intrusions, driven by glaciation and deglaciation events during the last 120 ka, can explain the U anomaly in the groundwater.
2007
Blomqvist, Runar, Juhani Suksi, Timo Ruskeeniemi, Lasse Ahonen, Heikki Niini, Ulla Vuorinen, and Kai Jakobsson. The Palmottu Natural Analogue Project, Summary Report 1992-1994. The behaviour of natural radionuclides in and around uranium deposits, Nr 8. Geological Survey of Finland, Nuclear Waste Disposal Research, Report YST-88, 82 pages. ISBN 951-690-580-3. ISSN 07833555; also Finnish Centre for Radiation and Nuclear Safety, STUK-YTO-TR 84, Helsinki 1995. ISBN 951-712-033-8. ISSN 0785-9325. The Palmottu U-Th mineralization at Nummi-Pusula, southwestern Finland, has been studied as a natural analogue to deep disposal of radioactive wastes since 1988. This report gives a summary of the results of investigations carried out during the years 1992-1994. The Palmottu Analogue Project aims at a more profound understanding of radionuclide transport processes in fractured crystalline bedrock. The essential factors controlling transport are groundwater flow and interaction between water and...
Uranium-Series Studies in Bedrock for the Safety Case of Deep Geological Disposal of Spent Fuel
Materials Research Society Symposium Proceedings, 2008
The possibility of glacial meltwaters disturbing the stable conditions around a nuclear waste repository can be studied by the USD method. The method is based on the perception of uranium isotope mobilisation relative to 230 Th occurring due to past meltwater intrusions. Uranium mobilisation occurs when redox potential increases. In shallow bedrock redox-potential may change under normal recharge. Similar changes probably occur when meltwater penetrates a deeper reducing aquifer. The role of redox conditions in U mobilisation appears dominant, as direct α recoil induced signals seem too weak to be observed for rock and water samples. The USD method can be used to provide support to the Finnish safety case in the area of complementary safety arguments and scenario formulations for transport calculations.
Applied Geochemistry, 2004
Precambrian U ore deposits, such as Oklo, Cigar-Lake or Palmottu, constitute invaluable analogues of nuclear waste repositories that provide direct evidence of U mobility or sequestration, over very large time intervals in geological formations. In this study, pervasive millimeter thick calcite veins filling microfractures in gneiss and granite surrounding the Palmottu U ore (Finland) were analysed, as fingerprints of past fluid circulation through the site. Stable Pb isotopes and short halflife 234 U-230 Th isotopes, all products of the U decay chain, have been chosen to investigate long term (over a Ga) and recent (within the last 500 ka) U migrations, respectively. Lead, U and Th isotopes have been analysed by thermo-ionisation mass spectrometry on mg-size bulk carbonate samples, and ion probe analyses of Pb isotopes performed on microinclusions of pyrite, monazite and coffinite. A striking contrast was found between the preservation of a well defined Pb-Pb isochron (1925 AE70 Ma, MSDW=2.47, N=17) on the one hand, and fractionated 230 Th/ 238 U ratios (230 Th/ 238 U < 1) on the other hand. This shows that although the veins were formed simultaneously to the ore itself, during the Fennoscandian orogeny, and behaved mainly as a closed system for several billion years, U migrations did occur in the last 0.5 Ma. The recent U mobility is probably related to changes in the groundwater circulation through the bedrock. Nowadays, the dispersion of the U is, however, restricted to within a few meters around the U ore. It is shown that complex processes of dissolution and re-adsorption (or re-crystallisation) took place within these tiny carbonate fracture fillings. Relatively unradiogenic 206 Pb/ 204 Pb ratios measured by SIMS in coffinite micro-inclusions demonstrate recent precipitation of this mineral. Coffinite being the main U-bearing phase in the carbonate fracture fillings, its crystallization probably played a major role in U redistribution within the fractures, and in the relatively restricted dispersion of U through the bedrock.
Applied Geochemistry, 2009
Editorial handling by M. Gascoyne a b s t r a c t Oxidizing conditions normally prevail in surface waters and near-surface groundwaters, but there is usually a change to reducing conditions in groundwater at greater depth. Dissolved O 2 originally present is consumed through biogenic and inorganic reactions along the flow paths. Fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front. An important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock, at an approximate depth of 500 m in Sweden, is to demonstrate that reducing conditions can be maintained for a long period of time. Oxygen may damage the Cu canisters that host nuclear waste; additionally, in the event of a canister failure, oxidizing conditions may increase the mobility of some radionuclides. The present study of the near-surface redox front is based on mineralogical (redox-sensitive minerals), geochemical (redox-sensitive elements) and U-series disequilibrium investigations of mineral coatings along open fractures. The fractures have been sampled along drill cores from closely spaced, 100 m deep boreholes, which were drilled during the site investigation work in the Laxemar area, south-eastern Sweden, carried out by the Swedish Nuclear Fuel and Waste Management Co. (SKB). The distribution of the redox-sensitive minerals pyrite and goethite in open fractures shows that the redox front (switch from mainly goethite to mainly pyrite in the fractures) generally occurs at about 15-20 m depth. Calcite leaching by recharging water is indicated in the upper 20-30 m and positive Ceanomalies suggest oxidation of Ce down to 20 m depth. The U-series radionuclides show disequilibrium in most of the samples, indicating mobility of U during the last 1 Ma.
1992
Isotope composition and concentration (δ 2 H, δ 18 O, 234 U/ 238 U, Ar, 40 Ar/ 36 Ar, 3 He/ 4 He and 20 Ne/ 4 He) were measured in the groundwaters (Tomsk-7). Water has a distinct cold climate isotope signature δ 2 H = -127..-140, δ 18 O = -17.0.. -18.2 in contrast of modern meteoric water δ 2 H = -117, δ 18 O = -15.9. Stable isotopes are in concord with noble gas temperature t NGT = 0..+4 o C and helium model ages t = 7-14 ka. Disequilibrium uranium 234 U/ 238 U up to 16 (activity ratio) obtained and explained of 234 U leaching by the melt water during the permafrost degradation.
Redox processes in uranium deposits — Lessons from Natural Analogue Studies
Uranium in the Aquatic Environment, 2002
The measurement and understanding of the processes that control the redox intensities (Eh) and capacities (RDC) ofuranium deposits are critical to assess the long-term stability of nuclear waste repositories, the migration of toxic metals from uranium mine tailings and other contaminated sites. W e present the redox modelling work performed in connection with the geochemical characterisation of five uranium deposits used as Natural Analogue systems: Po~os de Caldas (Brazil), Cigar Lake (Canada), EI Berrocal (Spain), Oklo (Gabon) and Palmottu (Finland). The data from these five sites indicate that Fe(III)/Fe(II) equilibrium controls the redox state of surface and intermediate groundwaters. However, the