Radiochim. Acta 98, 743–747 (2010) / DOI 10.1524/ract.2010.1776 © by Oldenbourg Wissenschaftsverlag, München
Understanding uranium behaviour at the Askola uranium mineralization By L. Jokelainen1 , ∗, M. Markovaara-Koivisto2 , D. Read3 , A. Lindberg4 , M. Siitari-Kauppi1 and K.-H. Hellmuth5 1 2 3 4 5
University of Helsinki, P.O. Box 55, 00014 Helsinki University, Finland Helsinki University of Technology, P.O. Box 6200, 02015 TKK, Finland Enterpris, The Old Library, Lower Shott, Leatherhead Road, Great Bookham, Surrey, UK Geological Survey of Finland, Betonimiehenkuja 4, 02150 Espoo, Finland Radiation and Nuclear Safety Authority, P.O. Box 14, 00881 Helsinki, Finland
(Received October 14, 2009; accepted in revised form March 8, 2010)
Uranium / Rock alteration processes / Uranium migration Summary. Understanding the behaviour of uranium is essential when assessing the safety of a spent nuclear fuel repository. The geochemical behaviour of uranium, including its reactive transport chemistry, is also a matter of concern when assessing the environmental impact of uranium mining. Subsurface uranium mobility is believed to be primarily controlled by dissolution and (co)-precipitation of uranium mineral solids and adsorption to mineral surfaces. This paper describes a modelling exercise based on characterisation of samples taken from drilled cores at the uranium mineralization at Askola, Southern Finland. In the modelling exercise, current conditions are assumed to be oxidizing and saturated with groundwater. PHREEQC was used for modelling in conjunction with the Lawrence Livermore National Laboratory database, chosen for its extensive coverage of uranium species and mineral phases. It is postulated that weathering processes near the surface have led to uranium dissolution from the primary ore, leaching out from the matrix and migrating along water-conducting fractures with subsequent re-diffusion into the rock matrix. Electron microscopy studies show that precipitated uranium occupies intra-granular fractures in feldspars and quartz. In addition, secondary uranium was found to be distributed within goethite nodules as well as around the margins of iron-containing minerals in the form of silicate and phosphate precipitates. Equilibrium modelling calculations predict that uranium would be precipitated as uranyl silicates, most likely soddyite and uranophane, in the prevailing chemical conditions beneath Lakeakallio hill.
pository will eventually become anoxic as O2 is consumed. However the precise time required for O2 depletion is not known [3, 4]. It has been reported to be in the range 7–280 a in the vicinity of the copper canisters [5], but there is a lack of reliable data concerning the bedrock around the bentonite buffer to support these estimates. Also early failure of the engineered barrier system could lead to direct interaction between the uranium and groundwater conditions still being oxidizing [6]. Furthermore several studies indicate that glacial melt-waters containing large quantities of dissolved O2 may intrude into the bedrock due to the high hydrostatic pressure under an ice sheet [7–9]. Thus naturally occurring accessory phases, including uranium minerals, need to be considered when evaluating this issue. Several studies [10, 11] have shown that uranium is precipitated as secondary uranium phases, e.g. uranophane, in oxic conditions. In this work, uranophane and other uranium silicates have been identified near a shallow uranium mineralization at Askola, Southern Finland [12]. The groundwater at the study site contains hexavalent uranium which originates from weathering of the primary uraninite ore and/or from dissolved secondary uranium phases. The aim of this study is to determine the conditions under which the mobile hexavalent uranium has precipitated as secondary uranium phases using drill cores from the Askola site. These conditions are reproduced by geochemical modelling using PHREEQC [13] and analytical data from the Askola mineralization. Supplementary groundwater data were obtained from the archives of the Geological Survey of Finland (GTK).
1. Introduction The concept for disposal of spent nuclear fuel in Finland is based on a deep underground repository. It is being excavated 400 m into granitic bedrock, which is regarded as sufficient depth to provide a stable environment [1, 2] over the long term (> 105 a) and to prevent radionuclide transfer to the biosphere. When deposition of the spent fuel is complete, drilled tunnels and cavities will be sealed and conditions in the re*Author for correspondence (E-mail:
[email protected]).
2. Materials and methods 2.1 Drill cores The uranium mineralization at Lakeakallio in Askola, Southern Finland was found in 1950 and a small-scale open cast exploration mine was established there in 1957–1958. According to exploration made by Geological Survey of Finland in late 1970’s, including several rock core drillings and geophysical measurements, the uranium mineralization exists in small, meter-scale lenses. Its host rock is granite and
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Table 1. Mineralogical composition (wt. %) of the drill core samples from Askola uranium mineralization. Minerals are point-counted from thin sections using 750 points. Mineral
Quartz K-feldspar Oligoclase Biotite Muscovite Sericite Chlorite Zircon Hydroxylapatite Pyrite Chalcopyrite Goethite Opaques Clay minerals
Sample/depth 303/15.55 m 303/19.25 m 311/21.45 m SiO2 KAlSi3 O8 (Na,Ca)(Si,Al)4 O8 K(Mg,Fe)3 (AlSi3 O10 )(OH)2 KAl2 (Si3 Al)O10 (OH,F)2 (Mg,Fe,Al)6 (Al,Si)4 O10 (OH)8 ZrSiO4 Ca5 (PO4 )3 (OH) FeS2 CuFeS2 FeOOH
Rock type
42.5 23.8 5.4 0.4 0.1 22.1 2 0.1 0.1 n.f. n.f. 0.1 2.9 0.5
40.9 19.7 8.5 4 0.8 25 0.4 + + + + n.f. 0.1 0.4
22.1 65.4 2.5 1.6 0.1 2.9 + + + + + n.f. 2.8 2.4
Alkali feldspar granite
Granite
Alkali feldspar granite
+ = optical observed; n.f. = minerals not found in the sample
pegmatite veins, usually garnet bearing. Other rock types in the area are migmatitic mica gneiss and hornblende gneiss. A local fracture zone is cutting all the rock types, but it is not controlling the behaviour of uranium. The main minerals of radioactive rock are K-feldspar, plagioclase (albite-oligoclase), quartz, biotite and garnet. Accessory minerals in the radioactive rock are monazite, zircon, uraninite, molybdenite and pyrite. These minerals have a clear tendency to occur together at grain boundaries and in micro fissure. Grains and accumulations of grains are often connected by micro fissures. Micro fissures going out from uraninite (in a pattern like radiation) are partly filled with secondary uranium minerals. Some carbonate in micro fractures might originate from alteration processes having affected plagioclase and causing chloritisation [14, 15]. For this study rock samples were taken from borehole 303 at lengths of 15.55 and 19.25 m and from borehole 311 at a length of 21.45 m. These inclined borehole lengths correspond to vertical depths of 10.49, 12.98 and 15.59 m, respectively. The mineralogical composition of the drill cores was determined by point counting using 750 points from three thin sections of the drill cores (Table 1).
Table 2. The median chemical composition of 21 drilled wells at Askola region (data obtained friom GTK). An excerpt of analyses results. Component HCO3 NO3 Ca Mg Na K Fe(tot) Cl SiO2 PO4 SO4 Al I U
Median (mg/L)
Std. deviation
128.0 0.2 14.05 6.3 34.05 2.4 0.05 25.6 11.7 0.02 17.9 0.02 0.009 0.02
75.4 0.1 16.5 5.3 26.3 4.8 2.6 19.9 5.1 0.003 13.6 0.09 0.005 0.1
Table 2, with the local bulk minerals present in the uranium mineralization (Table 1). The aim of this procedure was to approximate the actual pore water composition, which is different from the drilled wells median water composition.
2.2 Groundwater Askola area groundwater data were used as representative of groundwater present at the site. The data were collected from 21 private drilled wells. The median chemical composition of the well waters is presented in Table 2. The median pH was 7.7, E h = 0.250 V (derived from Fe2+ /Fe3+ ratio), temperature 10 ◦ C and alkalinity 2.1 mmol/dm3 . Since the exact groundwater data from the actual boreholes was not available, the initial solution composition representing the pore water had to be computed with PHREEQC. The composition of the initial solution was calculated by equilibrating the median water, with the composition presented in
2.3 Analysis The chemical composition of uranium minerals found in the samples was studied using electron microscopy and EDAX or EPMA. XRD analyses were performed on mineral grains from the fracture surfaces. However, the uranium phases inside the rock matrix filling the micro fissures and grain boundaries require more sophisticated analytical techniques for example µXRD and µXRF which were not available. SEM-imaging was carried out using a HITACHI S-4800 and chemical analyses by the EDAX attachment (Oxford instruments + INCA program for analyses) at the University
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Fig. 1. A conceptual model of the PHREEQC exercise.
of Helsinki. The accelerating voltage used in the SEM studies was 20 kV with a beam current of 10 µA. With these settings, beam size in the EDAX analyses was 8 µm. The sensitivity limit in EDAX is around 1000 ppm, (0.1%) depending on the element being analysed. EPMA-analyses were conducted at the Geological Survey of Finland with a Cameca SX100 instrument. The analyses were made with 15 keV acceleration voltage and 10–25 nA beam current.
2.4 Modelling The computations were made using PHREEQC for Windows version 2.16.03 with the built-in database llnl.dat taken from “thermo.com.V8.R6.230” prepared by Jim Johnson at Lawrence Livermore National Laboratory [13]. Before reducing minerals, pyrite and chalcopyrite, were introduced to the system the water was first equilibrated with the non-redox sensitive minerals present in Table 1, and the resulting initial solution was then allowed to react with pyrite and chalcopyrite (see Fig. 1). This arrangement describes the postulated recent evolution of the Askola mineralization, where hexavalent uranium ions in the groundwater are assumed to diffuse slowly into the rock matrix subsequently interacting with reducing minerals at a local scale. A similar procedure was used to study the effect of goethite in the drill core DH 303 15.55 m. To model the possible precipitation of hexavalent uranium in to the rock matrix, the concentration of uranium in the initial solution was increased progressively from 0.022 to 22 mg/dm3 over the pH range from 4 to 9. The variation of both pH and uranium concentration was selected because the true values of them from the test samples’ boreholes were not available. In the modelling exercise, the conditions in water conducting fractures were assumed to be oxic due to the shallow nature of the system. In the rock matrix, the observed presence of pyrite and chalcopyrite indicates the anoxic conditions. The modelling exercise was considered to be a closed system batch type experiment. The equilibrium be-
tween groundwater and the principal mineral phases was also assumed owing to the slow movement of the groundwater away from conductive fractures.
3. Results and discussion The samples DH 303 15.55 and 19.25 m are granites with nearly identical mineralogical composition (see Table 1). The difference is found in the accessory minerals: chalcopyrite and pyrite grains are abundant and associated with uranium phases in rock core 19.25 m, particularly in the 1 cm wide zone adjacent to water conducting fracture (WCF), whereas uranium was found with goethite nodules adjacent to the WCF and micro-fractures with calcite, in core 15.55 m. The uranium content is high due to co-precipitation (> 50 wt. %) at the rim of the goethite nodules next to the WCF. Deeper in the rock matrix, uranium content is lower (12 wt. %) and no enrichment at the outer rim of goethite was observed. The sample from another borehole, DH 311 21.45 m, is a coarse-grained alkali feldspar granite. Uranium occurs as a silicate surrounding both pyrite and, to a lesser extent, chalcopyrite and as an oxide or a hydroxide. Microfractures adjacent to WCF are filled with uranium silicate precipitates or a mixture of uranium silicate and pyrite. The sample was less altered than the DH 303 rock cores. No primary uranium phases could be found in SEM-studies in any of the three rock sample specimens. Fig. 2 shows the distribution of uranium in the sample drill core DH 303 19.25 m. Location A is in contact with the WCF, while B is at a depth of 1 mm and C at a depth of 13.4 mm from the WCF. In locations A and B, pyrite crystals are surrounded by secondary uranium precipitates but no uranium precipitates were found with pyrite in location C. The abundance of microcrystalline pyrite is an indicator of redox front progression from the WCF [8]. SEM/EDS analysis revealed that the uranium secondary phases around pyrite crystals are composed of uranium, silica and calcium, in stoichiometric proportions indicative of uranophane (ura-
Fig. 2. Pyrite crystals in sample drill core DH 303 19.45 m. Pyrite crystals are (A) in contact, (B) 1 mm away and (C) 15 mm away from WCF.
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Fig. 3. Saturation indices of soddyite and uranophane as a function of pH and uranium concentration.
Fig. 4. Saturation indices of uraninite and soddyite at equilibrium with chalcopyrite.
Fig. 5. Saturation indices of uraninite and soddyite when initial solution was equilibrated with pyrite.
nium content has been analysed to be higher than 50 wt % in these silicate phosphate mixtures). This is in agreement with the earlier studies [9]. The small proportion of phosphorus, thorium and REE found in the WDS analysis indicate the presence of monazite ((Ce,La,Nd,Th)PO4 ) adjacent to uranium silicate. Monazite and zircon are common accessory minerals in the samples. Uranium silicate fills micro-fissures and grain boundaries around biotite at a depth of 13.4 mm from the WCF. The uptake of uranium was enhanced by pyrites adjacent to the WCF but was not significant with the grains deeper in the rock. Modelling with PHREEQC was first carried out for the sample DH 303 15.55 m by equilibrating the initial solution with goethite. The model indicates that soddyite and uranophane are potential uranium secondary phases. Fig. 3 illustrates the saturation indices of soddyite and uranophane as a function of pH and concentration of uranium. Since the drill cores DH 303 and DH 311 from depths of 19.25 and 21.45 m, respectively contained trace amounts of pyrite and chalcopyrite, the initial solution was equilibrated with these minerals in two separate models. Figs. 4 and 5 show the saturation indices of selected uranium phases when the initial solution was equilibrated with chalcopyrite and pyrite, respectively. Fig. 4 shows the interesting behaviour of the model when the initial solution was equilibrated with chalcopyrite. When uranium concentration was increased to 22 mg/L, the E h of the system decreased below zero indicating reducing conditions. In reducing conditions a rise in saturation index was
found in the model for uraninite and the precipitation of U(IV) phase uraninite could occur. Similarly the decrease of the E h affects the saturation index of soddyite (U(VI) phase) decreasing it to have negative value, which does not promote precipitation. Because all other parameters were untouched in the model, the drop in the E h is supposedly a result of increased uranium concentration. When the initial solution is in equilibrium with pyrite, regardless of the pH only U(IV) phases precipitate and the saturation indices are controlled only by the concentration of uranium (Fig. 5). However, Wersin et al. have established that the reduction of U(VI) to U(IV) by pyrite is not uniform and U(VI) phase is always present [16]. In addition to soddyite, uranophane and coffinite PHREEQC suggests a possible precipitation of a rare uranyl silicate, haiweeite, when the initial solution was equilibrated with goethite, chalcopyrite and pyrite. According to Kienzler et al. [17] haiweeite could exist only at pH 9 and over which is not the case here.
4. Conclusions It is postulated that weathering processes near the surface have led to uranium dissolution from the primary ore, leaching out from the matrix and migrating along waterconducting fractures with subsequent re-diffusion into the rock matrix. Evidence for an alternative model – oxygen diffuses in and leads to in situ oxidation of U(IV) – can-
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not be derived from the presented investigation of the three specimens. SEM analysis confirmed that uranium was precipitated locally around pyrite crystals near the WCF to a depth of 1 cm. Uranium precipitates were also found in the micro-fractures and in contact with biotite to a depth of 2 cm from the WCF. In addition, uranium was co-precipitated with goethite near WCF. If reducing (sulphide) minerals are not present, uranium is precipitated as uranophane and soddyite according to the model, which is in broad agreement with observations and previous studies. However, in the presence of pyrite, equilibrium modelling calculations suggest reduction of U(VI) to U(IV) and precipitation as the U(IV) phase uraninite. Reduction of uranium and precipitation as uraninite is contradictory to the SEM and WDS analysis results, which indicate that the precipitated phase is uranophane (U(VI)). Reduction of U(VI) to U(IV) is known to be heterogeneous especially at high uranium concentrations when significant amounts of unreduced uranyl precipitates on the pyrite surface. Furthermore, the pyrite oxidation is strongly influenced by pH. At pH > 6 oxidation leads to precipitation of Fe(III), which in turn results in decreased reduction of uranium(VI) present [16]. This might explain the mechanisms, which makes the precipitation of U(VI) silicates around the reducing pyrite crystals possible. More detailed analyses (µXRD, µXRF, LA-ICP-MS) are in progress.
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Acknowledgment. Financial support for this study was provided by the Finnish Research Programme on Nuclear Waste Management (KYT). 13.
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