natural analogues

Applied Geochemistry 27 (2012) 490–500

Contents lists available at SciVerse ScienceDirect

Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

Real system analyses/natural analogues Ulrich Noseck a,⇑, Eva-Lena Tullborg b, Juhani Suksi c, Marcus Laaksoharju d, Václava Havlová e, Melissa A. Denecke f, Gunnar Buckau f a

Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Germany Terralogica AB, Sweden c University of Helsinki, Finland d Geopoint AB, Sweden e Nuclear Research Institute Rˇezˇ plc, Czech Republic f Karlsruhe Institute of Technology, Institut für Nukleare Entsorgung, Germany b

a r t i c l e

i n f o

Article history: Available online 19 September 2011

a b s t r a c t This paper gives an overview of the behaviour of U in two natural systems, the Forsmark site (a granitic system) in Sweden and the Ruprechtov site (a Tertiary sedimentary system) in the Czech Republic, which have been investigated in the frame of the FUNMIG project. It is not a full review paper on U geochemistry. It shows how different approaches and methods have been used to derive information on U solubility and speciation, on characteristics of key processes as well as on timescales of these processes and accordingly information on the long-term stability of U phases in the natural systems. The results are set in a wider context by relation to selected results from other sites. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

2. Relevance of uranium in the safety case

The robustness of a post-closure safety case for a geological repository for radioactive wastes is strengthened by the use of multiple lines of evidence leading to complementary, also qualitative, safety arguments that can compensate for shortcomings in any single argument. In this context real system analyses, i.e. the characterisation of conditions and processes occurring in natural systems at large spatial scales comprising the characterisation of sites foreseen as host rocks for radioactive waste repositories as well as sites similar to subsystems of a repository system, are of great importance. The latter ones are denoted as natural analogues (e.g. Miller et al., 2006). Within FUNMIG 3 natural systems have been investigated, namely the potential repository site at Forsmark in Sweden, as well as the analogue sites in the Opalinus clay at Mont Rusellin in Switzerland and at the sedimentary formation at the Ruprechtov Site in Czech Republic. The work covers a wide range of investigations, involving detailed hydrogeological, geological, mineralogical, geochemical, and environmental isotope characterisation. Since the area is too wide to be included in a single paper, it was decided to restrict the content here to the geochemical behaviour and migration of U in natural systems. Therefore, mainly the results from the investigations at Forsmark and Ruprechtov site are presented and discussed.

Uranium is the major constituent of spent fuel, which is foreseen to be directly disposed of in several national repository concepts worldwide. It does not necessarily dominate the releases to the surface but of course it is important to understand its behaviour. In several scenarios where groundwater gets into contact with the waste forms and radionuclides are released, U and its daughter products, particularly 226Ra have been shown to be of relevance with regard to potential radiation exposure in safety assessment studies, e.g. (SKB, 2006; Buhmann et al., 2008). Hence it is important to understand and if relevant include into modelling the processes controlling the transport of U in a repository system, like (co-)precipitation and sorption and how they are impacted by the geochemical conditions. For example in the Boom Clay the most important contributor to the limited mobility of U and most other actinides is its solubility (Maes, 2004). The migration behaviour of U released from a spent fuel repository is controlled by a complex set of coupled processes. Retardation and sorption are strongly affected under varying geochemical conditions depending on groundwater properties such as pH, Eh, pCO2, and the presence of complexing agents such as carbonates and natural organic matter, which determine the chemical speciation and hence the equilibrium distribution of U. In particular, the redox state of U is highly relevant, since U(IV) mineral phases are sparingly soluble in contrast to U(VI) phases. Moreover, U(VI) forms negative species in many geochemical environments compared to the dominating neutral tetravalent U(OH)4 species,

⇑ Corresponding author. Tel.: +49 531 8012247; fax: +49 531 8012200. E-mail address: [email protected] (U. Noseck). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.09.017

491

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

bottom). Above the kaolin layers the basin is mainly filled with Tertiary argillised pyroclastic sediments. The horizon of major interest is the so-called clay/lignite horizon at the interface of kaolin and pyroclastic sediments, with a high content of sedimentary organic C (SOC), irregular zones of U enrichment and partly aquiferous layers. The U anomalies with concentrations up to 500 mg/kg have been investigated in order to understand the major U mobilisation/immobilisation mechanisms that have occurred in the geological past and might still be active today, the chemical form and stability of immobile U phases and the factors controlling U mobility in this natural sedimentary system. In the currently assumed scenario U was mobilised from the granite, transported through the sediments as U(VI) and accumulated under the reducing conditions in the organic-rich clay/lignite layers (Noseck et al., 2008). Concerning the hydrogeology the general water flow in the Tertiary basin occurs from SW to NE (Fig. 1, top). However, water flow is restricted to distinct, sandy water bearing layers with hydraulic

resulting in quite different interaction behaviour with the mineral matrix. A large number of laboratory investigations have been performed during the last decades to understand these processes and to derive modelling approaches and data for safety assessment. The investigation of natural systems can underpin results from these experiments and in particular increase understanding on the long-term behaviour of U in complex natural systems. 3. The sites The natural analogue site at Ruprechtov, in the Czech Republic, which is not foreseen as a site for radioactive waste disposal, is a Tertiary basin bordered in the west and south by outcropping granite (Noseck and Brasser, 2006). Main features of the investigation area are the crystalline basement covered by kaolin layers of varying thickness (from a few up to several tens of metres, cf. Fig. 1,

Water flow NA13

NA14

LEGEND

NA12 NA10

NA6

NA5

NA4

Coal and carbonaceous clays in pyroclastics

NA9 PR4

Pyroclastic sediments (undiff.), argillized

RP5 NA7

HR4

NA8 RP3 RP4

Secondary kaolin (Kaolinite clays and sands) RP2

Primary kaolin (kaolinized granite)

RP1

Granite (slightly kaolinized) Granite (Krusné hory type)

100 m

SW

NE

Pyroclastic Sediments (argill.)

en t

Kaolin

MiddleLower Tertiary

ic hm

d an e-S nit g i y/L Cla

Upper Tertiary

Uen r

Granite

Carboniferous

Fault Zone General flow direction Fig. 1. Geological map with the conceptual model of groundwater flow pattern in the studied Ruprechtov aquifer system (top) and simplified geological cross section of the Tertiary basin at the Ruprechtov site (bottom).

492

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

conductivities of 105 m/s to 108 m/s and thicknesses of about 1– 2 m only, mainly at the interface of kaolin and pyroclastic sediments, i.e. in the direct vicinity of the clay/lignite horizon. These observations are based on laboratory experiments on selected drill cores, analysis of natural isotopes as well as pumping tests (Noseck et al., 2009a). The pyroclastic sediments and the underlying kaolin have a lower hydraulic conductivity, with typical kf-values in the range of 1010–1011 m/s. Fracture zones and areas with very low thickness of underlying kaolin might represent hydraulic connections between the clay/lignite horizon and the underlying granite. Further details on hydrogeology, geochemistry and immobile U forms can be found in Noseck et al. (2008, 2009a). The Forsmark site in Sweden is located NE of Forsmark village along the Baltic Sea coast in an area that contains three regionally significant deformation zones (Singö, Eckarfjärden and Forsmark). These zones dip steeply and strike WNW–ENE and NW–SE. In several parts of the area, outcrops are limited, especially away from the coast. The bedrock is dominated by different types of metamorphosed granitoid with subordinate felsic to intermediate metavolcanic rocks, metamorphosed diorite or gabbro, pegmatite or pegmatitic granite and amphibolite. These rocks formed between 1.89 and 1.85 Ga (Stephens et al., 2007). Several groundwater types which are now present in the bedrock at Forsmark can be associated with past climatic events in the late Pleistocene, including interglaciations, glaciations, deglaciations, and associated shore-level displacements in connection with marine/non-marine transgressions and regressions. Amongst these, the last glaciation and post glacial period is the most important for the groundwater development in the Fennoscandian Shield, especially in terms of land uplift and shore level displacement and the development of the Baltic Basin. There is groundwater and porewater evidence that indicates a pre-Pleistocene warmclimate derived meteoric water component. The hydrochemistry of the Forsmark area cannot be explained without recognising this older component. The present groundwaters, therefore, are a result of mixing and reactions over a long period of geological time. The interfaces between different water types are not sharp but reflect the variability in the structural-hydraulic properties (Laaksoharju et al., 2008). In conclusion, the groundwater flow is very slow except for in the larger fracture zones. The three major groundwater types distinguished are: Fresh water (<200 mg/L Cl), Brackish Marine (2000–6000 mg/L Cl and Mg > 100 mg/L) and Brackish to Saline non-marine groundwater with Cl between 4000 and 16,000 mg/L and Mg < 25 mg/L. Mixing between these three groups occurred in several of the sampled sections and mixing between fresh and brackish waters with Cl in the interval 200–2000 mg/L is labelled ‘‘Mixed Brackish’’. Water originating from mixing between Brackish Non-marine and Brackish to Saline non-marine groundwaters is called Transition type. The distribution of these groundwater types are shown in the conceptual model in Fig. 2 (Smellie et al., 2008). Elevated U concentrations in some of the sampled groundwater sections (up to 148 lg/L U), corresponding to a narrow range of chlorinity and usually at intermediate depth, coincide mostly, but not always, with the brackish marine (Littorina Sea) groundwaters. In addition, U-rich phases are present in some of the fracture coatings (max 2310 ppm U measured in bulk filling material), although to date it has only been possible to identify only one small grain of pitchblende (Sandström et al., 2008).

4. Uranium speciation and solubility in the natural systems At the Ruprechtov site, the U concentrations in groundwaters are generally low; in the more oxidising waters in boreholes from the infiltration area in granite, they range from 1.8 to 12 lg/L and

in the clay/lignite horizon they are below 2.1 lg/L and, with two exceptions, below 1 lg/L (Noseck et al., 2009b). At the Ruprechtov site, Eh-values have been determined in situ from granitic waters and from boreholes in the clay/lignite horizon, where the filter horizons are about 2–4 m long, in order to directly probe the groundwater from this horizon. The Eh-values have been measured directly by in situ probe with a Pt electrode and compared with on-site analyses in a flow cell isolated from the atmosphere. The in situ values are always significantly lower than those measured on-site (Noseck et al., 2009a). Although an isolated cell is used, the latter method is more susceptible to disturbances by contact with the atmosphere, which is probably responsible for the observed differences in Eh values. The in situ data usually declined during the measurements and became stable after a few days. The authors have confidence in the stable in situ values, since they are reproducible for each borehole. The constant values have been used for interpretation. The 3 representative boreholes NA6, NA12 and NA13 from the clay/lignite horizon show values between 160 and 280 mV (with corresponding pH values between 6.7  and 8), which seem to be determined by the SO2 4 =HS couple (Noseck et al., 2009b). At the Forsmark site, quite high concentrations of U with maximum values of 148 lg/L have been observed in several groundwater samples at depths between 170 and 650 m. Fig. 3 shows elevated U concentrations in selected boreholes from these depths in comparison to lower concentrations in surface and near surface waters and examples of very low U concentrations observed below 650 m. These findings are of interest for the siting and safety assessment programme as they indicate that U might be mobile in groundwaters at the Forsmark site even at a depth, where the repository may be constructed. Therefore, the objective of the investigations is to understand the observed mobility of U and its impact on the site understanding. Redox conditions (Eh-values), pH and pCO2 are the most important factors controlling U mobility (e.g. Langmuir, 1978). The direct measurement of Eh values in groundwaters is, however, technically difficult. A large effort has been made at Forsmark to measure reliable Eh-values (Laaksoharju et al., 2006). The selection of an Ehvalue for a borehole was based on a careful analysis of the values obtained with three different electrodes (Au, Pt and C) at depth and at the surface, the logging time, the pH, the conductivity and the dissolved O2. Taking into account all these issues, the following selection criteria have been applied:  logs longer than a week (logging times of selected representative values are between 13 and 68 days),  logs with stable and coincident readings (in a range smaller than 50 mV) by several electrodes in the long term; and  logs with simultaneous and stabilised pH values (in order to minimise the uncertainty associated with the pH). By application of these criteria, Eh values in a range between 140 and 250 mV are found at depths from 100 to 1000 m. Elevated U concentrations occur in slightly reducing groundwaters with values between 140 and 200 mV (pH range 7–8), whereas low concentrations are found in more strongly reducing waters with Eh-values between 200 and 250 mV. The weaker reducing waters are mainly found between 200 and 600 m depth, whereas more reducing conditions below 200 mV are met in shallower and deeper waters. From bulk fracture coating analyses (Sandström et al., 2008) it is obvious that enhanced U concentrations occur in part of the fracture systems at Forsmark. Most of the fracture samples analysed have U concentrations in the range 0.1–164 ppm, except for two samples from 630 to 640 m, which show contents of 2200 and 2310 ppm U, respectively. Out of the 70 fracture samples analysed,

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

493

Fig. 2. Visualisation of the hydrochemical data from Forsmark along cross-section WNW-ESE. Shown are: (a) the location of the boreholes and the sections which have undergone hydrochemical sampling, (b) the main fracture groundwater types which characterise the site, (c) the Cl distribution with depth along the major deformation zones and minor single open fractures, and (d) the Cl subdivisions of the rock matrix porewater. The groundwater flow directions are explained in the legend. The dotted lines in different colours crossing the section represent the approximate depths of penetration of (or extrapolation of) the various groundwater types along hydraulicallyactive deformation zones. (Cross-section length = 6790 m) (Smellie et al., 2008).

U µg/L 0

20

40

60

80

100

120

140

160

0

Elevation (m)

-200

-400

-600

-800

-1000

0-2000 mg/L Cl 2000-6000 mg/L Cl Brackish Marine Transition zone samples 4000-15000 mg/L Cl Brackish to Saline

-1200 Fig. 3. Elevated U concentrations from selected boreholes at depths between 170 and 632 m compared to concentrations in very deep boreholes and (near) surface waters at Forsmark site (Smellie et al., 2008).

nine show U concentrations higher than 24 ppm in combination with a U/Th ratio greater than 10, which strongly indicates deposition of the U in the fractures. Of these, nine samples represent depths ranging from 133 to 633 m. However, U contents above 10 ppm in fracture coating are measured down to 800 m depth. There are indications from U series measurements (Sandström et al., 2008) that the elevated U concentrations in the groundwater are related to contact with easy dissolvable U fractions in some of

these fracture coatings during favourable conditions (Fig. 4). Speciation-solubility calculations support this conclusion and indicate that the high U contents are the result of the control exerted by an amorphous (and very soluble) U phase present in the system, and the weakly reducing Eh values which may allow U complexation and re-equilibrium depending on Eh and dissolved carbonate. This is important considering the possible modification of the natural conditions undergone by these waters. The alteration of an

494

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

U in fracture fillings (mg/kg)

10000

1000

100

10

1 0.01

0.10

1.00

10.00

100.00

1000.00

U in groundwater (µg/L) Fig. 4. Uranium content in fracture fillings (ppm) versus U in groundwater samples (lg/L) from Forsmark. The fracture filling samples are from the corresponding drill core length. The precision of both water and fracture analyses is 15% (Sandström et al., 2008).

Fig. 5. pH/Eh-diagram activity = 103.

for

Ruprechtov

groundwaters.

cU = 103 mg/L,

CO2

1

pH = 8

0.8

U(IV) fraction

originally more reducing environment, and/or the increase of dissolved carbonate could have caused the increase in U-carbonate complexation, in turn enhancing the dissolution of U phases and increasing the contents of dissolved U with respect to that originally present in the system. Such a situation is not restricted to the Littorina type groundwaters, but may also occur if dilute recharge waters with significant HCO 3 content penetrate to greater depths (Laaksoharju et al., 2008).

0.6

0.4

0.2

4.1. Uranium speciation at Ruprechtrov site Uranium speciation calculations for the conditions at the Ruprechtov site have been performed with the Geochemist’s Workbench (GWB) (Bethke, 2006) and the updated NEA TDB database (Guillaumont et al., 2003). In Fig. 5, the stability of dissolved complexes in typical waters from the granitic infiltration area and from the U-enriched clay/lignite horizon at the Ruprechtov site are presented. It illustrates that the values for the more oxidising granite groundwater from the infiltration area are dominated by negative or neutral uranyl carbonato complexes, whereas the groundwaters from the clay/lignite horizon are located at the boundary of the stability fields for the neutral tetravalent aqueous U(OH)4 complex 2 and the hexavalent complexes UO2 ðCO3 Þ4 3 and UO2 ðCO3 Þ2 . Of most interest is the speciation in the clay/lignite horizon, where the secondary U phases are observed. The calculated fraction of U(IV) in a representative groundwater NA6 at pH 8 assuming equilibrium is shown as a bold line in Fig. 6. Even for the quite reducing conditions part of the dissolved U can be stabilised as U(VI) by formation of uranyl carbonato complexes. According to the thermodynamic calculations a U(IV) fraction of >60% is expected for borehole NA6 (Noseck et al., 2009b). The redox state of U in groundwater was studied in borehole NA6 applying a method described in Suksi and Salminen (2007). Groundwater was pumped under Ar-atmosphere into a reaction vessel where U(IV) precipitated, with NdF3 and U(VI) remaining in solution. The time difference between the groundwater pumping and NdF3-precipitation was on the order of a minute preventing effective U(IV) oxidation. The NdF3 precipitate and filtrate

0 -350

-300

-250

-200

Eh [mV] Fig. 6. Fraction of U(IV) dependent on Eh-value at pH 8, for conditions of NA6 groundwater for calculations without (bold line) and with consideration of (Me2+)nUO2(CO3)(42n) complexes (dotted line).

were taken to the laboratory where their U concentrations were determined. The U(IV) fraction was found to be in a range between 15% and 25% over three independent analyses (Noseck et al., 2009b). The measured U(IV) fraction was lower than calculated by GWB. The consideration of recently identified alkaline earth uranyl tricarbonato complexes of the form (Me2+)nUO2(CO3)(42n), which are not included in the NEA TDB from 2003, show a further stabilisation of the hexavalent U form, shifting the curve to lower Ehvalues, i.e. reducing the fraction of U(IV) closer to the experimental values (see discussion below). An important question with regard to the U-solubility is ‘Which mineral phase determines the U concentration in solution?’ In many natural systems uraninite and pitchblende are observed to occur in the more oxidised forms U4O9, or U3O7 (Langmuir, 1997). The U concentrations in solution are usually not determined by the crystalline phases UO2(c) or USiO4(c) but by these more oxidised forms, which are slightly less soluble than the amorphous forms of UO2 or USiO4 (Langmuir, 1997). For example, for several granitic groundwaters U4O9 or U3O7 have been suggested to be

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

responsible for the control of mobile U concentrations under reducing conditions (e.g. Cramer and Smellie, 1994; Blomqvist et al., 2000). At the Ruprechtov site the calculated saturation indices also show a strong supersaturation with respect to crystalline phases UO2(c) or USiO4(c) and also an oversaturation for U4O9 and U3O7 (Noseck et al., 2009b). The saturation indices are at or near to equilibrium with respect to the metastable amorphous UO2 form and nearly saturated/only slightly undersaturated by ningyoite. Both minerals have been identified as secondary minerals in several U-rich samples from the clay/lignite horizon. Recently, amorphous UO2 has also been reported to control U concentrations in granitic groundwaters at the Tono site (Iwatsuki et al., 2004; Arthur et al., 2006) at Eh-values below 260 mV and a pH-range between 8 and 9. Amorphous UO2 can be fairly insoluble and persistently metastable over a broad range of environmental conditions (Iwatsuki et al., 2004, and references therein). These results are in agreement with experimental evidence from experiments performed over a large pH-range under reducing conditions (Neck and Kim, 2001), that uraninite surfaces in contact with an aqueous solution appear to be coated with a thin layer of amorphous UO2, in neutral and alkaline solutions. Under these conditions the U concentration in solution is controlled by the amorphous surface layer.

4.2. Uranium speciation at Forsmark site As discussed above the analysis showed that in general Eh evolution with depth in Forsmark follows an atypical trend, with less reducing waters at 200–600 m correlated with higher U concentrations up to 122 lg/L and more reducing values at lower and higher depths featuring lower U concentrations. There is no clear correlation between Eh and U concentration, but groundwaters with Eh values lower than 210 mV show low U concentrations similar to values found for the Laxemar site in Sweden, which is also situated in crystalline Precambrian rocks. This suggests that the redox potential is one of the factors controlling dissolved U concentrations but implicates also the importance of other factors besides Eh. Uranium(IV)/U(VI) analyses were also performed in the U-rich groundwaters at the Forsmark site showing that U speciation in groundwater is clearly dominated by U(VI) with U(IV) fractions always below 5% (Suksi and Salminen, 2007). These waters are slightly less reducing than those from the Ruprechtov site with Eh-values between 140 and 200 mV and pH-values between 7 and 8. Uranium speciation calculations have been performed with PHREEQC and databases from WATEQ4F (Ball and Nordstrom, 2001), SKB-TDB (Duro et al., 2005) and NAGRA/PSI TDB (Hummel et al., 2002). The results show that the waters are very oversaturated with respect to the crystalline phases of uraninite and coffinite and also for the U oxides with mixed valences U4O9 and U3O7. The results suggest that the groundwaters with high U concentrations (10–122 lg/L) can be in equilibrium with amorphous phases of uraninite and coffinite but with a higher solubility than those considered in the databases (see below). The granitic rocks at Forsmark site do not have elevated U concentrations. The values around 4–5 ppm are typical for granitic rocks. However, analysis of fracture coatings show that U contents are almost always higher than in the bedrock (with a maximum value of 2310 ppm), particularly at depths between 400 and 650 m depths, where associated groundwaters also have high U-concentrations (Gimeno et al., 2008). Pitchblende has been observed in fracture fillings of one of these fractures (Sandström et al., 2008).

495

Therefore, the reason for the high U concentrations in these groundwaters seems to be a combination of easily accessible U in fracture minerals and possibly in the wall rock and also groundwater conditions that both mobilise U and maintain its mobility in solution. Uranium can remain mobile due to the relative mildly reducing conditions and presence of sufficient HCO3 to allow Ucarbonate complexation. 4.3. Discussion of thermodynamic data The U speciation in both systems is determined by the complexation of U(IV) with the neutral U(OH)4(aq) complex and to some extent by hexavalent uranyl carbonato complexes (impact depending on pH and alkalinity). There have been important developments since the compilation of the database for U by NEA (Grenthe et al., 1992). Amongst others it was recognised by the authors of the U database report that the stability of the U(OH)4(aq) complex was overestimated. Therefore, the complexation constant for the reaction

UðOHÞ4 þ 4Hþ ¼ U4þ þ 4H2 O

ð1Þ

has been corrected in the updated NEA TDB (Guillaumont et al., 2003). The apparently lower stability of this complex (log K = 10.05 for Reaction (1)) compared to the value from the NEA92 database (log K = 4.54) leads to lower redox potentials calculated for the U(IV)/U(VI) transition in the neutral and slightly alkaline pH range. The effect on U speciation of the recently identified trivalent carbonate complexes of uranyl with alkaline earth elements was tested using Ruprechtov conditions. In the Ca–HCO3 type waters at the Ruprechtov site the most important are the Ca complexes. The reactions 2 2þ UO2þ þ 3CO2 2 þ Ca 3 ¼ CaUO2 ðCO3 Þ3

ð2Þ

2þ þ 3CO2 UO2þ 2 þ 2Ca 3 ¼ Ca2 UO2 ðCO3 Þ3 ðaqÞ

ð3Þ

with log K values of 26.93 and 30.79 for Reactions (2) and (3) (Bernhard et al., 2001) have been included into the database. The calculations show a clear impact on the U speciation in the clay/lignite horizon. The strong complexation of uranyl at neutral/alkaline conditions leads to a further stabilisation of U(IV), which would explain the low U(IV) fraction measured in clay/lignite groundwater NA6 (discussed above). Therefore, this study gives evidence from a natural system that trivalent uranyl carbonate complexes with alkaline earth elements should be considered for U speciation and shows their strong impact on the redox speciation in Ca–HCO3 type waters as in the clay/lignite horizon at the Ruprechtov site. Consideration of this complex reduces the values for the saturation indices of UO2(am) in the calculations by 0.2–0.8, i.e. not changing the qualitative statement that amorphous UO2 determines the U concentration in the clay/lignite horizon. As in many other systems at the Ruprechtov and Forsmark sites, UO2 is a major U-bearing mineral phase and the results imply that amorphous UO2 phases also control the solubility at both sites. There is a lot of discussion on the solubility of amorphous UO2, which is due to the question of the degree of crystallinity of the UO2 phase. The solubility UO2 can be described by the reaction

UO2 þ 4Hþ ¼ U4þ þ 2H2 O

ð4Þ

The value from the updated NEA TDB used for the Ruprechtov calculations is log K = 1.505. The data in the WATEQ4F, SKB and NAGRA databases used for the Forsmark site are log K = 0.1, 1.5 and 0.0, respectively. However, the range of variation is conditioned by the different U speciation systems implemented in each database and, mainly, by the values included for the complexation reaction of U(OH)4 (Hummel et al., 2002). The solubility values for

496

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

UO2(am) obtained from the Forsmark groundwaters (between 2.6 and 4.0) are higher than any of the values included in the thermodynamic databases mentioned. Such a range of log K values have been discussed in Langmuir (1997) and Rai et al. (1997), where the lower value is representative for a relatively aged UO2(am) phase. This increased solubility of the amorphous U phase might be due to alterations in the natural system that could have enhanced the development of more oxidised surfaces of the previously existing UO2 phases with respect to U4O9. These alterations could also have caused U remobilisation reaching saturation with UO2(am) and provoked its precipitation (Laaksoharju et al., 2008). 5. Reactions involved in uranium mobilisation/immobilisation In order to use the results from analogue studies for testing models applied in performance assessment or to screen against features, events and processes (FEPs) used for scenario development in a safety assessment for radioactive waste repositories an in-depth understanding of the dynamic evolution of the geological system including the key processes involved in radionuclide mobilisation/immobilisation is of high importance. Part of the work at Ruprechtov was dedicated to the identification of reaction mechanisms leading to the observed U immobilisation. For the characterisation of such processes on a microscopic scale, both confocal l-XRF and l-XANES analyses were applied, which identified the U as U(IV). These results were in good agreement with the results from chemical separation and other spectroscopic methods like ASEM, which also identified U(IV) (Denecke et al., 2005). As demonstrated in Fig. 7 (left), the shape and intensities show the average valence state of the sampled volume to be U(IV). All three curves do not show the multiple scattering feature 10–15 eV above the white line (WL) characteristic for U(VI) nor do they show a significant decrease in the WL intensity, which would be expected for U(VI) as has been seen in the schoepite spectrum. From l-XANES and l-XRF it was also shown that As exists in two oxidation states, As(0) and As(V). The analyses of a number of tomographic cross sections of elemental distributions recorded over different sample areas show a strong positive correlation between U and As(V). By further development of the method, using the new planar compound refractive lens (CRL) array at the Fluoro–Topo-Beamline at the synchrotron facility ANKA of the Forschungszentrum Karlsruhe, a higher spatial resolution (focus beam spot size of 2  5 lm2 (V  H)) was achieved. The high resolution made it possible for the first time to discern an As-rich boundary layer surrounding Fe(II)-nodules, see Fig. 7, right (De-

necke et al., 2007). This suggests that an arsenopyrite mineral coating of framboidal pyrite nodules is present in the sediment. Uranium occurs in the direct vicinity of the As-rich layers. As a conclusion from these results a driving mechanism for U enrichment as secondary U(IV) minerals in the sediment was suggested. Mobile, groundwater-dissolved U(VI) was reduced on the arsenopyrite layers to less-soluble U(IV), which formed U(IV) mineral phases. Arsenic(0) was then oxidised to As(V) and the U remained associated with it. The results from these microscopic methods are supported by cluster analysis performed on the results from sequential extractions, which also indicate that U occurs in the tetravalent state, since the major part of the U is extracted in the respective steps for (reduced) U(IV) forms and the residual fraction (Havlová et al., 2006). By cluster analyses, performed to identify possible correlations between elements, a strong correlation of U with As and P was found (see Fig. 8), supporting the mechanism postulated above and the existence of U-phosphate mineral ningyoite identified by SEM-EDX. Further information on the processes involved in U immobilisation have been obtained from geochemical and isotope analyses. At the Ruprechtov site, the involvement of microbes in U immobilisation was shown to be quite important in an indirect way. In the geological past, pyrites have been formed by microbial SO4 reduction, still traceable by the typical framboidal shape of the pyrites in the clay/lignite layer. The reduction of sulphates to sulphides was shown in batch experiments with indigenous microbes and sediments extracted from drill cores. This microbial SO4 reduction is still active today demonstrated by the increase of 34S values in dissolved SO4 from the infiltration area compared to clay lignite layers (Noseck et al., 2009a). The microbial SO4 reduction seems to contribute to maintaining the reducing conditions and therewith to the long-term immobility of U (Noseck et al., 2009b). Microbial SO4 reduction involving organic matter and subsequent pyrite formation has also been identified as dominant buffering reactions at the Tono site that have maintained slightly alkaline and strongly reducing conditions for at least during the past several 100 ka (Arthur et al., 2006). The integration of all results showed that organic matter did not play such an important role by direct interaction with U, but sedimentary organic C (SOC) contributed and still contributes to maintain reducing conditions in the clay/lignite layers. It is supported by the finding that the highest accumulations of U were located slightly below the zones, which are highly enriched in organic matter. It can be concluded that SOC within the sedimentary layers

3.5 3.0

norm. Absorption

0 µm 2.5

40 µm 2.0

90 µm 1.5

schoepite 1.0 0.5 0.0 17.12

17.14

17.16

17.18

17.20

17.22

17.24

Energy [keV] Fig. 7. Results from l-XANES (left) and l-XRD (right) of a sample from borehole NA4 (Denecke et al., 2005, 2007).

497

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

NA14: 725 mg/kg U 0

Na As

P

U

K

Al Fe

NA15: 50 mg/kg U S

As U

0

P

S

K Na Fe Al

-10

-200

Similarity

Similarity

-100 -20

-30

-300 -40

Fig. 8. Cluster analyses for extended SE results of samples from the boreholes NA14 and NA15 (Havlová et al., 2006).

6. Uranium decay chain isotopes In order to get information on U mobilisation/immobilisation processes and therewith about the long-term stability of immobile U phases, the determination of isotopes of the U decay chain is of great value. It is not only the time scales but also information about the nature of groundwater flow that can be derived from the decay series analyses. In any naturally occurring U-containing material that has remained fully undisturbed for several millions of years, a state of secular radioactive equilibrium between the parent and the daughter nuclides in the radioactive decay chain will have been established, i.e. all radionuclides of a respective decay chain show the same activity. However, natural systems are seldom closed and small mass flow due to diffusion or advection occurs wherever water is in contact with the material. Under water–rock interaction geochemical processes such as chemical weathering, erosion, precipitation of minerals from aqueous solutions, and adsorption may cause fractionation between the mobile and immobile elements, resulting in a state of disequilibrium between parent and daughter nuclides, i.e. the parent–daughter activity ratio deviates from unity. Therefore, U disequilibrium series analyses allow judgment of whether U has been mobile in the geological past or not. The analysis and interpretation of U disequilibrium series in sediments and 234U/238U isotope ratios in groundwater is well documented, (e.g. Osmond and Cowart, 1976, 1992; Ivanovich et al., 1992) and has been applied in several natural analogue studies (e.g. Miller et al., 2000, and references therein). Such investigations at potential repository sites when they identify mobility and/or immobility of U also yield information about past groundwater flow. For example radiochemical studies in the Boom Clay showed that in general the U decay chain is in a state of secular equilibrium indicating no significant mobility of U (De Craen et al., 2004). It

also indicates that no advective water flow has occurred during the last 1 Ma in the Boom Clay, since not even more mobile 234U, formed by a-recoil and subsequent processes, is released from the bulk U. In a similar way analysis from the Tono U deposit showed that some migration of U occurred along faults and fractures, but the largest motion of U has been through the rock matrix over a distance of less than 1 m during the past 1 Ma (Nohara et al., 1992). These results are often also used as strong arguments for long-term stable geochemical conditions at the site, which increases confidence in the predictions of the site evolution in the future (NEA, 2008). At the Ruprechtov site, U disequilibrium series analyses were used to obtain information on the long-term stability of the immobile U phases described above (Noseck et al., 2008). The activities of 238U, 234U and 230Th were determined in the bulk samples from several boreholes from the U-enriched clay/lignite horizon. The results were plotted in a Thiel’s diagram (Thiel et al., 1983), where one can study features of U mobility in respective samples and, consequently, evaluate the dynamics of the system (Fig. 9). The diagram shows segments, which outline data with different U behaviour. The segments are determined by (i) the 234U/238U and 230 Th/238U equilibrium lines (i.e. activity ratios are unity) and (ii) the line obtained when a tangent is drawn on the closed system chain decay curves evolving towards radioactive equilibrium (230Th/238U = 1 and 234U/238U = 1) after sudden accumulation and removal of U at time zero. Data points plotting above the

2

50 ka 100 ka 234U/238U

was (and to some extent still is) microbially degraded. By this process dissolved organic C (DOC) is released, providing protons to additionally dissolve sedimentary inorganic C (SIC). Moreover SO2 4 is reduced leading (and lead in the geological past) to the formation of Fe sulphides, especially pyrite. Reducing conditions, being maintained amongst others by SO4-reducing bacteria, caused the reduction of As, which sorbed onto pyrite surfaces, forming thin layers of arsenopyrite. Uranium(VI), originally being released from the outcropping/underlying granite, was reduced to U(IV) on the arsenopyrite surfaces. The UO2 and U phosphates were formed by reaction of U(IV) with PO3 4 , released by microbial SOC degradation. These U(IV) minerals have been stable and immobile over geological time frames.

1,5

200 ka

U addition

S2 1

S1

U removal

0,5 0

0,5

1

1,5

230Th/238U Fig. 9. 230Th/238U and 234U/238U-activity ratios of bulk samples from the clay/lignite horizon plotted in a Thiel’s diagram (Noseck et al., 2008).

498 234

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

U/238U equilibrium line represent samples where bulk U has been affected, either by accumulating or removing U from the system. The segments S1 and S2 represent U series disequilibria, which cannot be created by closed system chain decay outlining samples where episodic and/or continuous U movement may have occurred, i.e. an open system. For the Ruprechtov samples nearly all data are plotted in the Uaddition area indicating a sink for U. A high fraction of data points accumulate in segment S1, which represents samples from which 234 U has been or is being selectively removed by the groundwater. Selective 234U release here means that bulk U is not affected in the respective conditions, i.e. being geochemically stable. The release of 234U from the system confirms water flow in or near the clay/lignite horizon. The ages of U phases in the respective samples are beyond the U series method. Thus, at least a significant fraction of U in the clay/lignite horizon is more than 1 Ma old, i.e. if any geological process affected the site, the clay/lignite horizon has been preserved geochemically undisturbed, at least for U. It can be further said that, because the number of data points are plotted in the bulk U addition area, some U accumulation may be still in progress. In order to better characterise the different U forms, the geochemical distribution and redox state of U were studied using sophisticated chemical separation methods. Uranium(IV)/U(VI)separation (Ervanne and Suksi, 1996) and sequential extraction (SE), coupled with analysis of the 234U/238U activity ratio, denoted as AR, in each phase, identified that the major part of U occurs in the tetravalent state, in agreement with the results from spectroscopic methods (see above). An important observation was that the tetravalent U form exhibits in nearly all samples an AR below one, with values generally in a range between 0.2 and 0.8 (Noseck et al., 2008). AR values significantly below unity are caused by the preferential release of 234U, which is facilitated by the preceding 234 Th a-recoil process contributing to 234U being born in the hexavalent state (see, e.g. Suksi et al. (2006) and references therein). In order to attain for the U(IV) phase low AR values of approx. 0.2, it must have been stable for a sufficiently long time to allow 238U to decay to 234U and form 234U(VI), i.e. no significant release of bulk U must have occurred during the last 1 Ma. This is in agreement with the hypothesis that the major U input into the clay/lignite horizon occurred during Tertiary, more than 10 Ma ago as described in (Noseck and Brasser, 2006). In conclusion, no evidence for significant U release from the clay lignite horizon in the geological past has been found. The low ARs in the U(IV) phase observed in this study are correlated with AR values >1 (between 1.5 and 4) in pore and groundwater from the clay/lignite horizon (Noseck et al., 2008). This is expected, because in anoxic sediment conditions bulk U release is strongly suppressed and the release of 234U(VI) formed by decay is favoured. It is in agreement with results from the Finnish study sites, where the 234U/238U release ratio has been suggested as an indicator for the redox conditions (Suksi et al., 2006). There, very high 234U/238U release ratios (down to 6.5) have been observed in strongly reducing groundwaters, whereas values around 1 occur in oxidising groundwater. The origin of the U-rich phases at Forsmark is largely unknown but it can be concluded that U has been circulating throughout the geological history of the site indicated by: (a) some of the pegmatites showing slightly enriched U values (a maximum value of 62 ppm is indicated by gamma spectrometric measurements, (b) redistribution and deposition of U irregularly along permeable structures during the Proterozoic is documented in the area just outside Forsmark, and (c) Palaeozoic, potentially U-rich alum shales that once covered the area may have contributed U to the system. Of special interest for understanding the present groundwater/mineral systems is the potentially late (Quaternary) redistribution of U, and this has been evaluated using U decay series

analyses of groundwater and contact fracture coatings. The main conclusions on the U system are given below (Smellie et al., 2008).  The highest U contents in both fracture coatings and groundwaters are found in open, water-conducting fractures. It is important to note that in other fractures at the same depth interval low U contents are found in both fracture coatings and groundwater samples.  The U present in groundwaters associated with fracture coatings is dominantly enriched in the oxidised U(VI) state. Assuming no anthropogenic effects, the U is believed to result from in situ dissolution of a mainly U(VI)-bearing mineral phase(s).  Transmissive fractures in the upper 150 m of the hanging wall and footwall bedrock show more pronounced deposition and leaching of U during the last 1 Ma, compatible with a dynamic and heterogeneous groundwater flow system.  The presence of oxidised U in groundwater in fracture zones at around 500 m depth does not mean that oxidised water has penetrated to this depth. Instead, it is shown that mildly reducing groundwaters with sufficient HCO3 (>30 mg/L) are capable of keeping U(VI) mobile, and this has resulted in variable accumulations occurring within the fracture zones down to maximum depths of around 500 m.  At some stage, dissolved U has become concentrated where there is a transition from high to low fracture transmissivity, corresponding to the transition from brackish marine (Littorina) to brackish non-marine groundwaters.  The deposition of U is not confined to any specific periods of recent groundwater circulation, for example, since the last deglaciation, but has been shown to have occurred in a few fractures within the last 1 Ma as indicated by U-series data.  Speciation-solubility calculations indicate solubility control by an amorphous U-phase for the groundwaters with the highest U contents (Gimeno et al., 2008). 7. Conclusions The geochemical behaviour of U has been studied in two hydrogeological systems, the first one, representing an example for a sedimentary formation covering potential repository host rocks and the second one, a granitic site designated for a spent fuel repository in Sweden. Several analytical methods for the characterisation of dissolved and solid phase U have been applied and further developed including U series disequilibrium measurements and U(IV)/U(VI) separation with 234U/238U isotope analysis. Innovative microscopic methods have been used in combination with macroscopic methods contributing to the identification of key processes at the Ruprechtov site. These new methods are important in building a safety case, particularly for yielding information about the (hydro)geological past of a potential repository site, which is important to increase confidence in the predictions of the site evolution in the future. Geochemical calculations indicate that at both sites amorphous UO2 controls the U concentration in solution with a higher soluble form suggested to occur in fractures at the Forsmark site. A further important result for the safety case is the evidence from a natural system that the trivalent carbonate complexes of U(VI) with alkaline earth elements – so far only determined in laboratory experiments – should be considered in the thermodynamic description of U speciation. The Ruprechtov natural analogue study serves as an example of a strong long-term barrier function of a sedimentary layer. The system provides evidence that U is and was effectively immobilised in a sedimentary layer of a formation, which was exposed to surface erosion in the geological past. All results indicate that U was efficiently immobilised in a reducing environment (controlled by the

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500 2 SO2 couple) over millions of years and no significant release 4 =S of immobilised U occurred within that time, although the U-bearing horizon is in the direct vicinity of water flowing layers. Even though part of the dissolved U exists in the U(VI) state, probably stabilised by carbonate complexes, the U concentrations in solution are kept quite low with values typically below 1 lg/L in the U-enriched horizon. The key processes involved in U immobilisation were identified. The observation of a strong impact of microbial processes contributing directly and indirectly to the U immobilisation is one further important result from the Ruprechtov study with respect to making a safety case. It shows the importance of addressing the role of microbes in the safety case. In the case of Ruprechtov, microbes have a ‘‘positive’’ effect, contributing to the long-term geochemical stability of the sedimentary layers and therewith to the long-term immobilisation of U. The results at Forsmark show the importance of investigations of U geochemistry at a potential repository site itself. In contrast to the situation at Ruprechtov and also to other potential argillaceous host rock sites like Mol, where U data in general indicate that it has not been mobile during the last 1 Ma, clear indications for U mobility in some of the waters are given. However, the in-depth analysis of the geochemical conditions at the site could explain the occurrence of elevated U concentrations, which in conclusion are due to (i) the occurrence of readily soluble amorphous UO2 phases in fracture fillings and (ii) the mildly reducing Eh-values, which allow U-carbonate complexation. An important conclusion for the safety case is that these waters will not be able (from a thermodynamic point of view) to dissolve a hypothetical spent fuel in contact with them. These waters are in equilibrium with the soluble U amorphous phases and, therefore, they are also strongly oversaturated with respect to crystalline uraninite (log K = 4.85 for Reaction (4)) or to the spent fuel material (log K = 1.6), see Bruno and Puigdoménech (1989) and Casas et al. (1998).

Acknowledgements This work has been financed by the German Federal Ministry of Economics (BMWi) under contract no’s 02 E 9551 and 02 E 9995, by RAWRA and Czech Ministry of Trade and Industry (Pokrok 1H-PK25), by SKB and by the European Atomic Energy Community Seventh Framework Programme [FP6/2002–2006] under Grant agreement n° 516514, Integrated Project FUNMIG. Financial support was given by The Swedish Nuclear Fuel and Waste Managemen Co (SKB). Comprehensive reviews by Bill Miller and Neil Chapman significantly improved the manuscript. References Arthur, C.R., Iwatsuki, T., Sasao, E., Metcalfe, R., Amano, K., Ota, K., 2006. Geochemical constraints on the origin and stability of the Tono uranium deposit, Japan. Geochem. Explor. Environ. Anal. 6, 33–48. Ball, J.W., Nordstrom, D.K., 2001. User’s manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. US Geol. Surv. Open File Rep. 91– 183. Bernhard, G., Geipel, G., Reich, T., Brendler, V., Amayri, S., Nitsche, H., 2001. Uranyl(VI) carbonate complex formation: validation of the Ca2UO2(CO3)(3)(aq.) species. Radiochim. Acta 89, 511–518. Bethke, C.M., 2006. The Geochemist’s Workbench Release 6.0. Hydrogeology Program, University of Illinois. Blomqvist, R., Ruskeeniemi, T., Kaija, J., Ahonen, L., Paananen, M., Smellie, J., Grundfelt, B., Pedersen, K., Bruno, J., Pérez del Villar, L., Cera, E., Rasilainen, K., Pitkänen, P., Suksi, J., Casanova, J., Read, D., Frape, S., 2000. The Palmottu natural analogue project. Phase II: transport of radionuclides in a natural flow system at Palmottu. Final report. EUR 19611 EN. Bruno, J., Puigdoménech, I., 1989. Validation of the SKBU1 uranium thermodynamic database for its use in geochemical calculations with EQ3/6. Mat. Res. Soc. Symp. Proc. 127, 887–896. Buhmann, D., Mönig, J., Wolf, J., 2008. Untersuchung und Ermittlung von Freisetzungsszenarien – Teilbericht zum Projekt ISIBEL: ‘‘Überprüfung und Bewertung des Instrumentariums für eine sicherheitliche Bewertung von

499

Endlagern für HAW’’. Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, GRS-233. Casas, I., de Pablo, J., Jiménez, J., Torrero, M.E., Bruno, J., Cera, E., Finch, R.J., Ewing, R.C., 1998. The role of pe, pH, and carbonate on the solubility of UO2 and uraninite under nominally reducing conditions. Geochim. Cosmochim. Acta 62, 2223–2231. Cramer, J., Smellie, J. (Eds.), 1994. Final report of the Cigar Lake Analogue Study. SKB Technical Report TR 94-04, Stockholm. De Craen, M., Wang, L., Weetjens, E., 2004. Natural evidence on the long-term behaviour of trace elements and radionuclides in Boom Clay. Final report of DS2.9 for the period 2000–2003, R-3926, SCK-CEN, Mol, Belgium. Denecke, M.A., Janssens, K., Proost, K., Rothe, J., Noseck, U., 2005. Confocal microXRF and micro-XAFS studies of uranium speciation in a Tertiary sediment from a waste disposal natural analogue site. Environ. Sci. Technol. 39, 2049–2058. Denecke, M.A., Somogyi, A., Janssens, K., Simon, R., Dardenne, K., Noseck, U., 2007. Microanalysis (micro-XRF, micro-XANES and micro-XRD) of a Tertiary sediment using synchrotron radiation. Micros. Microanal. 13, 165–172. Duro, L., Grivé, M., Cera, E., Doménech, C., Bruno, J., 2005. Update of a thermodynamic database for radionuclides to assist solubility limits calculation for PA. ENVIROS, Spain. Ervanne, H., Suksi, J., 1996. Comparison of ion-exchange and coprecipitation methods in determining uranium oxidation states in solid phases. Radiochemistry 38, 324–327. Gimeno, M.J., Auqué, L.F., Gómez, J.B., Acero, P., 2008. Forsmark area versions 2.2– 2.3. Contribution from the University of Zaragoza. SKB Report R-08-86, Stockholm, Sweden. Grenthe, I., Fuger, J., Konings, R.J.M., Lemire, R.J., Muller, A.B., 1992. The Chemical Thermodynamics of Uranium. OECD-NEA, North-Holland. Guillaumont, R., Fanghänel, Th., Fuger, J., Grenthe, I., Neck, V., Palmer, D.A., Rand, M.H., 2003. NEA-TDB, Chemical Thermodynamics. Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, vol. 5. Elsevier. Havlová, V., Laciok, A., Vopálka, D., Andrlík, M., 2006. Geochemical study of uranium mobility in Tertiary argillaceous system at Ruprechtov site, Czech Republic. Czech. J. Phys. 56 (Suppl. D), 1–6. Hummel, W., Berner, U., Curti, E., Pearson, F.J., Thoenen, T., 2002. Nagra/PSI Chemical Thermodynamic Data Base 01/01. Nagra Technical Report NTB 02-16, Nagra, Wettingen, Switzerland. Ivanovich, M., Latham, A.O., Longworth, O., Gascoyne, M., 1992. Applications to radioactive waste disposal studies. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium series Disequilibrium: Applications to Earth, Marine and Environmental Sciences, second ed. Clarendon Press, Oxford, pp. 583–630. Iwatsuki, T., Arthur, R., Ota, K., Metcalfe, R., 2004. Solubility constraints on uranium concentrations in groundwaters of the Tono uranium deposit, Japan. Radiochim. Acta 92, 789–796. Laaksoharju, M., Smellie, J., Tullborg, EL., Gimeno, M., Gómez, J., Gurban, I., Hallbeck, L., Auqué, L., Buckau, G., Gascoyne, M., Raposo, J., 2006. Hydrogeochemical evaluation of the Forsmark site, modelling stage 2.1. SKB-R06-69, Stockholm. Laaksoharju, M., Smellie, J., Tullborg, E.-L., Gimeno, M., Molinero, J., Hallbeck, L., Moliner, J., Waber, N., 2008. Bedrock hydrogeochemistry Forsmark. SKB Report R 08-47, Stockholm, Sweden. Langmuir, D., 1978. Uranium solution–mineral equilibria at low temperatures with application to sedimentary ore deposits. Geochim. Cosmochim. Acta 42, 547– 569. Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice Hall, Upper Saddle River, New York. Maes, N. (Ed.), 2004. Uranium retention and migration behaviour in Boom Clay. Status 2004. NIROND-TR Report 2007–10 E. ONDRAF/NIRAS, Brussels. Miller, W., Alexander, R., Chapman, N., McKinley, I., Smellie, J., 2000. Geological Disposal of Radioactive Wastes & Natural Analogues. Waste Management Series, vol. 2. Pergamon, Amsterdam. Miller, B., Hooker, P., Smellie, J., Dalton, J., Degnan, P., Knight, L., Noseck, U., Ahonen, L., Laciok, A., Trotignon, L., Wouters, L., Hernán, P., Vela, A., 2006. Network to review natural analogue studies and their applications to repository safety assessment and public communication (NAnet). Synthesis Report. EUR 21919. NEA, 2008. The Evolving Roles of Geoscience in the Safety Case: Responses to the AMIGO Questionnaire. A report of the NEA Working Group on Approaches and Methods for Integrating Geological Information in the Safety Case (AMIGO). Nuclear Energy Agency, Paris. Neck, V., Kim, J.I., 2001. Solubility and hydrolysis of tetravalent actinides. Radioachim. Acta 89, 1–16. Nohara, T., Ochiai, Y., Seo, T., Yoshida, H., 1992. Uranium series disequilibrium studies in the Tono Uranium deposit, Japan. Radiochim. Acta 58 (59), 409–413. Noseck, U., Brasser, Th., 2006. Radionuclide transport and retention in natural rock formations – Ruprechtov site. Gesellschaft für Anlagen- und Reaktorsicherheit, GRS-218, Köln. Noseck, U., Brasser, Th., Suksi, J., Havlova, V., Hercik, M., Denecke, M.A., Förster, H.J., 2008. Identification of uranium enrichment scenarios by multi-method characterisation of immobile uranium phases. J. Phys. Chem. Earth 33, 969–977. Noseck, U., Rozanski, K., Dulinski, M., Havlova, V., Sracek, O., Brasser, Th., Hercik, M., Buckau, G., 2009a. Carbon chemistry and groundwater dynamics at natural analogue site Ruprechtov, Czech Republic: insights from environmental isotopes. Appl. Geochem. 24, 1765–1776. Noseck, U., Havlova, V., Suksi, J., Cervinka, R., Brasser, Th., 2009b. Geochemical behaviour of uranium in sedimentary formations: insights from a natural analogue study. In: Proceedings of The 12th International Conference on

500

U. Noseck et al. / Applied Geochemistry 27 (2012) 490–500

Environmental Remediation and Radioactive Waste Management ICEM2009, Liverpool, UK. Osmond, J.K., Cowart, J.B., 1976. The theory and uses of natural uranium isotopic variations in hydrology. Atomic Energy Rev. 14, 621–679. Osmond, J.K., Cowart, J.B., 1992. Ground water. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium Series Disequilibrium: Applications to Earth Marine and Environmental Sciences, second ed. Clarendon Press, Oxford, pp. 290–330. Rai, D., Felmy, A.R., Sterner, S.M., Moore, D.A., Mason, M.J., Novak, C.F., 1997. The solubility of Th(IV) and U(IV) hydrous oxides in concentrated NaCl and MgCl2 solutions. Radiochim. Acta 79, 239–247. Sandström, B., Tullborg, E.-L., Smellie, J., MacKenzie, A., Suksi, J., 2008. Fracture mineralogy of the Forsmark site SDM-Site Forsmark SKB R-08-102. SKB, 2006. Long-term safety for KBS-3 repositories at Forsmark and Laxemar – a first evaluation, Main Report of the SR-Can project. SKB, Technical Report TR06-09, Stockholm.

Smellie, J., Tullborg, E.-L., Nilsson, A.-C., Sandström, B., Waber, N., Gimeno, M., Gascoyne, M., 2008. Explorative analysis of major components and isotopes. SDM-Site Forsmark, SKB R-08-84. Stephens, M-B., Fox, A., La Poinre., P., Simeonov, A., Isaksson, H., Hermanson, J., Öhman, J., 2007. Geology Forsmark. Site descriptive modelling. Forsmark stage 2.2. SKB R-Rep. (R-07-45), SKB, Stockhom, Sweden. Suksi, J., Salminen, S., 2007. Forsmark site investigation. Study of U oxidation states in groundwater with high U concentrations. SKB, Report P-07-54, Stockholm. Suksi, J., Rasilainen, K., Pitkänen, P., 2006. Variations in the 234U/238U activity ratio in groundwater–a key to characterise flow system? Phys. Chem. Earth 31, 556– 571. Thiel, K., Vorwerk, R., Saager, R., Stupp, H.D., 1983. 235U fission tracks and 238U series disequilibria as a means to study recent mobilisation of uranium in Archaean pyritic conglomerates. Earth Planet Sci. Lett. 65, 249–262.