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El estudio de arcillas espanolas para su utilizacion como material de sellado en almacenamientos de residuos radiactivos: 20 anos de progreso.

The study of Spanish clays for their use as sealing materials in nuclear waste repositories: 20 years of progress

1. Introduction

1.1. Deep geological disposal of high-level radioactive wastes

Radioactive wastes are considered to be any substance that contains or is contaminated with radionuclides in concentrations higher than those established by the competent authorities and for which no subsequent use is foreseen. Such wastes are produced during the generation of electricity using nuclear means, in the decommissioning of nuclear and radioactive installations and in the use of radioisotopes in industry, medicine, agriculture, research, etc. From the point of view of definitive disposal, radioactive wastes are generally classified into low and intermediate level wastes and high-level wastes. The latter have high specific short-lived emitter activities, contain appreciable concentrations of long-lived alpha-emitting radionuclides and are major heat producers.

The solution as regards protecting people and the environment against the radiations emitted by the radionuclides contained in high-level wastes consists in isolating them in such a way that throughout the period in which they remain active they cannot be released to the biosphere along any of the possible paths. The generally accepted option for the definitive disposal of high-level wastes (HLW) consists in their disposal in stable deep geological formations (500-1000 m) (Goguel et al., 1987), as was suggested by the United States National Academy of Science in the 1950's. The safety of this disposal concept is based on the existence of a series of superimposed natural and artificial barriers to guarantee isolation (multibarrier concept): natural barriers, constituted by the host rock, and artificial barriers, constituted by the solid matrix of the waste itself, the metallic canister and its backfill, the sealing materials placed around the canister and the material backfilling the drifts of the installation.

Crystalline rocks (granite, basalt), clays and salts have been proposed as the surrounding geological material. The system of barriers aims to seal the possible escape paths for the radionuclides to the environment, the most important of which is the circulation of groundwater. In this context, the basic functions of the sealing material between the canister and the host rock are to prevent or limit the entry of water to the wastes and to contribute to radionuclide retention. Other additional functions are to contribute to heat dissipation and to provide mechanical protection for the waste canisters. In view of these functions, Pusch (1979) proposed the use of sodium bentonite (rocks made up of clay minerals and belonging to the smectites group, in which montmorillonite is the most common species) compacted in the form of high density blocks as sealing material, since it provides the following characteristics:

--Very low permeability, reducing the percolation of groundwater, since water transport is the main radionuclide transfer mechanism.

--High exchange capacity, and therefore a high capacity for ion adsorption in the event of radionuclide release.

--Sufficient thermal conductivity, to prevent the generation of excessive thermal gradients.

--Mechanical resistance to withstand the weight of the canister.

--Mechanical properties guaranteeing the homogeneous nature of the barrier, this including plastic behaviour to prevent the formation of fissures as a result of differential displacements or the location of stresses, and swelling potential favouring the self-sealing of existing voids.

Other material requirements of the barrier that have been underlined in subsequent research (Yong et al., 1986) are as follows:

--Good compressibility, ensuring ease in processing, handling and transport to the disposal facility.

--Low shrinkage in response to the drying that will probably occur in the area surrounding the canister, in order to prevent the formation of a network of fissures.

--Non-excessive swelling pressure, to avoid damage to the system.

--Suitable deformability, ensuring that the pressures generated by the rock massif and by the hydration of the expansive component of the barrier are absorbed and reduced by deformation of the barrier itself.

--Physical and chemical stability, ensuring the longevity of the system in relation to the conditions of the disposal facility, i.e. high temperatures, chemical gradients and the presence of vapour (Pusch and Gray, 1989; Guven, 1990).

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Since the end of the 1980's, the agencies in charge of waste management in different countries have proposed other sealing materials, some based on the use of cement but most considering the use of bentonites, either nonsodic smectites or mixtures of expansive clay and aggregates in different proportions. As regards the aggregates, tests have been performed using crushed granite, crushed basalt, zeolites and quartz, quartz and graphite. The main objective of adding inert aggregates is to increase the thermal conductivity of the barrier, improve the mechanical resistance of the compacted blocks and reduce the cost of the material.

1.2. The Spanish case

The creation of ENRESA in 1984 marked the beginning of a new stage of research into the disposal of high-level radioactive wastes in Spain. In 1990, a series of activities was initiated with a view to defining a disposal system adequate for the Spanish case, within the framework of the deep geological disposal Project (Almacenamiento Geologico Profundo, AGP), the basic objective of which is to "avoid any type of radiological damage to mankind and his environment, using the waste concentration and confinement strategy". The geological formations considered are granite, clay and salt. The definitive disposal of the HLW is planned to be accomplished in canisters placed in the centre of the drifts of a disposal facility excavated at depth and surrounded by a sealing material, as shown in figure 1 for the granite case. The functions attributed to the sealing material surrounding the canister are those of reducing groundwater flows and the transport of corrosive substances, establishing a suitable physicochemical environment around the canisters and providing mechanical protection against possible movements of the rock. Consequently, the properties to be provided by the sealing material are fundamentally low permeability and diffusivity, enough thermal conductivity, high sorption capacity and long-term stability (ENRESA, 1995).

The research relating to deep geological disposal covers from the determination of areas suitable for the installation of disposal facilities to the study of the behaviour of the fuel and canister, including site characterisation (structural, hydrogeological, geochemical, hydrogeochemical and radionuclide migration) and theoretical studies of engineered barriers.

In the AGP Project, consideration has been given to compacted bentonite as the canister sealing material in the case of saturated formations (clay and granite) and to a mixture of sand and clay materials as the backfilling material for the galleries and cavities. The dry density considered for the bentonite receptacle surrounding the canister is 1.65 g/[cm.sup.3]. The layer of bentonite will measure 0.75 m in thickness and its temperature will not exceed 100[degrees]C. A series of suitability studies led to a bentonite exploited in the province of Almeria being selected as the reference sealing material. The following sections recapitulate on the prospecting, selection and study of the behaviour of materials suitable for the construction of engineered clay barriers in Spain, with a summary of the main results of each phase of the research.

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2. The study of clay barriers in Spain: first stages

2.1. Prospecting and initial selection (1987-1989)

The study of backfilling and sealing materials began in Spain in 1987, with a Project financed by the European Community, performed by CIEMAT with participation of ENRESA (Mingarro et al., 1991). This study had three objectives:

--To gain insight into the availability in Spain of clays suitable for use as backfilling and sealing materials.

--To study the possibility of using illitic clays as an alternative to smectitic clays, in order to minimise mineralogical unbalance with the granitic medium.

--To use ground granitic rock as an additive to the clay for backfilling material, this possibly reducing costs and contributing to the reconstitution of the environment of the disposal facility.

The prospecting of clay materials in Spain having been performed, study began on 30 samples from seven suppliers located in Gerona (samples 1 and 2), Barcelona (samples 3 to 11), Zaragoza (samples 12 to 20), Segovia (sample 21), Toledo (samples 22 and 23), Jaen (sample 24) and Almeria (samples 25 to 30) (Fig. 2). An initial semi-quantitative determination of the mineralogy allowed 50 percent of the samples to be excluded because of their content of phyllosilicates being lower than 65 percent (Fig. 3). In accordance with the mineralogy of the fraction of less than 2 [micro]m (Fig. 4), seven of these samples were selected; two were mainly made up of illite (samples 15 and 24) and the rest were smectites from the Cabo de Gata region (Almeria) supplied by Minas de Gador (samples 25, 26 and 28 to 30).

Finally, two bentonites were selected that had not been subjected to treatment in the factory: a bentonite known as "Serrata natural" (sample 26) from the Cortijo de Archidona deposit, in Almeria, and an illite from Zaragoza (sample 15), these being chosen for their low carbonate and colloidal mineral content and high degree of plasticity and compressibility (Fig. 5). Analysis of the mechanical, hydraulic, thermal and physicochemical properties of both, and their behaviour when mixed with crushed granite and when heated, allowed the following conclusions to be drawn (Mingarro et al., 1991):

--There are clays in Spain that might be used as backfilling and sealing materials.

--It is advisable to use smectitic clay and not illitic clay as a backfilling and sealing material, due to its greater swelling pressure, specific surface and cation exchange capacity (Fig. 6). The final dry density of the bentonite barrier should be above 1.6 g/[cm.sup.3] if montmorillonite is used and above 1.8 g/[cm.sup.3] if illite is used, in order to keep at adequate values its hydraulic and physico-mechanical properties.

--Crushed granite may be used as an additive to the smectite in the backfilling material, as long as it does not exceed 25 percent of the mixture. Above this percentage, swelling pressure decreases (Fig. 7) and hydraulic conductivity increases (Fig. 8) out of the acceptable ranges.

--The heating of the clays at temperatures above 100[degrees]C negatively modify their properties, since it reduces their plasticity, swelling capacity, strength and specific surface area.

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2.2. Study of two smectite-bearing areas

Parallel to the above, ENRESA promoted the study of two Spanish areas with important deposits of smectite clays that might be used as backfilling and sealing material for a future high-level radioactive waste disposal facility. These areas were the volcanic region of Cabo de Gata, in Almeria, and the Tertiary Basin of Madrid in the province of Toledo. Three deposits were selected from each zone, these being considered as potential points of supply of the material in the future, because of the characteristics already known and the estimated reserves.

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CSIC-Zaidin performed a preliminary characterization of 30 bentonite outcrops (10 samples per outcrop) in the Cabo de Gata region, that allowed the selection of the three deposits (Cortijo de Archidona, in the Serrata de Nijar, Morron de Mateo, in the Rambla de los Escullos and Los Trancos, in the northern area of the Sierra de Gata) for a comparative study. These deposits were selected on the basis of the following criteria: the smectite content should be higher than 70-75 percent and the minimum estimated reserves should be in the order of 106 tons. The bentonites from Cortijo de Archidona are very pure and have outstanding colloidal properties, the Los Trancos deposit --also of a great purity-- have the major bentonite reserves, and the bentonites from Morron de Mateo have important quantities of sand, what could avoid the addition of additives (Linares et al., 1993). The average mineralogical composition of the bentonites from the selected deposits is shown in Table 1 and the structural formulae of the smectites in Table 2.

The UAM group studied, in a first step, the mineralogical and physicochemical properties of 28 samples taken from three clay quarries located in the Tertiary Basin of Madrid: the Cerro del Aguila-Cerro del Monte deposit (green clays at Villaluenga de la Sagra), the Santa Barbara deposit (pink clays at Esquivias) and the Yuncos deposit (brown clays at Yuncos) (Cuevas, 1992). Table 3 shows some properties of these clays and their comparison with some limit values established at the beginning of the research. It is remarkable the unusual swelling pressure developed by some clays. The physicochemical and mechanical properties of the Madrid Basin clays were in agreement with their particular crystal-chemical nature. In fact, they are composed of smectites of the saponitestevensite group (Mg-smectites). Moreover, green clays were of saponitic nature (Al-substituted tetrahedralcharged), what is very rare in the sedimentary environment in which they formed (Table 4).

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CIEMAT received two samples from each selected deposit --each weighing 200 kg-- to study the granulometric distribution and specific gravity, mineralogy and geochemistry, Atterberg limits and total and external specific surface. Dynamic and static compaction tests were also performed. The superficial thermal conductivity was determined using compacted samples. The results obtained are to be found in Rivas et al. (1991), Villar and Dardaine (1990) and Perez del Villar (1989a, b), and some of them are shown in Table 5.

On the basis of the results obtained by the laboratories, one deposit was selected from each zone, the Cortijo de Archidona at Cabo de Gata, made up mainly of montmorillonite type smectites, and the Cerro del Aguila-Cerro del Monte deposit in the Tertiary Basin of Madrid, which is fundamentally constituted of saponite, accompanied by varying proportions of illite and sepiolite. The criteria taken into account in making the selection were those indicative of the suitability of the material for use as a HLW disposal facility backfill and sealing material: mineralogical purity, high specific surface, high liquid limit, high proportion of size fraction of less than 2 urn, high thermal conductivity and good compressibility, the latter being an additional requirement for the manufacturing of high density blocks. This aspect can be checked in figure 9 for the samples from Toledo and in figure 10 for the samples from Almeria (Rivas et al., 1991): for a given compaction energy (modified Proctor), the densities reached by the samples of Cerro del Aguila (MCA in figure 9) and Cortijo de Archidona (S in figure 10) are the highest.

2.3. Selection of one deposit (1989-1991)

Using the two selected reference clays, S-2 (Cortijo de Archidona in the Serrata de Nijar, Almeria) and MCA-C (Cerro del Aguila-Cerro del Monte, in the province of Toledo), the characterisation of their physicochemical properties and alterability was detailed. For this purpose, a 5000-kg sample from each deposit was used, dried at 60[degrees]C to water content of close to 10 percent and factory ground to a size of less than 5 mm. For use in the laboratory, the samples were ground to less than 2 mm (except in those cases in which the aim was to determine the effect of granulometry on a given property), and were stabilised at laboratory temperature and humidity conditions (18-25[degrees]C, 50-60 percent relative humidity). A part of the prepared sample was sent to the CSIC-Zaidin (S-2), UAM (MCA-C) and CEA (Commissariat a l'Energie Atomique, France) laboratories.

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The characterisation performed at CIEMAT included tests for the determination of different parameters and of the influence on different properties of the dry density and water content of the sample, the temperature of the determination, the maximum grain size used, the addition of quartz sand in different proportions and the preheating of the clay to different temperatures. The following studies were carried out on both clays:

--Dynamic compaction study by manual and semi-automatic beating in an axial press: determination of water contents and pressures necessary to reach particular dry densities, and the influence on final density of granulometry and of the addition of quartz sand in different proportions.

--Unconfined compressive strength: the influence of sample dry density and water content, of the granulometry used and of the addition of different proportions of quartz sand.

--Free swelling.

--Swelling pressure: analysis of the influence of dry density and of the proportion of quartz sand.

--Saturated hydraulic conductivity for different dry densities, proportions of quartz sand and measurement temperatures.

--Thermal conductivity: analysis of the influence of dry density, water content and the proportion of quartz sand.

--Oedometric tests: study of clay consolidation.

--Triaxial tests.

--Ion diffusion study: determination of distribution coefficients for [sup.137]Cs and [sup.60]Co.

--Determination of adsorption-desorption isotherms.

--Study of hydrothermal alterability: influence of treatment time, temperature and KCl concentration.

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The Atterberg limits, swelling pressure, unconfined compressive strength and hydraulic conductivity were also determined for samples subjected previously to heating at 100, 200 and 300[degrees]C (Fig. 11). Also determined were the modifications caused by heating as regards mineralogy, specific gravity, specific external surface and microstructure observed through electron microscopy.

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In addition, hydrothermal alteration experiments were conducted by CSIC on the montmorillonite and by UAM on the saponite. The objective of these studies was to test the stability of each type of clay under heating up to 175[degrees]C in an aqueous solution environment dominated by KCl salts (0.01-0.5 M). The main reactivity response expected was the adsorption of [K.sup.+] cations in the interlayer space of the smectite and, subsequently, its dehydration. This process, together with some crystal-chemical rearrangements, is the illitization reaction, in which the interlayer space collapses due to dehydration and causes the loss of many of the useful properties of smectites (swelling, water retention, cation adsorption).

In the case of the montmorillonite from Cabo de Gata, the potassium ionic exchange was the only process observed, and no other changes were detected in the bentonites after the hydrothermal treatment (Linares et al., 1992, 1993). The saponite from the Madrid Basin was stable up to 175[degrees]C, showing no sign of illitization or crystallochemical changes (Cuevas et al., 1992, 1994). The high hydrothermal stability of saponite had been previously reported by Whitney (1983).

These studies complemented the results obtained at CIEMAT (Rivas et al., 1991; Perez del Villar et al., 1991; Villar and Dardaine, 1990), and allowed the montmorillonite from the Cortijo de Archidona deposit, reference S-2, to be definitively selected, mostly for its better compaction properties, although both clays (montmorillonites and saponites) in fact fulfilled the requirements for use as a barrier material. Good compressibility was taken as selection criterion because it is a key property for the manufacturing of blocks, which is an essential aspect of the installation of the barrier. The conclusions drawn during this phase were as follows:

--Both uniaxial and dynamic compression were adequate for the production of high density compacted blocks.

--The grain size distribution did not modify the final density of the blocks.

--Saturated hydraulic conductivity and swelling pressure depend on the dry density of the clay (pd), in accordance with an exponential relation. A dry density of 1.60 g/[cm.sup.3] satisfies the hydraulic requirements of the barrier.

--The hydraulic conductivity of the clay increases with temperature, this increase possibly being explained in terms of reduction of the kinetic viscosity of the water.

--The thermal conductivity of the material increases with dry density and water content.

--The addition of quartz sand did not imply any major improvement in the properties of the material. Although many properties of the bentonite (hydraulic conductivity, swelling capacity, thermal conductivity) improve with the increase of dry density (see above), and the addition of quartz increases the density of the manufactured blocks for a given compaction energy, it was seen that most of the properties of the bentonite/sand mixtures depend actually on the clay dry density, the quartz acting just as an inert material (Villar and Rivas, 1994). Figure 12 shows how the thermal conductivity of blocks of a given dry density does not increase with the addition of quartz.

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Simultaneously, industrial type uniaxial compaction tests were performed on both clays under a contract with CEA (Martin and Dardaine, 1990; Martin et al, 1990). Also carried out was a clay barrier validation test in a vertical shaft, performed at the Fanay-Silord mine (France) using the S-2 clay (Astudillo et al, 1993). Figure 13 shows the final arrangement of blocks of compacted bentonite before their installation at the vertical shaft. The vertical disposal was one of the concepts considered at that moment.

2.4. Detailed studies on bentonite S-2 (1991-1995)

2.4.1. Thermo-hydro-mechanical and geochemical behaviour studies

The second phase of characterisation of the S-2 clay from the Cortijo de Archidona deposit began in 1991. For these studies, 24 t of bentonite were used, supplied by Minas de Gador in October 1990 and prepared at its Almeria plant by drying at 60[degrees]C and grinding to a size of less than 5 mm. This material was further ground to less than 2 mm for use in the laboratories.

This phase included characterisation of the properties and behaviour of the material under the conditions to which it would be subjected once emplaced in the disposal facility, i.e. when subjected to simultaneous heating and hydration on opposing fronts over long periods of time. For this, the effects caused by thermo-hydraulic flux in compacted blocks and the hydro-mechanical behaviour of the material in unsaturated conditions were studied (Villar, 1995a).

The first issue was addressed by means of tests consisting basically in subjecting cylindrical blocks of compacted clay, confined in hermetically sealed and non-deformable cells, to simultaneous heating and/or hydration on opposing fronts. The dimensions of the clay blocks used for these tests were between 8 and 14 cm in length and between 3 and 15 cm in diameter. Figure 14 shows the main components of a thermo-hydraulic test, with the cell in the central part.

This research provided information about the temperature field inside the clay, the variables affecting water intake, the water content and dry density distribution inside the clay, the chemical and physical changes occurred (changes in cationic exchange capacity, salinity, specific surface), the generation and evolution of saline fronts and the variation of hydro-mechanical properties of the bentonite (Villar et al., 1996, 1997).

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Furthermore, study of the material in the unsaturated state focussed on determination of the free volume suction/water content relation of the clay with different values of water content and density, and on the performance of oedometric tests with controlled suction. The results obtained show a hardening of the bentonite due to the effects of suction and the repercussion of the vertical load applied during hydration of the sample on the reversibility of the strain induced (Villar, 1995b; Villar and Martin, 1996).

Likewise, the characterisation of the clay continued, with new aspects being addressed, such as the determination of porosimetry distribution and of the specific heat depending on temperature. New determinations of specific gravity, cation exchange capacity and exchangeable cations, thermal conductivity, etc. were undertaken. Other issues addressed for the first time were extraction and analysis of the interstitial water and bacteriological analysis (Villar et al., 1997; Cuevas et al., 1997).

2.4.2. Hydrothermal alteration studies

For its part, during the period 1992-1995, CSIC-Zaidin carried out a study of the hydrothermal alteration of bentonites in potassium solutions (Linares et al., 1996). This investigation was focused on the mineralogical stability of smectite to assess its conversion to illite under repository conditions. This reaction requires potassium fixation in the smectite interlayer space and reorganisation of the smectite structure into the illite one. Potassium, temperature and time favour the reaction. The smectite fraction (<20 [micro]m) of S-2 samples was treated with potassium solutions (0.025-1 KCl mol/L) at 60-200[degrees]C for 1 day to 1 year. The analysis of the equilibrium solutions and the characterisation of the final products allowed monitoring the chemical and mineralogical changes undergone by the smectite.

The potassium fixation in the interlayer space of the natural smectite occurs only in solutions concentrated in potassium, which is not the situation in granite formations. On the other hand, a kinetic equation was derived for the smectite-to-illite conversion and different scenarios were considered to determine the evolution of the bentonite mineralogy. For extreme temperature conditions (120[degrees]C) and [10.sup.-5] mol/L in [K.sup.+] (granite solution), after 10.000 years the amount of bentonite remaining in the bentonite is higher than 60 percent (Fig. 15), which is sufficient to maintain the bentonite properties. This conclusion reveals that this bentonite is mineralogically stable enough under repository conditions.

Although saponitic clays from the Madrid Basin had been discarded as reference materials for their worst compressibility, a complementing study was carried out by the UAM group financed by ENRESA in parallel to the detailed study developed by CSIC on the montmorillonitic clay. MCA-C sample was hydrothermally treated for 30 to 365-day periods with granitic groundwater solutions at temperatures of 45 to 200[degrees]C. The material, containing traces of sepiolite and illite, transformed into a mono-mineral saponitic clay at 125-200[degrees]C. The time for conversion was estimated between 20 and 7 years for temperatures of 45 and 200[degrees]C, respectively. In addition to the establishment of a simple kinetic law for the reaction conversion (Cuevas et al., 1998), microstructural (SEMTEM) and surface changes were also studied (Cuevas et al., 2001). In general, the mineralogical and textural changes observed at temperatures below 125[degrees]C were of minor importance. It can be concluded that this type of clays would be suitable in alkaline environments imposed by the presence of concrete in some disposal concepts, since saponitic clays are thermodynamically stable in alkaline solutions.

3. Studies of the bentonite as engineered barrier

3.1. Study of the clay under repository conditions: The FEBEX Project

In the R+D plans performed prior to 1994, which have been partially summarised above, ENRESA studied sources of supply of the materials to be used in the clay barrier, along with its thermal, hydraulic, mechanical and geochemical behaviour. Likewise, integral characterisation studies were performed on granitic massifs. The next step in gaining insight into the feasibility of the AGP concept, and with a view to making progress regarding understanding and evaluation of the near-field behaviour, was the performance of a large-scale experiment, the FEBEX Project (ENRESA, 2000). The purpose of the FEBEX Project was the study of the near-field components of a high-level radioactive waste disposal facility in crystalline rock, in accordance with the Spanish concept (Fig. 1), in which the waste canisters are placed horizontally in galleries, surrounded by a clay barrier made up of high density compacted bentonite blocks (ENRESA, 1995). More specifically, three objectives were mapped out: 1) demonstration of the feasibility of handling and constructing a system of engineered barriers; 2) study of thermo-hydro-mechanical (THM) processes in the near field; and 3) study of thermo-hydro-geochemical (THG) processes in the near field.

The FEBEX Project, coordinated by ENRESA --assisted by NAGRA (Switzerland) in certain aspects-- and co-funded by the European Commission, has included the participation of numerous European organisations and has been developed in two phases: FEBEX I, from 1994 to 1998, and FEBEX II, from 1999 to 2004. The Project consisted of three main parts: an in situ test under natural conditions and at full scale; a test on an almost full-scale mock-up; and a series of laboratory tests aimed at providing information complementary to the two large-scale tests. All these activities served as a support for a far-reaching programme of modelling work (ENRESA, 2000).

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In the two large-scale tests, the thermal effect of the wastes is simulated by means of heaters, while hydration is natural in the in situ test and controlled in the one performed on the mock-up. Both tests are monitored, this allowing the evolution of the temperature, total pressure, water content, water pressure, displacements and other parameters to be obtained continuously in different parts of the barrier and the host rock, this information being used to check the predictions of the thermo-hydro-mechanical and geochemical models. The in situ test is performed in a gallery excavated in the granite of the underground laboratory managed by NAGRA at Grimsel (Switzerland), whereas the mock-up test is being performed at the CIEMAT installations (Madrid). Figure 16 illustrates the process of assembling the bentonite blocks for the mock-up test.

The laboratory tests included characterisation tests and tests for the acquisition of parameters, as well as THM and THG tests, the objective of which is to measure the changes undergone by the bentonite in response to actions analogous to those taking place in the clay barrier and their repercussion on subsequent behaviour. In addition, these tests provide support for the THM and THG modelling, serving to check its predictive capacity. The results obtained in laboratory tests can be found in Villar et al. (1998), ENRESA (2000), Villar (2000, 2002), Fernandez (2003), Missanaeia/. (2004) and Lloretela/. (2004). On the THM side, they refer to 1) the study of the behaviour of the bentonite under high suction and pressure variations, 2) the water retention capacity of the clay, 3) the effect of temperature and salinity on the hydro-mechanical properties, 4) the influence of the hydraulic gradient on permeability. On the THG side, the studies concern 1) the pore water in the bentonite barrier, 2) the geochemical processes taking place at the bentonite/aqueous solution interface, 3) the processes of sorption and transport of radionuclides in the bentonite.

The clay used for all the Project experiments, both the large-scale and laboratory tests --the FEBEX clay--, comes from the Cortijo de Archidona deposit (Almeria), the same from which the S-2 bentonite was taken five years before. A bentonite deposit, even though very homogeneous, has horizontal as well as vertical variations. For a project such as FEBEX, it was important to use material as homogeneous as possible and determine its properties so as to reduce the uncertainties in the subsequent modelisations and the final interpretation of the results of the entire experiment. However, for a performance assessment of a repository, it is necessary to know the range of the relevant properties of a massive source of supply of bentonite. Thus, the comparison with the information and data sets gathered in previous research phases is very useful.

3.2. Study of the long-term evolution of the bentonite engineered barrier: natural analogues

Given that numerous laboratory studies were performed on bentonite properties as candidate material for the engineered barrier, it seemed reasonable to start in 1997 the study of natural analogues from appropriate bentonite deposits, in order to increase our knowledge about the long-term thermo-hydraulic-mechanical-chemical evolution of the buffer material (Barra I project). A natural analogue offers the possibility to study how nature behaves both under different conditions and over different periods of time. Analogues are very useful in helping to understand the phenomena that can occur over long spans of time. For that, a series of bentonite deposit in the Cabo de Gata region (Almeria) -from where the reference clay comeswere selected and studied by CIEMAT, CSIC, Enviros, QuantiSci and ENRESA. These bentonite deposits were Pozo Usero, Morron de Mateo, Cala de Tomate, El Toril, San Jose, El Corralete, Cala Raja, Curva de Felipe y Cortijo de Archidona (Fig. 17), which were classified, according to the analogue processes expected, as follows: Cala de Tomate and Morron de Mateo for the temperature effect, Pozo Usero for pressure effect, El Corralete, Cala Raja and El Toril for the effect of the intrusion of oxidant water, and Cortijo de Archidona, El Corralete y San Jose for the effect of the intrusion of high salinity water (Arcos et al., 2001).

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The results obtained (Pelayo et al., 2000; Arcos et al., 2001) during the first phase of the project advised to reduce the number of bentonite deposits considered, as well as the analogue processes to be studied during the second phase of the project (Barra II project). Thus, the deposits and processes selected and studied were Morron de Mateo for the thermal effect and Cala de Tomate and Cortijo de Archidona for the saline effect. The results, reported in Arcos et al. (2004), Perez del Villar et al. (2005a, b), Fernandez (2003) and Fernandez et al. (2005), are summarised below.

The thermal effect induced by the Morron de Mateo volcanic dome on the adjacent bentonitised tuffaceous beds was studied as a natural analogue of the thermal behaviour of the bentonite engineered barrier. This investigation was focussed on the detection of the chemical differences between smectites from proximal and distal (El Murciano area) zones to the dome, as well as to test whether the temperatures calculated based on the oxygen and hydrogen isotopic values (Delgado, 1993) correspond to their formation or transformation (Fig. 18, Fig. 19).

[FIGURE 17 OMITTED]

The initial hypothesis was that the chosen smectites could be formed under marine conditions, being later transformed and isotopically re-equilibrated with hot fluids in the vicinity of the intrusion. To check this hypothesis, a detailed mineralogical, chemical, geochemical and isotopic study was performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome. The results show that distal smectites are dioctahedral Al-smectites, similar to those from other deposits in the region, whereas proximal smectites are dioctahedral Fe and Mg-rich smectites (except in just one proximal sample in which both di- and trioctahedral smectites coexist), showing two evolutionary trends on a Fe-Mg-Al ternary diagram (Fig. 20a). Similar features are observed when their structural formulae are plotted on the muscovite-celadonite-pyrophylite diagram (Fig. 20b). Thus, they plot in the smectite domain with interlayer charge less than 1, which is mainly due to octahedral substitution for distal smectites, whereas for proximal ones is caused by both octahedral and tetrahedral substitutions. In this ternary diagram, the domains of both proximal and distal smectites are partially overlapped. Further, proximal biocalcarenites are enriched in Fe-rich dolomite in relation to the distal ones.

The [sup.87]Sr/[sup.86]Sr and [delta][sup.13]C values in carbonates and [delta]D in smectites indicate equilibrium with seawater. In contrast, [delta][sup.18]O values of carbonates and smectites indicate that these minerals were transformed and re-equilibrated between 40 and 90[degrees]C, and between 55 and 66[degrees]C, respectively, independently of their location with respect to the dome.

[FIGURE 18 OMITTED]

These data suggest that the transformation of calcite into Mg-Fe-carbonates and the occurrence of Fe- and Mg-rich smectites close to the dome resulted from a chemically induced process at similar temperatures and, at least, under transitional redox conditions. The compositional differences among samples suggest that Fe, Mg and minor Mn were supplied by a probable contaminant front originating from the dome, migrating through the sediments and becoming more diluted away from the source. The absence of a well-defined thermal gradient in the system could be due to the small size, semi-closed and shallow character of the basin, as well as to its high underlying volcanic activity.

From a physicochemical point of view, no significant differences in the total exchange cations and BET specific surface area values are observed in relation to the crystallochemical differences observed between proximal and distal smectites.

As we have seen above, the candidate bentonite to build the bentonite-engineered barrier is an Al-smectite similar to that of the distal zone (El Murciano area) of the Morron de Mateo volcanic dome (Astudillo, 2001). Furthermore, the thermal peak induced by radioactive decay of the nuclear spent fuel after burial, estimated to reach 100[degrees]C, is another important physical variable of the engineered system, since this parameter will have a large influence on the hydration state of the bentonite barrier. This temperature is also similar to that calculated for the studied natural system.

After sealing of a deep geological repository, it is expected that the steel container will be affected by two corrosion phases. The first will occur immediately post-closure, under aerobic conditions, and as a result of the oxidation by the air occluded in the bentonite barrier. These effects will be weak and restrictive in space. In contrast, during the second phase, which will start when the barrier is totally saturated (approximately 103-104 years), the oxidation of the steel container will occur, resulting in the formation of both magnetite ([Fe.sub.3][O.sub.4]) and Fe-oxyhydroxides or siderite (FeC[O.sub.3]), depending on the physicochemical conditions in the system and the production of hydrogen. In any case, the probable coexistence of Fe(II) and Fe(III) indicates that transitional redox conditions can prevail in the engineered system.

With respect to the natural system all these physicochemical conditions were met as a result of the volcanic dome intrusion. So, relatively high temperature, Fe(II)Fe(III) concentration, total hydration of the smectitised tuffaceous bed and transitional redox conditions worked all together close to the Morron de Mateo volcanic dome. Consequently, among the main processes expected in the bentonite engineered barrier, the alteration and/or transformation of Al-smectites are the most relevant, as a result of the interaction among smectite, temperature and geochemical conditions.

[FIGURE 19 OMITTED]

Based on these analogies, the neoformation of Fe (Mg)rich smectites observed in the proximal zones of the natural system would be expected in the bentonite barrier of the repository, in which an enhanced Fe concentration close to the container can be induced as a result of its partial oxidation under intermediate redox conditions. In this scenario, no variations in the physicochemical properties, i.e. total exchange cations and BET specific surface area, of the bentonite barrier will occur, as this study demonstrated. However, under a stronger or longer interaction, the neoformation of trioctahedral smectites such as saponite-stevensite, Fe(II)-rich saponite or dioctahedral nontronite (as a function of the Mg and Fe(II)/Fe(III) activities in the environment, respectively), corrensite and Fe-Mg-rich chlorite may occur as recorded in some active geothermal and diagenetic systems (Yamada and Nakasawa, 1993; Beaufort et al., 1995, 2001; Bril et al., 1996; Robinson and Santana de Zamora, 1999; Mayayo et al., 2000; Hover and Ashley, 2003). Under these circumstances, significant variations in the physicochemical and physicomecanical properties of the bentonite engineered barrier of a HLW deep geological repository could be expected.

Though the studies performed on the bentonites from the Cala de Tomate deposit were primarily focussed on the thermal effect (Fig. 21), the results do not allowed the establishment of any analogy in relation to this expected effect. Thus, the geology of the site showed that the contact between the Cala de Tomate volcanic dome and the bentonitised tuff is re-taken by faults, and that the bentonitisation process was mainly favoured by this faulting event.

The mineralogy of bentonites indicates that the major mineral is an Al-rich dioctahedral smectite (montmorillonite type), as in other bentonite deposits in the region (Reyes, 1977; Reyes et al., 1987; Linares et al., 1993; Delgado, 1993), including the Cortijo de Archidona deposit, from which the Spanish reference bentonite comes. The most significant and differential feature of the smectites from the Cala de Tomate deposit is the nature of their exchangeable cations, since Mg-Ca and Na-Mg-smectites coexist: Na-Mg-smectites are present in samples located in the deepest parts of the exploration exposures, whereas Mg-Ca-rich smectites are present in their upper parts with respect to the topographic surface.

[FIGURE 20 OMITTED]

[FIGURE 21 OMITTED]

The isotopic signature of these smectites agrees with those formed in equilibrium with meteoric waters at <25[degrees]C, whereas the isotopic signature of minor carbonates in some bulk samples suggests that these smectites were under submarine condition after their formation. As a consequence, it can be stated that these smectites were formed under meteoric diagenesis, particularly in the faulting zones originated after intrusion of the Cala de Tomate dome. However, the environmental conditions under which these smectites were primarily formed seem to be incompatible with the presence of Na, which is the main exchangeable cation in some specimens. This fact can be explained considering that the deposit was under submarine conditions after its formation. The high concentrations of [Na.sup.+] and [Cl.sup.-] determined in soluble salts of the bulk samples seem to corroborate this hypothesis.

After emersion, the interaction between Na-Mg-smectites and fresh water produced cation exchange processes in the upper parts of the deposit. Thus, Na-Mg-smectites became Mg-Ca-smectites in the surface, whereas Na-Mg-rich smectites prevailed in the deeper parts of the bentonitised faulting zones.

The transformation of Ca-rich smectites into Na-rich smectites can also be expected in the bentonite barrier of a deep geological repository in the case that a Ca-rich bentonite is used and it is invaded by Na-rich saline water. Under these hypothetical conditions, Ca-rich bentonite becomes Na-rich and some of the most important features of the bentonite-engineered barrier, such as the reological and colloidal properties could be modified. Deep Na-rich saline waters and brines, regardless their origin, are frequent in crystalline basements suitable for radwaste disposals, such as the Canadian and Fennoscandinavian shields and the crystalline basement of Central and Western Europe, particularly in the Bohemian Massif and the Vienne district in W-France (Kloppmann et al., 2002).

The Cortijo de Archidona deposit was selected as a natural analogue of the behaviour of bentonite influenced by natural saline waters, since it has been affected by different water intrusions during its recent geological history. The chemistry of the pore water can lead to important changes in the bentonite properties due to different processes such as illitisation, cementation, dissolution-precipitation of accessory minerals and cation exchange.

The Cortijo de Archidona deposit is related to two large fractures (NE-SW and N-E), which affect the volcano-sedimentary sequence (Fig. 22). This bentonite originated from the alteration of dark-coloured vesicular rhyodacitic glasses and light-coloured ignimbrites. The alteration process was favoured by the intense brecciation of these materials in the fracture zones, which in turn also allowed the passage of the alteration solutions. The bentonite mineralogy and microfabric, the pore water composition and the physico-chemical properties of different core samples taken from six 20-m depth boreholes were determined. Special emphasis has been given to the role of the water-bentonite interaction processes involving accessory nonclay minerals, which affect the bentonite cementation and its buffer capacity for pH and redox potential of the nearfield pore waters.

Three zones can be distinguished in the bentonite deposit: i) the smectitic zone; ii) the contact zone with the volcanic rocks; and iii) the transitional zone between both of them. The smectitic zone is located at the central part of the deposit, where the major mineral phase (78-97 mass percent) is a dioctahedral Al-Fe rich smectite. The contact zone with the volcanic rocks is characterised by the presence of bentonite with important amounts of pyroclastic materials (>50 mass percent) and other clay minerals, apart from smectite, such as illite, chlorite and fibrous minerals of sepiolite-palygorskite type. The transitional zone between the bentonite massive zone and the bentonite in contact with the volcanic rocks corresponds to a bentonite with a 30-50 mass percent of pyroclasts.

[FIGURE 22 OMITTED]

Some properties like the chemical stability, the long-term durability, the swelling capacity and the high exchange capacity of the smectite of the Cortijo de Archidona deposit have been preserved during its geological history. The bentonite in the smectitic zone is almost water saturated and the clay minerals are stable. However, two types of heterogeneities have been found: 1) the chemical composition of the pore waters; 2) the composition of the exchangeable sites. The chemical composition of the pore waters, obtained by squeezing, varies from Na-Mg-S[O.sub.4] to Na-Cl water type, being Na-Cl type in the smectitic zone. The pore water salinity is higher in the upper part of the deposit and decreases downwards (Fig. 23), indicating a possible diffusive transport of seawater through the bentonite. The ionic strength varies from 0.23 to 0.02 M and the pH is neutral (6.8-8.0), which implies the stability of the clay minerals. According to the chemical composition of the pore waters and the stable isotopes study, two bentonite post-formational processes have been found. First, a seawater intrusion, since the Cl/Br and S[O.sub.4]/Cl ratios are similar to those of seawater. Subsequently, an infiltration of new Ca-Mg rich meteoric waters. These waters interacted with the bentonite and modified the pore water chemistry and the concentration of cations in the exchangeable sites. The composition of the exchange complex is Ca-Mg type, with high contents of sodium. At the top of the deposit, where the salinity is higher, the content of sodium as exchangeable cation increases, replacing calcium. In any case, the bentonite has preserved a high concentration of solutes and its mineral stability. These Ca-Mg-rich meteoric waters caused the aforementioned new post-formational alteration of the pyroclastic rocks.

4. Summary and conclusions

The study of clays for their use as sealing materials in high-level radioactive waste repositories started in Spain in the late 80's. Smectites and illites were proposed at the beginning, as well as mixtures of them with quartz sand or crushed granite. Among the smectites, montmorillonites from the Cabo de Gata region (Almeria) and saponites from the Madrid Basin (Toledo) were found to be suitable materials to construct clay barriers, since their mineralogical, geochemical, physicochemical, thermal, hydraulic and mechanical properties are the adequate ones. For its best compaction properties, a bentonite from the Cortijo de Archidona deposit (Almeria) was selected as Spanish reference material. Detailed and specific studies were carried out in this clay, concerning particularly its alterability and its behaviour under conditions similar to those of a repository: temperature gradients, water flow and unsaturated conditions during the transient stage. As a continuation to the initial stages of the investigation, two new researching lines were opened: one of them devoted to the study of the bentonite under conditions as close as possible to those in the repository, at the laboratory and in large-scale tests; and the other to the study, by means of natural analogues, of the long-term evolution of the barrier. Due to the social and environmental repercussion of the radwaste disposal, the research on the areas presented is still ongoing.

[FIGURE 23 OMITTED]

Acknowledgements

This paper is dedicated to Pedro Rivas, a pioneer in the study of some of the issues herein presented and the head of many of the research projects compiled. He has participated intensely in all the described investigations.

The useful suggestions and comments from the referees A. Meunier and R. Pusch have improved the paper and are gratefully acknowledged.

Received: 10/04/05 / Accepted: 25/09/05

References

Arcos, D., Bruno, J., Linares, J., Martinez, J., Caballero, E., Jimenez, C., Fernandez, A. M., Pelayo, M., Villar, M.V., Rivas, P., Perez del Villar, L. (2001): Bentonitas como analogo natural del campo proximo: proyecto Barra I. Publicacion Tecnica ENRESA 08/01, 223-243. Madrid.

Arcos, D., Bruno, J., Perez del Villar, L., Fernandez, A. M., Pelayo, M., Cozar, J. S., Delgado, A., Reyes, E., Raya, J., Fernandez-Soler, J. M., Cardellach, E., Grandia, F., Tsige, M., Hernan, P., Lucini, M. (2004): Analogos naturales del comportamiento a largo plazo de la bentonita de la barrera de ingenieria (Proyecto Barra II). Publicacion Tecnica ENRESA 06/04, 276-292. Madrid.

Astudillo J. (2001): El almacenamiento geologico profundo de los residuos radiactivos de alta actividad. Principios Basicos. ENRESA. 200 p. Madrid.

Astudillo, J., del Olmo, C., Rivas, P., Martin, P.L., Villar, M.V, Perez del Villar, L., Pardillo, J., Dardaine, M., Lajudie, A. (1993): Field test for the demonstration of the emplacement feasibility of clay buffer material as engineered barrier in crystalline formation (Volume 3). Development of the industrial manufacturing processes and field validation of the buffer material concept. Nuclear Science and Technologies. EUR 15692/3. European Commission. 232 p. Luxembourg.

Beaufort, D., Papapanagiotou, P., Patrier, P., Traineau, H. (1995): Les interstratifies I-S et C-S dans les champs geothermique actifs: sont-ils comparables ceux des series diagenetiques? Bulletin des Centres de Recherches ExplorationProduction Elf-Aquitaine, 19: 267-294.

Beaufort, D., Berger, G., Lacharpagne, J.C., Meunier, A. (2001): An experimental alteration of montmorillonite to a di+trioctahedral smectite assemblage at 100 and 200[degrees]C. Clay Mineralogy, 36: 211-225.

Bril, H., Papapanagiotou, P., Patrier, P., Lenain, J.F., Beaufort, D. (1996): Fluid-rock interaction in the geothermal field of Chipilapa (El Salvador): Contribution of fluid inclusion data. European Journal of Mineralogy, 8: 515-531.

Caballero, E., Reyes, E., Linares, J., Huertas, F. (1985): Hydrothermal solutions related to bentonite genesis, Cabo de Gata region, Almeria, SE Spain. Mineralogy and Petrography Acta, 29-A: 187-196.

Cuadros, J., Linares, J. (1996): Experimental kinetic study of the smectite-to-illite transformation. Geochimica et Cosmochimica Acta, 60 (3): 439-453.

Cuevas, J. (1992): Caracterizacion de esmectitas magnesicas de la Cuenca de Madrid como materiales de sellado. Ensayos de alteracion hidrotermal. Publicacion Tecnica ENRESA 04/92. 183 p. Madrid.

Cuevas, J., Medina, J.A., Leguey, S. (1992): Saponitic clays from Madrid Basin: Accesory minerals influence in hydrothermal reactivity. Applied Clay Science, 7: 185-199.

Cuevas, J., Pelayo, M., Rivas, P., Leguey, S. (1993): Characterization of Mg-clays from the Neogene of the Madrid Basin and their potential as backfilling and sealing material in high level radioactive waste disposal. Applied Clay Science, 7: 383-406.

Cuevas, J., Leguey, S., Pusch, R. (1994): Hydrothermal stability of saponitic clays from the Madrid Basin. Applied Clay Science, 8: 467-484.

Cuevas, J., Villar, M.V., Fernandez, A.M., Gomez, P., Martin, P.L. (1997): Pore waters extracted from compacted bentonite subjeted to simultaneous heating and hydration. Applied Geochemistry, 12: 473-481.

Cuevas, J., Garralon, A., Ramirez, S., Leguey, S. (1998): Kinetic approach to the mineral reaction processes during hydrothermal treatment of a saponitic clay. Clay Minerals, 33: 409-421.

Cuevas, J., Garralon, A., Ramirez, S. y Leguey, S. (2001): Hydrothermal alteration of saponitic bentonite: mineral reactivity and surface properties. Clay Minerals, 36: 61-74.

Delgado, A. (1993): Estudio isotopico de los procesos diageneticos e hidrotermales relacionados con la genesis de bentonitas (Cabo de Gata, Almeria). Ph. D. Thesis, CSIC-Zaidin. 413 p. Granada.

ENRESA (1995): Almacenamiento geologico profundo de residuos radiactivos de alta actividad (AGP). Disenos conceptuales genericos. Publicacion Tecnica ENRESA 11/95. 105 p. Madrid.

ENRESA (1998): FEBEX. Bentonite: origin, properties and fabrication of blocks. Publicacion Tecnica ENRESA 4/98. 146 p. Madrid.

ENRESA (2000): FEBEX Project. Full-scale engineered barriers experiment for a deep geological repository for high level radioactive waste in crystalline host rock. Final Report. Publicacion Tecnica ENRESA 1/2000. 354 p. Madrid.

Fernandez, A.M. (2003): Caracterizacion y modelizacion del agua intersticial de materiales arcillosos: estudio de la bentonita de Cortijo de Archidona. Ph.D. Thesis. Univ. Autonoma de Madrid. 505 p. Madrid.

Fernandez, A.M., Arcos, D., Pelayo, M., Tsige, M., Fernandez-Soler, J.M., Rivas, P., Perez del Villar, L. (2005): Natural analogues in the performance assessment of a nuclear waste disposal: the Cortijo de Archidona deposit and the salinity effect. Presented at the 2nd International Meeting on Clays in Natural & Engineered Barriers for Radioactive Waste Confinement (ANDRA, Tours 14-18 March). Abstracs 293294.

Fernandez-Soler, J.M. (1992): El volcanismo calco-alcalino de Cabo de Gata (Almeria). Estudio volcanologico y petrologico. Ph. D. Thesis Universidad de Granada. 243 p. Granada.

Fernandez-Soler, J.M. (2002): Thermal Effect: Morron de Mateo outcrops. Geological setting Memoir. Barra II Project Report. 41p.

Goguel, J., Candes, P., Izabel, C. (1987): Stockage des dechets radioactifs en formations geologiques. Criteres techniques de choix de site. Rapport du groupe de travail preside par le professeur Goguel. Ministere de l'Industrie, des P. and T. et du Tourisme. 245 p.

Guven, N. (1990): Longevity of bentonite as buffer material in a nuclear-waste repository. Engineering Geology, 28(3-4): 233-248.

Hover, V.C., Ashley, G.M. (2003): Geochemical signatures of paleodepositional and diagenetic environments: a STEM/AEM study of autogenic clay minerals from an arid rift basin, Olduvai Gorge, Tanzania. Clays and Clay Minerals, 51: 231-251.

Kloppmann, W., Girard, J-P., Negrel, Ph. (2002): Exotic stable isotope compositions of saline waters and brines from the crystalline basement. Chemical Geology, 184: 49-70.

Linares J., Huertas F., Barahona E. (1992): Conversion time from smectite to illite. A preliminary study. Applied Clay Science, 7: 125-130.

Linares, J., Huertas, F., Reyes, R., Barahona, E., Caballero, E., Yanez, J., Guardiola, J.L. Romero, E., Delgado, A., Rodriguez, J., Martin-Vivaldi, M.T. (1993): Caracterizacion de bentonitas como materiales de sellado en almacenamientos de residuos radiactivos. Publicacion Tecnica ENRESA 1/93. 327 p. Madrid.

Linares, J., Barahona, E., Huertas, F., Caballero, E., Cuadros, J., Huertas, F.J., Jimenez de Cisneros, C., Linares, C., Rodriguez, J., Martin-Vivaldi, M.T., Civantos, M.J. (1996): Alteracion hidrotermal de las bentonitas de Almeria. Publicacion Tecnica ENRESA 6/96. 151 p. Madrid.

Lloret, A., Romero, E., Villar, M.V. (2004): FEBEX II Project. Final report on thermo-hydro-mechanical laboratory tests. Publicacion Tecnica ENRESA 10/04. 180 p. Madrid.

Martin, P.L. (2004): Desarrollo y diseno de ensayos termo-hidromecanicos integrados para la validacion del concepto espanol de almacenamiento de residuos nucleares de alta actividad. Ph.D. Thesis. Universidad Politecnica de Madrid. 408 p. Madrid.

Martin, P.L., Dardaine, M. (1990): Resultados preliminares de compactacion uniaxial industrial sobre dos bentonitas espanolas seleccionadas: MAC-C de la Cuenca de Madrid y S-2 de la zona de Almeria. Resultados de la Fase I. Contrato CEA/ENRESA A2845. Rapport Technique RDD 98/17. 57 p. Fontenay-aux-Roses.

Martin, P.L., Dardaine, M., Lajudie, A. (1990): Resultados de compactacion uniaxial industrial sobre una bentonita espanola seleccionada: S-2 de la zona de Almeria. Resultados de la Fase I. Contrato CEA/ENRESA A2845. Rapport Technique DSD 90/18. 53 p. Fontenay-aux-Roses.

Mayayo, M. J., Bauluz, B., Gonzalez Lopez, J.M. (2000): Variations in the chemistry of smectites from the Calatayud Basin (NE Spain). Clay Mineralogy, 35: 365-374.

Mingarro, E., Rivas, P., Villar, L.P. Del, Cruz, B. De La, Gomez, P., Hernandez, A., Turrero, M.J., Villar, M.V., Campos, R., Cozar, J. (1991): Characterization of clay (bentonite)/ crushed granite mixtures to build barriers against the migration of radionuclides: diffusion studies and physical properties. Task 3-Characterization of radioactive waste forms. A series of final reports (1985-1989). Nuclear science and technology series, 35. Commission of the European Communities. 136 p. Luxembourg.

Missana, T., Fernandez, A.M., Garcia-Gutierrez, M., Mingarro, M., Rivas, P., Caballero, E., Huertas, F.J., GarciaPalma, S., Jimenez de Cisneros, C., Linares, J., Rozalen, M.L., Baeyens, B., Bradbury, M.H., Muurinen, A., Cormenzana, J.L. (2004): FEBEXII Project. THG Laboratory Experiments. Publicacion Tecnica ENRESA 09/2004. 137 p. Madrid.

Pelayo, M., Martinez, J.A., Perez del Villar, L. (2000): Effects of acid solutions on the mineralogical and geochemical characteristics of the bentonite from the " El Toril" deposit (Almeria, Spain). Proceedings of the 1st Latin American Clay Conference, Funchal 2000, II: 255-260.

Perez del Villar, L. (1989a): Caracterizacion mineralogica y geoquimica de algunas muestras de bentonita procedentes de la Cuenca terciaria de Madrid (Yuncos, Toledo). CIEMAT report. 29 p. Madrid.

Perez del Villar, L. (1989b): Caracterizacion mineralogica y geoquimica de algunas muestras de bentonita procedentes de la provincia de Almeria. CIEMAT report. 29 p. Madrid.

Perez del Villar, L., De La Cruz, B., Cozar, J.S. (1991): Estudio mineralogico, geoquimico y de alterabilidad de las arcillas de Serrata de Nijar (Almeria) y del Cerro del Monte (Toledo). CIEMAT report. 51 p. Madrid.

Perez del Villar, L., Delgado, A., Reyes, E., Pelayo, M., Fernandez-Soler, J.M., Cozar, J.S., Tsige, M., Quejido, A.J. (2005a): Thermochemically induced transformations in Al-smectites: A Spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal. Applied Geochemistry, 20(12): 2252-2282.

Perez del Villar, L., Pelayo, M., Fernandez, A.M , Cozar, J.S., Delgado, A., Reyes, E., Fernandez-Soler, J.M., Tsige, M. (2005b): Cation exchange processes in the Cala de Tomate bentonite deposit (Spain): implications for the bentonite behaviour as an engineered barrier. Presented at the 2nd International Meeting on Clays in Natural & Engineered Barriers for Radioactive Waste Confinement (ANDRA, Tours 14-18 March).

Pusch, R. (1979): Highly compacted sodium bentonite for isolating rock-deposited radioactive waste products. Nuclear Technology, 45: 153-157.

Pusch, R., Gray, M.N. (1989): Sealing of radioactive waste repositories in crystalline rock. OECD-NEA/CEC. Proceedings of the Workshop on Sealing of Radioactive Waste Repositories. Braunschweig.

Reyes, E. (1977): Mineralogia y geoquimica de las bentonitas de la zona norte de Cabo de Gata (Almeria). Ph. D. Thesis. Universidad de Granada. 650 p. Granada.

Reyes, E., Caballero, E., Huertas, F., Linares, J. (1987): Bentonite deposits from Cabo de Gata region, Almeria, Spain. Field Book Guide. Euroclay-87. Excursion A: 9-32.

Rivas, P., Villar, M.V., Campos, R., Pelayo, M., Martin, P.L., Gomez, P., Turrero, M.J., Hernandez, A.I., Cozar, J.S., Mingarro, E. (1991): Caracterizacion de materiales de relleno y sellado para almacenamiento de residuos radiactivos: bentonitas espanolas. CIEMAT-DT-TG report. 196 p. Madrid.

Robinson, D., Santana de Zamora, A., (1999): The smectite to chlorite transition in the Chipilapa geothermal system, El Salvador. American Mineralogy, 84: 607-619.

Villar, M.V (1995a): Thermo-Hydro-Mechanical characterization of the Spanish reference clay material for engineered barrier for granite and clay HLW repository: Laboratory and small mock-up testing. Publicacion Tecnica ENRESA 03/95. 251 p. Madrid.

Villar, M.V. (1995b): First results of suction controlled oedometer tests in highly expansive montmorillonite. In: E.E. Alonso, P. Delage (eds): Unsaturated soils. Proceedings of the 1st International Conference on Unsaturated Soils (Paris). Balkema. Rotterdam. 207-213.

Villar, M.V. (2000): Caracterizacion termo-hidro-mecanica de una bentonita de Cabo de Gata. Ph. D. Thesis. Universidad Complutense de Madrid. 396 p. Madrid.

Villar, M.V. (2002): Thermo-hydro-mechanical characterisation of a bentonite from Cabo de Gata. A study applied to the use of bentonite as sealing material in high level radioactive waste repositories. Publicacion Tecnica ENRESA 01/2002. 258 p. Madrid.

Villar, M.V, Dardaine, M. (1990): Contribucion a la seleccion de una arcilla espanola como material de relleno de un almacenamiento de combustible irradiado en roca dura. Resultados de la Fase I. Contrato CEA/ENRESA A2845. Rapport Technique DRDS no.] 244. CEA. 51 p. Fontenay-aux-Roses.

Villar, M.V., Rivas, P. (1994): Hydraulic properties of montmorillonite-quartz and saponite-quartz clay mixtures. Applied Clay Science, 9: 1-9.

Villar, M.V., Martin, P.L. (1996): Suction controlled oedometer tests in montmorillonite clay. Preliminary results. In: S.P. Bentley (ed.): Enginering geology of waste disposal. Geological Society Engineering Geology Special Publication No. 11: 309-311. The Geological Society. London.

Villar M.V., Cuevas J., Martin P.L. (1996): Effects of heat/water flow interaction on compacted bentonite: Preliminary results. Engineering Geology, 41: 257-267.

Villar, M.V., Fernandez, A.M., Cuevas, J. (1997): Caracterizacion geoquimica de bentonita compactada: Efectos producidos por flujo termohidraulico. Version 1. Technical report CIEMAT/IMA/54A15/6/97. FEBEX report 70-IMA M-0-2. 51 p. Madrid.

Villar, M.V., Martin, P.L., Pelayo, M., Ruiz, B., Rivas, P., Alonso, E., Lloret, A., Pintado, X., Gens, A., Linares, J., Huertas, J., Caballero, E., Jimenez de Cisneros, C., Obis, J., Perez, A., Velasco, J. (1998): FEBEX. Bentonite: origin, properties and fabrication of blocks. Publicacion Tecnica ENRESA 05/98. 146 p. Madrid.

Whitney, G. (1983): The hydrothermal reactivity of saponite. Clays and Clay Minerals, 16: 297-304.

Yamada, H., Nakasawa, H. (1993): Isothermal treatments of regularly interstratified montmorillonite-beidellite at hydrothermal conditions. Clays and Clay Minerals, 41: 726-730.

Yong, R.N., Boonsinsuk, P., Wong, G. (1986): Formulation of backfill material for a nuclear fuel waste disposal vault. Canadian Geotechnical Journal, 23: 216-228.

M.V. Villar (1), L. Perez del Villar (1), P.L. Martin (1), M. Pelayo (1), A.M. Fernandez (1), A. Garralon (1), J. Cuevas (2), S. Leguey (2), E. Caballero (3), F.J. Huertas (3), C. Jimenez de Cisneros (3), J. Linares (3), E. Reyes (3), A. Delgado (3), J.M. Fernandez-Soler (4), J. Astudillo (5)

(1) Dpto. Medioambiente, CIEMAT, Avda. Complutense, 22, 28040 Madrid (Spain) e-mail: mv.villar@ciemat.es, l.pvillar@ciemat.es, p.lmartin@ciemat.es, m.pelayo@ciemat.es, anamaria. fernandez@ciemat.es, antonio.garralon@ciemat.es

(2) Dpto. Quimica Agricola, Geologia y Geoquimica, UAM, Ciudad Universitaria de Cantoblanco, 28049 Madrid (Spain) e-mail: jaime.cuevas@uam.es, santiago.leguey@uam.es

(3) Estacion Experimental del Zaidin, CSIC, Prof. Albareda, 1, 18008 Granada (Spain) e-mail: ecabal@eez.csic.es, javier.huertas@eez.csic.es, cisneros@eez.csic.es, jlinares@eez.cisc.es, emilio.reyes@eez. csic.es, antonio.delgado@eez.csic.es

(4) Dpto. Mineralogia y Petrologia, UGR, Avda. Fuentenueva, s/n, 18002 Granada (Spain) e-mail: jmfsoler@ugr.es

(5) ENRESA, Emilio Vargas, 7, 28043, Madrid (Spain) e-mail: jasp@enresa.es
Table 1.- Average mineralogical composition of the bentonites from
the deposits selected in the Almeria area (CSIC-Zaidin, Linares et
al., 1993).

Tabla 1.- Composicion mineralogica media de las bentonitas de los
depositos seleccionados en el area de Almeria (CSIC-Zaidin, Linares
et al., 1993).

 Mineral (%)         Los           Cortijo de        Morron de
                   Trancos         Archidona         Mateo (1)

Smectite        97 [+ or -] 2    92 [+ or -] 4    72 [+ or -] 11
Cristobalite     1 [+ or -] 1     2 [+ or -] 1     6 [+ or -] 4
Quartz           1 [+ or -] 1     2 [+ or -] 1     3 [+ or -] 2
Plagioclase      7 [+ or -] 1     3 [+ or -] 1     9 [+ or -] 4
Calcite             Traces        2 [+ or -] 2     4 [+ or -] 4
Biotite             Traces        1 [+ or -] 1        Traces

(1) Also different quantities of pyroxenes, amphiboles, K-feldspars,
zeolites and pyrolusite

Table 2.- Structural formulae calculated on a [O.sub.20][(OH).sub.4]
basis of the smectites from the deposits selected in the Almeria
area (CSIC-Zaidin, Linares et al, 1993).

Tabla 2.- Formula estructural calculada sobre [O.sub.20][(OH).sub.4]
de las esmectitas a partir de los depositos seleccionados en el area
de Almeria (CSIC-Zaidin, Linares et al., 1993).

Deposit           [Si.sub.IV.sup.4+]   [Al.sub.IV.sup.3+]

Los Trancos       7.74                 0.26
C. Archidona      7.78                 0.22
Morron de Mateo   7.61                 0.38

Deposit           [Al.sub.VI.sup.3+]   [Fe.sub.VI.sup.3+]

Los Trancos       2.93                 0.24
C. Archidona      2.78                 0.33
Morron de Mateo   2.57                 0.51

Deposit           [Mg.sub.VI.sup.2+]   [X.sup.+]   [X.sub.VI]

Los Trancos       0.84                 0.85        0.58
C. Archidona      1.03                 0.82        0.59
Morron de Mateo   1.10                 0.91        0.52

Deposit           [[SIGMA].sub.VI]

Los Trancos       4.12
C. Archidona      4.15
Morron de Mateo   4.19

Table 3.- Properties of clays from the Madrid Basin (Max: maximum
value, Min: minimum value, Avg: average value) and limit values of
these properties established for sealing and backfilling purposes
(Cuevas et al., 1993).

Tabla 3.- Propiedades de las arcillas de la Cuenca de Madrid (Max:
valor maximo, Min: valor minimo, Avg: valor medio) y valores limite
establecidos de estas propiedades, para arcillas con propositos de
relleno y sellado (Cuevas et al., 1993).

                                                   Cerro del Aguila

            Parameter                Limit value   Max    Min    Avg

Fraction size <2i m (%)                  >80       98     85     93
Smectite content (%)                     >60       96     58     82
Amorphous content (%)
Si[O.sub.2]                              <1        0.4    0.1    0.2
Fe2[O.sub.3]                                       --     --     --
[Al.sub.2][O.sub.3]                                --     --     --
CEC (mmol/kg)                            >60       112    53     77
Organic matter (%)                      <0.2       0.09   0.04   0.07
CaC[O.sub.3] (%)                         <6        --     --     --
CEC (meq/100g)                                                   77
Specific surface BET ([m.sup.2]/g)                               155
Swelling pressure (1) (MPa)                                      6

                                      Santa Barbara

            Parameter                Max    Min    Avg

Fraction size <2i m (%)              93     76     85
Smectite content (%)                 kerolite-stevensite
Amorphous content (%)
Si[O.sub.2]                          0.4    0.1    0.2
Fe2[O.sub.3]                         0.04   0.02   0.03
[Al.sub.2][O.sub.3]                  --     --     --
CEC (mmol/kg)                        57     38     49
Organic matter (%)                   0.58   0.27   0.37
CaC[O.sub.3] (%)                     0.3    --     0.1
CEC (meq/100g)                                     49
Specific surface BET ([m.sup.2]/g)                 257
Swelling pressure (1) (MPa)                        12

                                           Yuncos

            Parameter                Max    Min    Avg

Fraction size <2i m (%)              96     79     88
Smectite content (%)                 86     69     81
Amorphous content (%)
Si[O.sub.2]                          0.1    0.04   0.06
Fe2[O.sub.3]                         0.23   0.07   0.14
[Al.sub.2][O.sub.3]                  --     --     --
CEC (mmol/kg)                        86     59     73
Organic matter (%)                   0.11   0.05   0.09
CaC[O.sub.3] (%)                     8.0    6.0    5.0
CEC (meq/100g)                                     73
Specific surface BET ([m.sup.2]/g)                 12
Swelling pressure (1) (MPa)                        4

(1) For dry density of 1.4 g/[cm.sup.3]

Table 4.- Structural formulae calculated on a [O.sub.20][(OH).sub.4]
basis of smectites from the deposits selected in the Madrid Basin
(Cuevas et al, 1993).

Tabla 4.- Formula estructural calculada sobre una base de
[O.sub.20][(OH).sub.4] de esmectitas a partir de depositos
seleccionados en la Cuenca de Madrid (Cuevas et al., 1993).

Deposit            [Si.sub.IV.sup.4+]   [Al.sub.IV.sup.3+]

Cerro del Aguila   7.26                 0.74
Santa Barbara      7.69                 0.31
Yuncos             7.75                 0.25

Deposit            [Al.sub.VI.sup.3+]   [Fe.sub.VI.sup.3+]

Cerro del Aguila   0.52                 0.33
Santa Barbara      0.09                 0.07
Yuncos             0.43                 0.20

Deposit            [Mg.sub.VI.sup.2+]   [Ti.sub.VI.sup.4+]   CHO

Cerro del Aguila   4.56                 0.04                 -0.16
Santa Barbara      5.62                 0.00                 -0.28
Yuncos             4.80                 0.02                 -0.42

Deposit            [[SIGMA].sub.VI]   [Ca.sup.2+]   [K.sup.+]

Cerro del Aguila   5.47               0.39          0.12
Santa Barbara      5.78               0.29          0.01
Yuncos             5.46               0.30          0.07

Table 5.- Physical properties of samples from Almeria and Toledo
areas.

Tabla 5.- Propiedades fisicas de las muestras de las areas de Almeria
y Toledo.

                                   Liquid     Specific     Hygroscopic
Reference (1)   Phyllosilicates-   limit-   surface (2)-      water
                       %-            %-     [m.sup.2]/g-   conient-%-

MY-A                   82           220         304           16.1
MY-B                   84           133         212           13.1
MCA-A                  90           237         216           16.1
MCA-B                  90           129         385           15.8
MCA-C                  91           188         336           15.5
MSB-A                  91           153         223           16.0
MSB-B                  94           137         150           17.0
E-5                    74           122         341           18.6
E-6                    66            85         332           13.3
T-3                    95            97         198           20.6
T-4                    98            90         152           19.5
S-1                    96           139         553           19.0
S-2                    86           213         517           15.8

                     Dry          Thermal
Reference (1)   density (3)-    conductivity-
                g/[cm.sup.3]-      W/m-K-

MY-A                1.58            0.87
MY-B                1.79            1.07
MCA-A               1.66            0.87
MCA-B               1.78            1.11
MCA-C               1.79            1.07
MSB-A               1.65            1.01
MSB-B               1.64            0.99
E-5                 1.72            1.03
E-6                 1.78            1.11
T-3                 1.64            1.15
T-4                 1.64            1.15
S-1                 1.73            1.24
S-2                 1.82            1.21

(1) MY: Yuncos, MCA: Cerro del Aguila-Cerro del Monte,
MSB: Santa Barbara, E: Morron de Mateo, T: Los Trancos,
S: Cortijo de Archidona

(2) Methylene blue method

(3) After uniaxial compaction at 100 MPa with hygroscopic
water content
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Author:Villar, M.V.; del Villar, L. Perez; Martin, P.L.; Pelayo, M.; Fernandez, A.M.; Garralon, A.; Cuevas,
Publication:Journal of Iberian Geology
Article Type:Report
Date:Jan 1, 2006
Words:11344
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