w *£.''• 7 / J, i. AECL-7792 ATOMIC ENERGY £2?A L'ENERGIE ATOMIQUE OF CANADA LIMITED \fijT DU CANADA,LIMITEE THE CONVERSION OF SMECTITE TO ILLITE IN HYDROTHERMAL SYSTEMS: A LITERATURE REVIEW LA CONVERSION OE LA SMECTITE EN ILLITE DANS LES SYSTEMES HYDROTHERMIQUE: EXAMEN DES TRAVAUX PUBLIES R.M. Johnston Whiteshell Nuclear Research Etabiissement de recherches Establishment i>ucleaires de Whiteshel! Pinawa, Manitoba ROE 1LO June 1983 juin Copyright © Atomic Energy of Canada Limited, 1983 ATOMIC ENERGY OF CANADA LIMITED THE CONVERSION OF SMECTITE TO ILLITE IN HYDROTHERMAL SYSTEMS: A LITERATURE REVIEW by R.M. Johnston Whiteshell Nuclear "esearch Establishment Pinawa, Manitoba ROE 1L0 1983 June AECL-7792 LA CONVERSION DE LA SMECTITE EN ILLITE DANS LES SYSTÈMES HYDROTHERMIQUE: EXAMEN DES TRAVAUX PUBLIÉS par R.M. Johnston RESUME Dans les systèmes schisteux diagénétiques naturels, la smectite se transforme en illlte et en illite-smectite à couche mixte en moins d'un mil- lion d'années, en présence de températures se situant entre 75°C et 200°C. De ce fait, certaines questions se posent quant à la stabilité des tampons de bentonite à base de smectite par rapports aux conditions propres â une enceinte d'évacuation nucléaire• Les données expérimentales et géologiques obtenues indiquent que la réaction dépend de la disponibilité des K*" et que le taux de réaction des systèmes pauvres en K* (tels que les enceintes d'é- vacuation) peut être bien inférieur â ce que l'on observe dans le schiste. La présence de Na+, de Ca2+ et de Mg2+ dans le système ralentie la réaction et peut même l'arrêter complètement à des températures inférieures. On a proposé deux mécanismes de réaction différents dont les élé- ments positifs et les implications sont ici passés en revue. L'Énergie Atomique du Canada, Limitée Établissement de recherches nucléaires de Whiteshell Pinawa, Manitoba ROE 1L0 juin 1983 AECL-7792 THE CONVERSION OF SMECTITE TO ILLITE IN HYDROTHERMAL SYSTEMS: A LITERATURE REVIEW by R.M. Johnston ABSTRACT In natural diagenetic shale systems, smectite converts to illite and mixed-layer illite-smectite in less than a million years at tempera- tures between 75°C and 200°C. This has raised questions as to the stability of smectite-based bentouite buffers under nuclear waste disposal vault con- ditions. Experimental and geological evidence indicate that the reaction is dependent on the availability of K , and that the rate of reaction in K -poor systems (such as the disposal vault) may be much lower than that + 2+ 2+ observed in shale. The presence of Na , Ca and Mg in the system slows the reaction and nay halt it altogether at lower temperatures. Two different reaction mechanisms have been proposed; the evidence for, and implications of, each are discussed. Atomic Energy of Canada Limited Whiteshell Nuclear Research Establishment Pinawa, Manitoba ROE 1L0 1983 June AECL-7792 CONTENTS 1. INTRODUCTION 1 1.1 DISPOSAL VAULT CONDITIONS 1 2. STRUCTURE AND CHEMISTRY OF CLAY MINERALS 3 2.1 SMECTITES 3 2.2 ILLITE 4 2.3 MIXED-LAYER CLAYS 5 3. SMECTITE-ILLITE TRANSFORMATION 6 3.1 GEOLOGICAL EVIDENCE, 6 3.1.1 Shales ,' 6 3.1.2 Bentonites , 8 3.1.3 Mechanisms ; 8 3.2 EXPERIMENTAL EVIDENCE 9 3.2.1 Effects of Chemistry of T-O-T Layers 10 3.2.2 Effect of Ir.terlayer Cations 11 3.2.3 Reaction Series 12 3.2.4 Activation Energies 13 3.3 DISCUSSION 14 4. CONCLUSIONS 16 REFERENCES 18 TABLES 22 FIGURES 26 1. INTRODUCTION The proposed use of bentonlte* as a buffer material In the dis- posal of high-level nuclear wastes in the Canadian Nuclear Fuel Haste Management Program [1,2] raises the question of the long-term stability of bentonite under disposal vault conditions. The suitability of bentonite (or a bentonite/filler mixture) in terms of both chemical and physical properties has been documented in publications from the Swedish Haste Management Program [3-9]. The advantages of bentonites over other clay materials are their high cation exchange capacities and their swelling properties. Evidence from sedimentary shale sequences, however, Indicates that smectite (the major component of bentonite) may not be stable at temperatures In excess of 75°C to 100°C and may react to form llltte [10]. Such a transformation would involve a decrease in the swelling and cation exchange properties of the buffer, and would release interlayer water, thereby enhancing mass transport. Thus, a thorough understanding of dla- genetic clay reactions is necessary before any prediction can be made con- cerning the ultimate performance of bentonite buffers under the thermal and chemical conditions of a waste disposal vault. 1.1 DISPOSAL VAULT CONDITIONS The current Canadian concept of nuclear fuel waste disposal involves the emplacement of either Irradiated fuel or immobilized high- level wastes in a mined vault 500 to 1000 m below the surface in granitic or gabbroic plutons of the Canadian Shield [11]. Bentonite buffer will surround the waste package in either borehole or in-room emplacement, the remainder of the vault being back-filled with bentonite mixed with sand, gravel or crushed host rock [1,12]. * Bentonite is a general term for highly plastic colloidal clay, composed largely of smectite, frequently formed by the alteration of volcanic ash. + Smectite refers to a specific group of clay minerals characterised by high cation exchange capacity and swelling properties (described in 2.1). - 2 - The geothermal gradient provides an ambient rock temperature of 25 °C at a depth of 1 km. Radioactive decay heat will increase this, but temperatures can be controlled by areal heat loading of the vault, and a reference maximum of 150°C at the surface of the waste package is pro- jected. For immobilized wastes, high temperatures will be sustained for less than 1000 years, while for irradiated fuel the duration of the thermal transient will be tens of thousands of years. Hydrostatic pressure in the vault is expected to be about 10 MPa (100 bars), assuming the vault floods. Provided the vault retains its integrity, lithostatic pressure should have no effect - should the overburden contribute, total pressure would not exceed 30 MPa. The buffer will be chosen to keep the swelling pressure less than the hydrostatic pressure. Pusch concluded that the swelling pressure is strongly dependent on the bulk density of the bentonite [9]. For highly compacted bentonite 3 (bulk density greater than 2000 kg/m ) pressures up to 10 to 20 MPa could develop, but for bulk densities less than 1800 kg/m , the swelling pressure is less than 10 MPa. Surface groundwaters in equilibrium with granitic and gabbroic rocks have relatively low total dissolved solids [13], and are usually Na -HCOg dominated (examples are given in Table (la)). These represent the least aggressive groundwaters to which the buffer would be exposed. Recent investigations [14-17] have shown that saline groundwaters are common at depths greater than 500 m in the Canadian Shield. These brines are calcium-sodium-chloride dominated, with low magnesium and potassium, and total dissolved solids as high as 200 g/L. (Selected analyses are given in Table l(b)). In summary, the most severe conditions that the buffer is likely to experience in the vault are (1) Temperatures up to 150cC. (2) Pressures in the range 10 to 30 MPa (100 to 300 bars). (3) Calcium-sodium-chlorine brine groundwaters. - 3 - 2. STRUCTURE AND CHEMISTRY OF CLAY MINERALS* The basic structural units of layer silicates, including clay minerals, are tetrahedral (T) sheets (predominantly silicon-aluminium) and octahedral (0) sheets (mainly aluminium-magnesium-iron) as shown in Figures 1 and 2, respectively. The octahedral sheets may be either trioctahedral (X,{OH),., where X = Mg + or Fe +], or dioctahedral 3+3+ , where D = vacancy, Y = Al or Fe ], corresponding to the structures of brucite and gibbsite, respectively. In both the smectites and illite, the octahedral sheet is sandwiched between two tetrahedral sheets to form a T-O-T [18] layer, as shown in Figure 3. This layer is also known as a 2:1 layer [19]. The ideal formulae for T-O-T layers are for the dioctahedral case (corresponding to pyrophyllite) and for the trioctahedral case (corresponding to talc) Substitutions are possible in both the octahedral and tetrahedral 3+ 4+ 2+ 3+ positions (e.g. Al for Si in tetrahedral sites, and Mg for Al in octahedral sites), resulting in a charge deficiency on the T-O-T layers. This charge is usually negative and is satisfied by the presence of inter- layer cations, which are held electrostatically between the T-O-T layers, strengthening the weak Van der Waal's forces holding the layers together. The nature and extent of substitution and the type of interlayer cation determine the properties of the various clay mineral groups. 2.1 SMECTITES Both di- and trioctahedral smectites occur, compositions being restricted for the most part to site occupancies of 4.00 to 4.44 cations per 01Q in the trioctahedral smectites, and 5.76 to 6.00 cations per 01Q * Descriptions of clay mineral structures are taken from references 18 to 21. - 4 - for the dioctahedral group. Layer charge is in the range -0.2 to -0.5 per 010, and may be developed primarily b;' substitution on the octahedral sites, as in montmorillonlte, or on the tetrahedral sites, as in beidel- lite.* A continuum exists between the two types. Ideal formulae for the major smectites are given in Table 2. The most common Interlayer cations are Na+ and Ca2+, but others such as K+, Mg2+, Fe2+, Rb+, Cs+, Ba2+ and H+ may be fourd. Interlayer cations are exchangeable, which explains the high cation exchange capacity of smectites (80 to 150 meq/100 g).