High-level nuclear waste management: a geochemical perspective

T. T. VANDERGRAAF Geochemistry Section, Geochemistry and Applied Chemistry Branch, Whiteshell Nuclear Research Establishment,

Atomic Energy of Canada Limited, Pinawa, Man., Canada ROE ILO

Received February 23, 1988

Revised manuscript accepted February 20, 1989

Atomic Energy of Canada Limited is investigating the concept of the disposal of high-level radioactive waste in an underground vault in an intrusive crystalline rock formation. The environmental impact of such a disposal is, to a large extent, dictated by geochemical processes involving rock-forming minerals, groundwater, and fission products and actinides in the waste. These various geochemical processes impact on the transport of contaminants, including radionuclides and chemically toxic elements, from a used-fuel disposal vault towards the biosphere. The extent and importance of the geochemical processes on contaminant transport are discussed. The predominant processes controlling the velocity of contaminant transport are the various geochemical interactions of the dissolved contaminant species with the minerals lining the surfaces of conductive fractures and fracture systems.

Key words: radionuclide, uranium, nuclear contaminant, transport, sorption, diffusion, geochemistry, fission products, granite.

~ n e r ~ i e atomique du Canada LimitCe Ctudie un concept reliC A I'Climination des dCchets radioactifs de haut niveau dans une voiite souterraine situCe dans une formation de roche cristalline intrusive. L'impact environnemental d'une telle Climination est, dans une grande mesure, reliC au processus giochimique impliquant les constituants des roches, la nappe phrkatique, les produits de la fission et les actinides contenus dans les dCchets. Ces divers procCdCs gkochimiques ont un effet sur la diffusion des contaminants, y compris les radionuclides et les ClCments chimiquement toxiques, de la voiite d'klimination des carburants usCs A la biosphkre. L'Ctendue et l'importance des procCdCs gCochimiques sur la diffusion des contaminants sont discutCes. Les procCdCs prkdominants qui rCgissent la vitesse de diffusion des contaminants sont les diverses interactions gCochimiques des espkces de contaminants dissous avec les minCraux qui recouvrent la surface des fractures et des systkmes de fractures conductifs.

Mots clts : radionuclide, uranium, contaminant nuclCaire, transport, sorption, diffusion, gCochimie, produits de la fission, granite.

[Traduit par la revue] Can. J. Civ. Eng. 16, 498-503 (1989)

Introduction Atomic Energy of Canada Limited (AECL) is examining the

concept of the disposal of used nuclear fuel wastes in an underground vault. This concept involves the removal of the wastes from the environment and the environmental impact of the disposal of these wastes on future generations will be within present-day acceptable limits.

The disposal options for high-level nuclear fuel wastes, like those for any type of waste, are rather limited: wastes can be disposed of by burial, by dispersion or dilution into the biosphere or atmosphere,- or by removal from the earth's environment, i.e., by transporting them into space. Space disposal is not viable because of the limited reliability of space rockets. Dilution into the biosphere or dispersion into the atmosphere is not only environmentally and ethically unaccept- able but unnecessary, since the high-level waste produced in a nuclear power reactor is concentrated in a relatively small volume. This leaves burial as the only viable option for final disposal.

Following the recommendations given in a comprehensive survey by the Geological Survey of Canada of the rock types available in Canada as potential host rock for the disposal of radioactive wastes, AECL has developed the concept of disposal in an intrusive rock formation in the Canadian Shield. The wastes will be encapsulated in corrosion-resistant contain- ers and emplaced in cylindrical holes, bored in the floor of a

buffer, and the mined cavity backfilled with a mixture of crushed rock, sand, and clay, and all access shafts sealed.

In time, the disposal vault will be saturated with ground- water, and the containers will corrode, leading to a leaching of the used fuel and a subsequent dissolution of the various radionuclides. The only credible way in which radionuclides can re-enter the biosphere is by transport via flowing ground- water through an existing or reactivated fracture network in the host rock. The extent and the geochemical mechanisms affect- ing this transport are the topics of discussion in this paper.

Groundwater composition Analysis of groundwater obtained from representative intm-

sive rock formations in the Canadian Shield and from existing mines (Fritz and Frape 1982; Frape et al. 1984; Gascoyne et al. 1987; Gascoyne and Vandergraafl) have shown a widespread occurrence of saline groundwater. The salinity generally in- creases with depth. At the depth proposed for a waste disposal vault, the groundwater is expected to consist of a mixed Na/Ca chloride with a concentration of approximately 1 mol/L. After the vault has been backfilled and sealed, it will in time become saturated with groundwater. The composition of this ground- water will depend on its source, and this will in turn depend on the hydraulic conductivity of the rock mass around the vault. If the vault is saturated with water representative of that at depth, it will be saline. If, on the other hand, surface waters and

7 mined at a of 500-1 Oo0 m' The 'pace between the l ~ ~ ~ ~ ~ ~ ~ ~ , M., and VANDER(juAF, T T Groundwater composi- container and the rock will be filled with a bentonite/sand tions in intrusive crvstalline rocks in the Canadian Shield. Atomic

Energy of Canada Lii ted Technical Record. unrestricted,-unpublished NOTE: Written discussion of this paper is welcomed and will be report available from Scientific Document Distribution Office, Atomic

received by the Editor until December 3 1, 1989 (address inside front Energy of Canada Limited Research Company, Chalk River, Ontario cover). KOJ 1JO. In preparation.

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groundwater from shallower depths saturate the vault, the groundwater in the vault will, at least initially, be much less saline.

The groundwater is expected to react first with the backfill material because it will have a relatively high porosity, and eventually with the buffer which has a much lower porosity. The initial result of these interactions wilI be that the more soluble ions are leached from the crushed rock, leading to an increase in the pH of the groundwater, due to a substitution of alkali metal ions on the surface of the geological material by hydrogen ions from the groundwater. The ionic strength of the groundwater may increase slightly. Eventually, ion exchange will take place between the Ca++ ions in the groundwater and the more mobile alkali metal ions in the backfill and buffer material, leading to an increase in the (Na+ + K+)/Ca++ ion ratio (Wanner 1986). The chloride concentration is not expected to increase, as there is no source for chloride ions, unless a gabbroic pluton is chosen as the host rock (Kamineni 1987). If carbonate-rich surface waters enter the vault, the carbonate concentration will be constrained by the solubility of calcite. Similarly, the solubility of amorphous quartz is expected to control the silica concentration. Even though the vault is backfilled, there will be a considerable quantity of entrapped air, and the conditions in parts of the vault will initially be oxidizing. The oxidation of ferrous-iron- containing minerals and of organic material, present in the buffer and backfill and surrounding rock, could eventually result in reducing conditions, and a concomitant increase in the Fe++ ion concentration.

Radiolysis of the groundwater by the radioactive material in the vault will produce hydroxide and oxide radicals (Tait et al. 1986). These oxide radicals will in turn react with ferrous-iron- containing minerals and organic material, and these reactions will thus compete with those of the entrapped oxygen, resulting in an increase in time to reach reducing conditions.

Because of the above-mentioned uncertainties in the eventual groundwater composition, research into processes that are governed by groundwater composition have been and are being carried out with widely varying groundwater composition. It should also be pointed out that rock-water interactions are often slow in the temperature regime expected in the vault (< 100°C), and may take thousands of years to reach equilibrium, espe- cially when some of the more complex aluminosilicates are involved.

Host-rock mineralogy During the survey of the intrusive rock formations in the

Canadian Shield, over 1500 plutons were tabulated (McCrank et ~1.'). Of these, approximately 75% are granitic and 20% gabbroic in nature. The granitic plutons are generally larger and more extensive than the gabbroic plutons. Because of this preponderance of granitic plutons, the Canadian Nuclear Fuel Waste Management Program has concentrated its research into these intrusive rock formations.

Granitic plutons consist mainly of quartz, plagioclase and potassium feldspar, and biotite mica, with titanite, pyroxene, chlorite, epidote, ilmenite, hornblende, and muscovite present as accessory or trace minerals. Since the vault will be backfilled

'MCCRANK, G . F. D., MISIURA, J. D., and BROWN, P. A. 1981. Plutonic rocks in Ontario. Atomic Energy of Canada Limited Technical Record TR-114. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1JO.

with crushed rock excavated from the vault during construction, reactions between groundwater and these minerals are expected to occur, with the formation of low-temperature alteration minerals, such as calcite, illite clays, amorphous silica, and iron oxides and iron oxyhydroxides, including goethite and limonite.

Groundwater movement is expected to occur almost exclu- sively through existing or reactivated fracture networks. Inves- tigations into plutons at various locations on the Canadian Shield have provided data on the evolution of fracture networks and on the composition of the alteration minerals coating these fractures. Thus, in the Eye-Dashwa Lakes pluton near Ati- kokan, northwest Ontario, four distinct fracture coatings have been identified, each representing a distinct temperature region over which they were formed (Kamineni and Stone 1983). Similarly, analysis of core material taken from the Lac du Bonnet pluton in the Whiteshell Research Area has shown that a limited number of minerals is found coating the water-bearing fractures (Kamineni et a1.3).

The possibility that partially open fractures may be opened further as a result of groundwater-rock interactions must also be considered. The temperature in the vault is expected to reach approximately 100°C due to the decay heat of the radionuclides in the waste. Thus, a thermal gradient will be set up, and thermal cycling of groundwater may occur. If mineral dissolution does occur in the high-temperature region, fracture apertures and overall permeability of the rock mass could increase in the immediate vicinity of the vault. However, the deposition of alteration minerals in the cooler zone at some distance from the vault, due to decreased solubility at lower temperatures, will result in a decrease of the fracture aperture. Fractures at this distance may seal. Since a large number of geochemical reactions must be considered, and since equilibrium may not be reached, results obtained with equilibrium calculations such as SOLMNQ (Goodwin and Munday 1983), PHREEQE (Park- hurst et al. 1980), and EQ3 (Wollery 1983) need to be used in parallel with kinetic and pathways codes, and compared with results obtained in mass-transport experiments (Charles 1978; Bourg et al. 1985).

Radionuclide behaviour The radionuclides in the wastes will eventually be leached

from the waste container. If the integrity of the buffer is preserved, the radionuclides will only be able to pass through the buffer into the vault by diffusion. This process of diffusion is described in detail by Cheung and Oscarson (1987).

The behaviour of the radionuclides in the vault is strongly dependent on the groundwater composition, the temperature, and the mineralogy in the vault. The alkali metal radionuclides, 135Cs and 137Cs, and the halide, '"I, are highly soluble and do not form sparingly soluble inorganic compounds. The cesium isotopes, however, are known to sorb strongly onto clay materials and phyllosilicates via an ion exchange mechanism, providing the ionic strength of the groundwater is low. Alkalim earth radionuclides, such as 9 0 ~ r and the radium isotopes, also sorb, but not nearly as strongly as cesium. The rare earth

3 ~ ~ ~ 1 ~ ~ ~ 1 , D. C., DUGAL, J. J. B . , and EJECKAM, R. B. 1984. Geochemical investigations of granitic core samples from boreholes at the Underground Research Laboratory near Lac du Bonnet, Manitoba. Atomic Energy of Canada Limited Technical Record TR-221. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1JO.

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elements sorb very strongly and also tend to be highly insoluble under the range of conditions expected to occur in the vault. The behaviour of multivalent radionuclides, such as 9 9 ~ c and 75Se, is more complex: under oxidizing conditions, these isotopes form poorly sorbing anionic species. However, under reducing conditions, technetium forms a sparingly soluble Tc(1V) oxide (Paquette et al. 1982) and has been observed to bond to magnetite (Haines et al . 1987). Similarly, selenium has been observed to sorb under a variety of conditions, especially on phyllosilicate minerals (Ticknor et al.4). Thus, if reducing conditions can be maintained, these multivalent radionuclides will exist only in very low concentrations in the groundwater. The chemical behaviour of the actinides also reflects their multivalent oxidation states. In general, in their reduced state, the actinides tend to be sparingly soluble; in a higher oxidized state, they are more soluble and tend to form complexes with anionic species, such as C03' and C1- (Lemire and Tremaine 1980; Lemire 1984). The reactions between complex-forming ligands and the actinides, and the redox reactions themselves, are quite rapid and can be predicted quite well with thermody- namic calculations, providing the thermodynamic data exist. A considerable amount of effort is being spent, under the auspices of the Nuclear Energy Agency of the OECD (Organization for Economic Cooperation and Development), to collect and evaluate all published thermodynamic data on the actinides of interest and on a select number of fission products.

Thus, the solubilities of the radionuclides, leached from the wastes, can reasonably well be predicted. A certain fraction of the released radionuclide inventory will be precipitated or sorbed on mineral surfaces, depending on the radionuclide and on the composition of the groundwater. The fraction that remains in solution may diffuse into the connected pore space in the rock matrix, or may be transported by flowing groundwater towards the biosphere.

Radionuclide transport The dissolved radionuclides can only travel from the vault by

diffusion into the connected pore volume in the rock, by dispersion in stagnant water in fractures, and by migration in flowing groundwater. Studies have shown that diffusion of nonsorbing species, such as 1 2 9 ~ , occurs readily through the connected pore space of the rock matrix (Katsube et a1.5). If the starting concentration of an element in solution is held high, in the order of 10-I mol/L, diffusion of sorbing species can occur over appreciable distances (Skagius and Neretnieks 1983; Birgersson and Neretnieks 1982). In addition, experiments have shown that, if sorption of a radionuclide can be depressed, by increasing the ionic strength of the groundwater, diffusion of even trace concentrations of '37Cs and of 9 0 ~ r will take place

4 ~ 1 ~ ~ ~ ~ ~ , K. V., HARRIS, D. R., and VANDERGRAAF, T. T. 1988. Sorption/desorption of selenium in contact with fracture-filling min- erals. Atomic Energy of Canada Limited Technical Record TR-453. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1JO.

'KATSUBE, T. J., MELNYK, T. W . , and HUME, J. P. 1986. Pore strxture from diffusion in granitic rocks. Atomic Energy of Canada Limited Technical Record TR-38 1. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1JO.

over distances of 10 cm in periods as short as 6 months (Grondin et

Radionuclide transport through fracture systems is governed by the flow velocity of the groundwater and by the sorptive behaviour of the radionuclide involved. Under equilibrium conditions, the extent of sorption of a radionuclide can be expressed by a surface (k,) or a bulk (kd) sorption coefficient, defined as the ratio between the concentration of a sorbed radionuclide and that in solution. If the sorption process is in equilibrium, the velocity of the radionuclide with respect to that of the transport medium can be expressed as a simple function of the sorption coefficient and of the fracture aperture.

Radionuclide sorption, and hence radionuclide transport, is a function of the groundwater composition, the radionuclide itself, and the geological material. Progress has been made in understanding the phenomenon of sorption (Davis et al. 1978; Langmuir 1981), and it has become clear that sorption cannot, in general, be represented by a simple equation. Consequently, much of the work that is carried out in this area continues to be empirical in nature. It has been found that sorption is often irreversible, at least on a laboratory time scale, and that more than one process must be invoked to mathematically describe the sorption process (Walton et al . 1983). Sorption is often concentration dependent and can be described by Freundlich, Langmuir, or Dubinin-Raduskhevich isotherms. Evidence has been found of the incorporation of a radionuclide into hematitie during the transformation of fenihydrate into the iron oxide (Walton et al . 1987), further complicating the picture.

For predictive modelling of radionuclide transport from a vault through the geosphere, sorption is being expressed as an empirical function of a number of independent and pseudo- independent variables, such as pH, ionic strength of the groundwater, Eh, and mineralogy of the sorbing surface. This approach has the advantage that sorption coefficients are used that are consistent with the geochemical environment, as defined by the groundwater composition and the mineralogy of the environment. The data used are obtained from the literature (Vandergraaf7), from laboratory measurements (Ticknor et al.'), and from the ISIRS (International Sorption Information Retrieval System) Data Bank of the Nuclear Energy Agency of the OECD in Saclay, France. The models used to calculate the extent of radionuclide migration and the data used in these

6 ~ ~ o ~ ~ ~ ~ , D. M., DREW, D. J., and VANDERGRAAF, T. T. Radio- nuclide diffusion studies in granitic rock. Atomic Energy of Canada Limited Technical Record. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1 JO. In preparation. - -

'VANDERGRAAF, T. T. 1982. A compilation of sorption coefficients for radionuclides on granites and granitic rocks. Atomic Energy of Canada Limited Technical Record TR-120. Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1JO.

'TICKNOR, K. V., VANDERGRAAF, T. T., and KAMINENI, D. C. Radionuclide sorption on mineral and rock thin sections. Part I: Sorption on selected minerals. Atomic Energy of Canada Limited Technical Record TR-365 (1985); Part 11: Sorption on granitic rock. Atomic Energy of Canada Limited Technical Record TR-385 (1986). Unrestricted, unpublished report available from Scientific Document Distribution Office, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1 JO.

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models are being evaluated by radionuclide migration experi- uraninite, a uranium oxide, to remain emplaced in a porous ments at scales ranging from 2.5 cm to 1 m inthe laboratory (Vandergraaf et al. 1987). These experiments also form a basis for future field migration experiments in the Underground Research Laboratory in the Lac du Bonnet pluton near Pinawa, Manitoba.

In addition to migration as a dissolved species, radionuclide transport can also occur as a radiocolloid or pseudocolloidal particle, that could behave quite differently to free ions, e.g., not sorb. A radiocolloid is somewhat arbitrarily defined as having a diameter of 1-450 nm, and may be formed during the dissolution of the used fuel in the vault, or by precipitation of an actinide, for example, in a thermal gradient. A pseudocolloid is defined as a colloidal particle, such as a silica, iron oxide, or boehmite particle, on which one or more radionuclides are sorbed. In either case, the end result is a radioactive particle that has at least the potential of being transported through the fracture network in the geosphere. Laboratory experiments (Vilks and Drew 1987) have shown that radiocolloids can be readily formed under laboratory conditions, and that these colloids may migrate more rapidly than would be predicted by the transport equations that assume sorption. High ionic strength solutions depress the formation of actinide colloids. Other experiments (Ho and Miller 1985) have shown that uranium will sorb readily on hematite colloids. Based on the evidence available at this point, it is more likely that, under actual disposal conditions, pseudocolloids will be more impor- tant than colloids. Also, since colloids are much larger than dissolved ions, colloids may only be transported through fractures with a sufficiently large aperture. The diffusion velocity of colloids through groundwater is extremely low; hence, transport can only occur via flowing groundwater. Since groundwater velocities in and around a disposal vault are expected to be very low, in the order of centimetres to decimetres per year, colloids should coagulate in these nearly stagnant waters and eventually precipitate. Experiments are being conducted to measure the rates of these processes.

Geochemical evidence The concept to dispose of nuclear fuel wastes must be shown

to be acceptably safe before selection of a suitable disposal site can begin. The assessment of the disposal concept is based on an understanding of the chemical, physical, and geochemical processes that can involve radionuclides in the geosphere. Information on these processes is being obtained from both laboratory experiments and from examination of the geological record itself. This information is adding to our understanding and can increase confidence that the proposed disposal concept is acceptably safe.

Investigations into the alteration halos around existing water-bearing fractures in intrusive, granitic rock formations in the Canadian Shield have shown that, although these fractures were open to flowing groundwater and that considerable alteration occurred in the rock mass adjacent to the fracture, many trace elements have been preserved and have not moved appreciably (Kamineni 1986). Some of these trace elements have chemical properties similar to those of specific actinides, and the information that they provide can be used to validate and increase our understanding of geochemical processes involving these actinides.

The discovery of rich uranium deposits in the Athabasca basin in northern Saskatchewan has shown that it is possible for

sandstone aquifer for periods of up to lo9 (~ramer-1986). One of these deposits, at Cigar Lake, is being studied to verify our understanding of some of the processes that are expected to be operative in a fuel waste disposal vault.

Other natural analogs exist. For example, the Oklo natural nuclear reactor in Gabon, in Equatorial Africa (Maurette 1976), has been used as a natural site to study the long-term migration of naturally produced fission products and actinides. Similarly, the mineralogy surrounding areas of low-temperature hydro- thermal activity can be used to follow the long-term effects of the transport of mineral-forming elements in a thermal gradient on rock permeability. All these naturally occurring analogs are being used to increase our understanding of the relevant geochemical processes and in the validation of the models used in predicting the environmental assessment of a disposal vault.

Implications for waste management and disposal in general There are important differences between high-level nuclear

fuel wastes and other forms of more typical chemically toxic waste. Nuclear fuel wastes are concentrated in relatively small volumes. It is expected that, in Canada by the year 2000, there will be approximately 34000Mg of used fuel occupying a volume of approximately 4600m3. This amount of waste represents the by-product of the production of 6100 TWh of electrical energy, or some 50 times the total 1984 electrical energy production of Ontario. In contrast, many of the more conventional wastes, such as fly ash, are much more volumi- nous, contain toxic materials such as cadmium, arsenic, and mercury in relatively low concentrations, or are often dispersed in soils or in other media.

In spite of these differences, the physical, chemical, and geochemical principles that govern radionuclide behaviour and migration in the geosphere govern those of chemically toxic wastes equally well. Thus, the laws governing ion exchange processes, complexation, and redox reactions operate equally well on nonradioactive toxic heavy metals as on radioactive fission products and actinides. The framework used in the assessment of the environmental impact of the disposal of nuclear waste can therefore be applied to disposal scenarios for other wastes. Consequently, there are important lessons to be learned from high-level nuclear fuel wastes disposal programs such as the Canadian Nuclear Fuel Waste Management Pro- gram. The main purpose of any disposal technique is to ensure that the burden on the environment and on future generations is acceptably small. With this main premise, it becomes important to isolate the wastes from the environment, or to decrease the concentrations of the wastes to insignificant levels. For organic material such as PCB, dioxin, etc., the material should be decomposed into innocuous organic compounds, unless it can be shown that the material can be decomposed by biological activity or by chemical reactions along the pathways between the disposal site and the biosphere. Heavy metals have always been part of the environment and, in trace quantities, appear to be essential to biological growth. The main operative words in this case are concentration and availability. As is the case with radioactive wastes, high concentrations of heavy metals must be kept from the environment, and this is best accomplished by isolation at a considerable distance from the biosphere, coupled with the incorporation of the toxic material in a leach-resistant waste form.

Regardless of the mode of disposal, reentry of at least some of

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the toxic waste into the biosphere should be considered. Any assessment of the environmental impact of toxic waste disposal must be based on a sound knowledge of the chemistry of the toxic material, the hydrogeochemistry of the groundwater in and around the disposal site, and the geochemistry of the geological barriers used to isolate the wastes from the bio- sphere. This knowledge is, in many cases, still rudimentary, and a long, sustained, multi-faceted research program will be needed before this knowledge can be considered sufficient.

Summary and Conclusions The fate of radionuclides in a used-fuel wastes disposal vault

is to a great extent dictated by the geochemical conditions in the vault. In turn, these conditions will evolve as a direct result of the groundwater composition and the geochemical processes that occur between the groundwater, the material used to backfill the vault, and the mineralogy of the host rock. The migration of dissolved radionuclides and, to some extent, pseudoradiocolloids will occur through existing fracture net- works, and will be governed by the extent of sorption and diffusion of radionuclides. These processes in turn are affected by the groundwater and the mineralogy of the fracture networks. Of the processes cited, the interactions of dissolved radio- nuclides and chemically toxic materials with the alteration minerals lining conductive fractures and fracture networks are the most important in controlling the transport of these contaminants through the geosphere. The environmental assess- ment of the disposal concept is based on an understanding of these processes, gained from laboratory observations and from the examination and analysis of natural processes that are analogous to those expected to occur in and around a disposal site.

Although there are differences between the radioactive waste generated in the production of nuclear energy and more conventional toxic wastes, in every case, the transport of these wastes is governed by the chemistry of the toxic material, the hydrogeochemistry of the groundwater, and the geochemistry of the geological barrier between the emplaced wastes and the biosphere. A sound understanding of these disciplines is essential to assessing the safety of the chosen method of disposal.

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