The alteration of a granitic rock around a repository for spent nuclear fuel has been simulated considering the geothermal effect through an increase of temperature. The results of the simulation were considered in terms of mass transfers and their volumic consequences. The dissolution of primary minerals by alteration tends to increase the porous volume due to fissures and cracks, while the precipitation of secondary minerals and particularly here calcite and clay minerals will tend to decrease the porosity by sealing the cracks. The results of the simulations show that an increase of temperature from 10 to 100°C will favour the alteration of the granitic rock by the solutions circulating today in the rock. This rock is characterized by a very low fissure porosity and the water velocity is consequently particularly low. These conditions favour intense water-rock interactions and the water-rock system will probably reach the stability field of secondary clay minerals predicted by the calculations: the porosity will decrease. A combination of these thermodynamic mass balance calculations and hydrodynamic considerations, with a kinetic approach of mineral dissolutions gives a first attempt to calibrate the modelling in the time scale.

The humic materials isolated from ground waters of a borehole in Fjällveden (Sweden) were characterized by elemental and functional group analyses. Spectroscopic properties, molecular weight distributions as well as acid-base properties of the fulvic and humic fractions were also studied. The ground water humic substances were found to be quite similar in many respects (but not identical) to the Swedish surface water humics concentrated from the Gτ(b, A)öta River but appeared to be quite different from the American ground water humics from Biscayne Florida Aquifer or Laramie Fox-Hills in Colorado. The physico-chemical properties of the isolated humic materials are discussed.

Microbial effects are one of the possible perturbations to the expect-ed performance of a deep geological repository which must be examined as part of a comprehensive safety analysis. Recent literature surveys (eg [1, 2] ) and reconnaisance sampling studies [3, 4] have concluded that: a) microbial contamination of a repository is inevitable b) even for high level nuclear waste, conditions in the near-field are insufficiently extreme to ensure complete sterilisation.

The chemistry of near field, i.e. the pH, Eh and potential complexing ions, determines the solubility of radionuclides from a waste source. It is often assumed that outside the repository this chemistry is that of the appropriate groundwater. However, the chemical environment around the repository itself is perturbed both by the near field components, e.g. wasteform, backfill etc., and by the act of building the repository.

General progress in modelling the behaviour of cement matrices and their interactions with wastes is reviewed in terms of appropriate model-ling concepts and their formulation in terms of parameters Which are susceptible to evaluation. The redox behaviour, particularly of slag-cement blends is shown to differ sharply from that of Portland cement matrices. Experimental methods of measuring Eh and poising capacities are described and the composition of pore fluids determined. Slag cements contain appreciable soluble Sn− species as well as S2O32− and SO42−

This paper describes an experimental and theoretical analysis of the near-field thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonite-sand buffer. Effects of heat and moisture diffusion on the thermal performance of clay-based buffer and backfills were studied by means of laboratory scale experiments. The bentonite-sand buffer exhibited cracking due to desiccation. The impact of such cracking on the perforamnce of buffer as an engineered barrier is being investigated.

An integrated finite difference computer code ‘TRUCHAM’ was written to analyze the transient heat and moisture flow regime in a disposal vault and was validated by a scale model heater experiment. It is hoped that with proper characterization of the material properties ‘TRUCHAM’ can be employed as a useful tool in assessing the thermal response of the prototype disposal vault. Further work is needed to integrate the effects of heat and moisture on the shrinkage and expansion of clay-based buffer and backfills.

A procedure has been established to assess the effectiveness of various excavation techniques using a computerized borehole dilatometer system developed by the authors[l]. This system, which incorporates data acquisition and computer capabilities, allows data to be collected and analyzed quickly in the field. The zone of disturbance is determined by conducting borehole tests at relatively close intervals and profiling the results with depth from the excavation wall. A similar concept of using deformation moduli measurements from plate bearing tests to detect changes in bedrock conditions is discussed by Koopmans[2]. In general, lower moduli are obtained in disturbed zones which have become stress relieved due to rock relaxation and/or the presence of rock discontinuities while higher values are obtained in relatively undisturbed zones which have not become stress relieved due to an absence of rock relaxation and/or rock discontinuities.

This paper summarizes computer simulation studies on water transport in German rock salt. Based on JOCKWERS's experimental investigations on water content and water liberation, the object of these studies was to select a water transport model, that matches the water inflow which was measured in some heater experiments in the Asse Salt Mine. The main re-sult is, that an evaporation front model, with Knudsen-type vapor transport combined with fluid transport by thermal expansion of the adsorbed water layers in the non evaporated zone, showed the best agreement with experi-mental evidence.

Rock salt contains crystal water of the hydrated minerals, water in the form of brine inclusions or adsorbed to the crystal boundaries, as well as primary gases. Further gases are produced or liberated due to the disposed high-level, heat producing wastes resulting from the thermal and radiolytic decomposition of the main and secondary constituents.

All volatile components which are present in a final repository have an influence on the disposal horizon, promote the spreading of possibly liberated radionuclides, increase corrosion of the containment and may lead to a pressure increase in a disposal borehole. Therefore a qualita-tive and quantitative determination of the components which could be lib-erated from the rock salt as a result of the disposal of high-level wastes is of significance for a final repository.

Clay's impermeability is an essential feature for the quality of a possible repository. However it results in a substantial pore water pressure rise induced by heating whose magnitude and dissipation rate depend on the clay's deformational response to heating. Experiments on thermal strains and effective stress are reported and the mathematical model implemented in a computer code is described. It allows study of a repository performance due to waste heating in both continental and subseabed clay formation.

The properties of water saturated bentonite systems are highly dependent on the pore water chemistry, water content and temperature. In this study relative changes in the physical state of pore water are studied by means of the nuclear magnetic resonance technique (NMR). It is found that the relative water mobility increases with increasing water content and/or increasing temperature, while an increase in the electrolyte content lowers the water mobility.

For a porous water-saturated material where diffusion in the pore water, sorption on the solid material and diffusion of the sorbed ions (surface diffusion) occur, a diffusion equation can be derived where the apparent diffusivity includes two terms. One represents diffusion in the pore-water, the other surface diffusion.

In this research diffusion mechanisms were studied. The apparent diffusivities of strontium, cesium and cobalt in compacted sodium bentonite were measured by a non-steady state method. The sorption factors were adjusted using different sodium chloride solutions, groundwater and addition of EDTA for saturation of the bentonite samples. The corresponding sorption factors were measured by a batch method.

The results suggest that cations diffuse also while being sorbed. A combined pore diffusion-surface diffusion model has been used to explain the transport and the corresponding diffusivities have been evaluated. The surface diffusivities (Ds) of Sr and Cs were 8-9·10-12 m2/s and 4-7·10-13 m2 /s respectively. The pore diffusivity eD of Cs was 3.5.10-11 m2 /s which has been used also for Sr.

The sorption mechanism of Co seems to be different from that of Sr or Cs and the results allow no specific conclusions of the diffusion mechanism of Co. The apparent diffusivity of Co ranged from 2·10-14 to 7·10-14 m2/s.

The anionic Co-EDTA seems to follow some other diffusion mechanism than the cations.

In a final repository for radioactive waste in crystalline rock, water flowing in the fractures may transport the radionuclides eventually leached from the waste. To be able to predict the migration of the radionuclides the processes involved must be understood. To quantify the processes, data from flow and transport in real fractures under realistic conditions are needed. Models used for prediction must include descriptions of the important processes and mechanisms.

Radionuclides escaping from a repository for high level nuclear waste in crystalline rock may eventually be carried by the flowing water in fissure zones. In such zones the rock is broken in blocks of varying sizes and shapes. Also, the water velocity may vary very much along the flow path. The integrated finite difference method (IFDM) is proposed for numerical calculations of radionuclide transport in such zones. A method for lumping blocks of various sizes and shapes into a single PSEUDOBODY is tested by comparing it with an exact analytical solution which can account for the diffusion into blocks of any size distribution. The errors obtained in using the PSEUDOBODY-approach are found to be small. Furthermore, a method for determining an “average” Peclet number in a strongly varying velocity field is tested and found to give comparatively small errors.

In Switzerland high-level radioactive wastes are planned to be disposed of in deep-lying crystalline formations. The modelling approach for geospheric transport in the framework of a HLW safety assessment is described. The processes taken into account are advection and dispersion in water conducting veins or fractures. Out of these, nuclides may diffuse into a spatially limited zone of altered rock. Sorption of nuclides on the rock is taken into account. Diffusion out of fractures and sorption in the adjacent altered rock may often be represented by an effective surface sorption on the fracture walls. For transport of the 237-Np nuclide chain we present a sensitivity analysis. The performance of the geologic barrier depends most critically on the extent of the altered zones.

Five thermal-convection loops, constructed of granite, were operated for 350 d with a hot-side temperature of 60°C and a cold-side temperature of 20°C, to examine the effects of a thermal gradient on the convective transport of seven radionuclides. Radionuclide concentrations on the surface of the loop flow channels indicated that 125Sb, 60Co and 54Mn and precipitated iron oxyhydroxides were concentrated in the hot side of the loops. Concentrations of 144Ce and 99Tc were higher in the cold side of the loops. Both of these patterns were exhibited by 75Se. The concentration of 137Cs was not significantly affected by the thermal gradient. Radionuclide solubility and sorption were affected by variations in pH, Eh and ionic strength. Incorporation of the radionuclide into mineral lattices was found to play a key role in retarding the transport of all radionuclides tested.

The 234U/230Th ratio serves as an in-situ indicator of the redox state in groundwater aquifers. The higher this ratio, the more U there is in the +6 valance state and thus a less reducing environment. Radium sorption is retarded in the shallow aquifer and is dependent on the CaSO4 content and the redox state. Relative to Ra, U and Th are highly sorbed. The total retardation factor for Th is ∼1400 and mean sorption time for 228Th is ∼10 days in the shallow zone. The desorption rate of Ra is significantly slower in the shallow than in the deep aquifer. There is no effect of colloids in brines.

The sorption of some actinides (Th, U, Np, Pu and Am) and fission products (I, Cs) was measured on two types of Standard Portland cements as well as on samples from old (70 years) hydro power dam constructions using a batch technique. Pore water compositions were analysed, and artificial pore water solutions were used as aqueous phases in the experiments. Measurements were also performed on five other concrete types (not reported in detail in this paper) to illustrate the effects of the cement matrix composition on the sorption behaviour of the radionuclides.

The sorption of actinides in the trivalent (americium), tetravalent (thorium) pentavalent (neptunium) and hexavalent (uranium) states was high in all the studied concrete systems.

Generally, the sorption of cesium was low due to the low exchange capacity of the cement and the high concentration of competing cations in the pore waters.

The sorption of iodine was much higher than in most silicate minerals of geologic origin.

The differences between the various concrete systems were generally minor in terms of their sorbing capacities.

Experiments have been carried out into the leaching of ILW conditioned in cement and bitumen in concentrated salt solutions. Although difficult to observe with real waste leachates, investigations into the leaching of a simulated waste in cement have indicated leached transuranic levels of ca. 10−9 M ; the amount of activity leached by a quinary salt solution being 102 - 103 times higher . This has been interpreted in terms of a pH effect. For the real waste, spectroscopy has indicated a significantly larger release of Cs from cement than from bitumen. For all waste samples a notable absence of colloidal material was observed ; an observation which can be explained in terms of the high solution ionic strengths and the corresponding influence upon radionuclide solvation.

Transuranic mobility studies through salt and sand from a salt dome in Northern Germany have shown the presence of at least two types of species of wildly differing mobility ; one migrating with approximately the same velocity as that of the solvent front and the other strongly retarded. Actinide recoveries (i.e. that passing through the columns) could be strongly influenced by either changing the system pH or by the addition of a competitor such as Ce ; the latter effect pointing to a competitive sorption.

The solubility of some of the radiologically important actinide elements have been determined in a water whose chemistry was representative of that believed to exist in potential repositories in the U.K. The solubilities of the actinides have been determined as a function of the pH of the water. The results of these experiments have been compared with the results of predictions made using the PHREEQE geochemical modelling code in order to test and validate the data base used in the model. In the light of these comparisons, suggestions are made for alternative values for the thermodynamic data and for further studies.