SummaryThe radionuclideIt was found that it is very difficult to create the same conditions present in the deep repository in the laboratory, which is why the reduction of the pertechnetate anion to its lower oxidation state (Tc(IV)) was not achieved. The retention of the pertechnate anion on bentonite is very low (

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The aim of the DOPAS project is to address the design basis, reference designs and strategies for the plugs and seals to be used in geological disposal facilities. The Czech 'Experimental Pressure and Sealing Plug' (EPSP) experiment has the following objectives: to develop, monitor and verify the functionality of an in situ experimental plug, and to determine and describe in detail the materials to be used with respect to experimental plug construction. The functionality of the experimental plug will be verified by means of conducting pressure tests after the construction phase of EPSP. The internal bentonite section of the EPSP will be constructed from bentonite pellets with a dry density of 1850 kg m 23 ; the pellets will disintegrate following saturation and swelling to form a homogeneous material with a dry density of around 1400 kg m 23 . This process leads to the question of the pore space of the bentonite material: that is, whether it will be possible to employ a numerical model for homogeneous compacted bentonite powder porosity instead of that of the double pore/void space of the bentonite pellets used in the experiment. For this reason, a laboratory physical model (PHM -physical hydraulic model) was incorporated into the DOPAS project for the purpose of describing the saturation and disintegration of the bentonite pellets. The aim of this paper is to describe the results of the space distribution of relative humidity in a sample consisting of bentonite pellets with a view to subsequent numerical modelling and to extend the current knowledge concerning the behaviour of the bentonite pellets used in the EPSP experiment. The results show that following water saturation and the swelling of the bentonite the pellets form a homogeneous mass in the same way as does the compacted bentonite powder. After a period of 160 days, the pellets attained 100% relative humidity at a distance of 12.5 cm and swelling pressure recorded at the end of the model was seen to slowly increase over this time period.

In this study, the uncertainties associated with through-diffusion, the steady-state method used for determination of the pore diffusion coefficient (D p ), were analysed and evaluated. The diffusion of strontium through the sample of compacted bentonite was studied to evaluate various sources of uncertainties and compare their contribution with uncertainty of D p . Different diffusion experiment arrangements were used to test the effect of solution accessibility to the filter membranes. Three main general factors influencing the uncertainty of D p at given conditions were identified: (a) the compacted sample preparation and its physical properties; (b) the properties of the diffusion cell; and (c) the strontium concentration analyses. Some of the identified sources can be treated as fixed with limited extent (e.g. the deformation of components of the diffusion cell), but some sources must be treated as variable with wide extent depending on the experimental conditions (e.g. the physical properties of the compacted bentonite sample -moisture, density, porosity). The variable uncertainty sources have to be taken into account, especially in repeatability and reproducibility tests. It was concluded that each of the diffusion cells with compacted bentonite sample represents a unique diffusion system. It is preferable to present the results of diffusion experiments as a range of values rather than as one average value with an uncertainty.

The main aim of this study was to investigate diffusion of technetium 99 Tc under different conditions. Because technetium represents one of the most dangerous fission products due to its very long halftime and high mobility in aerobic conditions diffusion experiments of technetium (as 99 TcO 4 anion) in Czech bentonite from Rokle locality have been carried out. For performance and evaluation of experiments the through-diffusion method was chosen and apparent (D a ) and effective (D e ) diffusion coefficients were evaluated. The effects of particle mesh-size, dry bulk density and aerobic or anaerobic conditions on diffusion were studied. In the presence of oxygen, technetium occurs in oxidation state VII, as an anion, soluble and mobile in the environment. However, under reducing conditions it occurs in a lower oxidation states, mainly as insoluble oxides or hydroxides. Aerobic experiments were carried out under laboratory conditions and anaerobic experiments were performed in a nitrogen atmosphere in a glove box, to simulate the real underground conditions.