ALBERT

Description: In order to evaluate the long-term behaviour of the engineered barriers in geological disposal sites for transuranic element-bearing (TRU) waste, an evaluation by numerical analysis is required. Although chemical and hydraulic/mechanical analyses have been conducted independently until now, essentially both type of phenomena occur simultaneously and produce synergistic effects. Therefore, we focused attention on the buffer (bentonite) engineered barrier and conducted a study of which involved incorporating hydraulic/mechanical phenomena into the chemical analysis of bentonite alteration. The simulations employed weakly-coupled chemical and hydraulic/mechanical effects to study the behaviour in one dimension. The results showed that the dissolution of the montmorillonite is suppressed in the buffer section nearest the cement material. Moreover, in order to achieve a fully coupled analysis in future, the present study also identifies issues that need to be resolved.

Description: Disposal of Finnish spent nuclear fuel is planned to be based on the KBS-3 repository concept. The role of the bentonite buffer in this concept is essential, and thus the behaviour of the bentonite has to be known. The experiments in this paper concentrated on providing information about the effects of carbon dioxide CO 2 (g) partial pressure on compacted sodium bentonite, giving an insight into the buffering capacity. The experimental setup consisted of a hermetic box which had a CO 2 -adjusted atmosphere, and the bentonite was in contact with this atmosphere through water reservoirs. The results indicated that it is possible to measure online the changing pH in the porewater inside compacted bentonite using IrOx electrodes. It was found that the pH fell if the CO 2 partial pressure increased above atmospheric conditions. The experimental results indicated a greater fall in pH than in our model in the test cases where CO 2 was present. The pH in the experiment with 0 P CO 2 remained nearly constant throughout the 5 month period. On the other hand, the pH dropped to near 6 with 0.3 P CO 2 and to 5.5 with 1 P CO 2 .

Description: Electron paramagnetic resonance (EPR) spectra of different particle size fractions of four kaolins from diverse sources in North America, Europe and Asia have been investigated in order to characterize their paramagnetic properties and heterogeneity. There were major differences in the sources of the EPR signals from transition metals; V and Mn were structural, Fe was both structural and as associated oxides, and Cu was in the form of an adsorbed ion. The radiation-induced free radical signals commonly known as the A- and B-centres were observed in three of the deposits; however, in addition to the previously reported 27 Al hyperfine structure associated with the B-centre, we also observed much smaller 27 Al hyperfine structure on the g feature of the A-centre. The other kaolin sample produced four free radical signals that have not previously been reported in kaolins. Each had substantial 1 H hyperfine splitting; three are interpreted as corresponding to defect centres associated with Si-OH groups, and the other to a Si hole surrounded by protonated O atoms. The EPR spectra changed progressively with particle size, and measurements on the Asian specimens after grinding showed major differences in the Fe 3+ signals from the same particle size fractions separated from the natural samples, thus supporting previous reports that grinding results in major structural changes in the minerals.

Description: Water-saturated bentonite is planned to be used in many countries as an important barrier component in high-level nuclear waste (HLW) repositories. Knowledge about the microstructure of the bentonite and the distribution of water between interlayer (IL) and non-interlayer (non-IL) pores is important for modelling of long-term processes. In this work the microstructure of water-saturated samples prepared from MX-80 bentonite was studied with nuclear magnetic resonance (NMR) and small-angle X-ray scattering spectroscopy (SAXS) coupled with chloride exclusion modelling. The sample dry densities ranged between 0.7 and 1.6 g/cm 3 . The NMR technique was used to get information about the relative amounts of different water types. Water in smaller volume domains has a shorter relaxation time than that in larger domains due to the average closer proximity of the water to the paramagnetic Fe at the layer surfaces. The results were obtained using 1 H NMR T 1 relaxation time measurements with the short inter-pulse CPMG method. The interpretation of the NMR results was made by fitting a sum of discrete exponentials to the observed decay curves. The SAXS measurement on bentonite samples was used to get information about the size distribution of the IL distance of montmorillonite. The chloride porosity measurements and Donnan exclusion calculations were used together with the SAXS results to evaluate the bentonite microstructure. In the model, the montmorillonite layers were organized in stacks having IL water between the layers and non-IL water between the stacks. In the modelling, the number of layers in the stacks was used as fitting parameters which determined the IL and non-IL surface areas. The fitting parameters were adjusted so that the modelled chloride concentration was equal to the measured one. The NMR studies and SAXS studies coupled with the Cl porosity measurements provided very similar pictures of how the porewater is divided in two phases in bentonite.

Description: The use of HF digestion to concentrate minerals of the plumbogummite group is discussed. Prolonged treatment is shown to result in solution of plumbogummite minerals, and in precipitation of HCl-insoluble, complex fluorides. These yield X-ray diffraction spacings that may be confused with those of plumbogummite minerals. The presence of fluorides in digestion residues is confirmed by X-ray diffraction, DTA and by identifying the products formed when such residues are heated to 600°C.