[en] Extensive use of cement and concrete is envisaged in the construction of geological repositories for low and intermediate-level radioactive wastes, both for structural, and encapsulation and backfilling purposes. Saturation of these materials with groundwater may occur in the post-closure period of disposal, producing a hyperalkaline pore fluid with a pH in the range 10-13.5. These pore fluids have the potential to migrate from the repository according to local groundwater flow conditions and react chemically with the host rock. These chemical reactions may affect the rock's capacity to retard the migration of radionuclides released from the repository after the degradation of the waste packages. The effects of these chemical reactions on the behaviour of the repository rock as a barrier to waste migration need to be investigated for the purposes of assessing the safety of the repository design (so-called 'safety assessment' or 'performance assessment'). The objectives of the work reported here were to: identify those processes influencing radionuclide mobility in the geosphere which could be affected by plume migration; review literature relevant to alkali-rock reaction; contact organisations carrying out relevant research and summarise their current and future activities; and make recommendations how the effects of plume migration can be incorporated into models of repository performance assessment. (author)

[en] Four interactions experiments involving a simulated borosilicate waste glass, granite and deionised water have been carried out at 100⁰, 150⁰, 200⁰ and 350⁰C at a total pressure of 50 MPa to simulate the near-field geochemistry of a high level waste repository in granite. Experiments were conducted in gold-titanium cell, direct sampling autoclaves for run durations of 200 days (100⁰, 150⁰ and 200⁰C) and 30 days (350⁰C), during which time solution samples were extracted for the analysis of 25 chemical species. Solid phases retrieved at the end of the experiments were examined using X-ray diffraction and scanning electron microscopy. The high temperature speciation characteristics and degrees of mineral saturation of the fluids were investigated using the geochemical software packages, EQ3 and SOLMNEQ. (author)

[en] Extensive use of cement and concrete is envisaged in the construction of geological repositories for low and intermediate-level radioactive wastes, both for structural, and encapsulation and backfilling purposes. Saturation of these materials with groundwater may occur in the post-closure period of disposal, producing a hyperalkaline pore fluid with a pH in the range 10-13.5. These pore fluids have the potential to migrate from the repository according to local groundwater flow conditions and react chemically with the host rock. These chemical reactions may affect the rock's capacity to retard the migration of radionuclides released from the repository after the degradation of the waste packages. The effects of these chemical reactions on the behaviour of the repository rock as a barrier to waste migration need to be investigated for the purpose of assessing the safety of the repository design (so-called 'safety assessment' or 'performance assessment'). The objectives of the work reported here were to: identify those processes influencing radionuclide mobility in the geosphere which could be affected by plume migration; review literature relevant to alkali-rock reaction; contact organisations carrying out relevant research and summarise their current and future activities; and make recommendations how the effects of plume migration can be incorporated into models of repository performance assessment. (author)

[en] A monzogranite was reacted with water in hydrothermal solution equipment at 100 deg C, 50 MPa for 203 days, during which time six fluid samples were extracted at run temperature and pressure for chemical analysis. Fluid samples were analysed using ICP, INAA and standard wet chemical techniques and data are presented for 18 chemical species. Solids were examined using XRD and SEM. The high temperature speciation of the fluid phase chemistry was investigated using the geochemical software package EQ3/6. The evolution of the fluid phase chemistry is discussed in terms of mineral-fluid equilibria and the kinetics of mineral dissolution and precipitation reactions. The implications of these results to the modelling of the near-field geochemical environment of a high-level radioactive waste repository in granitic rock are discussed. (author)

[en] Clay-based buffer and tunnel backfill materials are important barriers in the KBS- 3 repository concept for final disposal of spent nuclear fuel in Finland. Significant changes can be expected to occur to the properties and behaviour of buffer and backfill, especially during re-saturation and through the thermal period. Reactions will occur in response to thermal and chemical gradients, induced by the thermal output of the spent fuel and at interfaces between different barrier materials, such as cement/clay, steel/clay etc. Processes of ion exchange, mineral dissolution and precipitation, and swelling can lead to significant re-distribution of mass and evolution of physical properties so that reliable predictive modelling of future behaviour and properties must be made. This report evaluates the current status of modelling of buffer and backfill evolution and tries to assess the potential future capabilities in the short- to medium-term (5-10 years) in a number of technical areas: (1) Non-isothermal (T-H-M-C-B) modelling and the potential for cementation, (2) The consistency of models, (3) Swelling pressure, (4) Cement-bentonite interactions, (5) Iron-bentonite interactions, (6) Mechanical (shear) behavior, and (7) Bentonite erosion

[en] Bentonite is important as a near-field buffer and backfill for a spent fuel/high level waste (SF/HLW) repository in Opalinus Clay, because of swelling and low solute transport rates. These properties should be preserved in the long-term (up to a million years). A number of processes could perturb them, such as thermal gradients from the decay heat of waste packages and chemical gradients due to thermodynamically unstable materials (steel, concrete). The potential interactions of bentonite with engineered components have been assessed. They are characterized by a complex interplay between fluid transport, clay ion exchange and dissolution, secondary mineral growth, and consequent changes in physical properties (porosity, permeability, swelling pressure). The near-field evolution will be curtailed well within the timeframe of a million years by mass transport constraints (porosity decreasing to zero) or mass balance limitations (reactants completely consumed). For bentonite alteration at 100 ka limited by mass transport constraints, there will be a thin (5 cm thick; 1 vol.-% total bentonite) alteration layer around the canister, derived partly through thermal redistribution of minerals and aqueous solutes, and partly due to interaction of the steel canister with bentonite. This results in a thin zone with zero porosity and zero swelling pressure (montmorillonite totally altered) around the canister, but with an unaltered hydraulic conductivity (potential minor fracturing cancels out the effects of decreased porosity). The mineralogical composition of the thin zone consists of a layer of calcite, gypsum/anhydrite and magnetite on the canister, with montmorillonite in the altered bentonite replaced by Fe-silicates such as cronstedtite, berthierine and chlorite. Beyond this inner alteration zone is an annulus of 68 cm (92 vol.-%) of unaltered bentonite. The potential interaction of metallic engineered structures other than the canister with bentonite is relatively minor, consisting of only 2 vol.-% of the total bentonite in the canister zone. Removing the rails prior to closure reduces this effect to 1 vol.-%. At the external margin of the bentonite, alteration of the buffer and sealing element adjacent to the concrete liner is 2 cm in thickness (4 vol.-%) at 100 ka. The hydraulic conductivity of this zone is decreased. The porosity and swelling pressure decrease to zero over a few hundreds to a thousand years due to alteration of montmorillonite. The mineralogical composition is characterised by a sequence of calcite, C-(A)-S-H minerals, Ca-zeolites, sepiolite and saponite clays, with C-S-H minerals forming nearest the cement contact, and other minerals such as zeolites and clays forming further towards the canister. The concrete liner itself may degrade via conversion of portlandite and C-S-H gel to ettringite with a consequent increase in porosity, reaching a few percent over 100 ka. For the sealing element, there are steel reinforcement arches, along with steel mesh, anchors and rails. A maximum of 5 vol.-% of bentonite could be transformed to non-swelling silicates. Removing the rails prior to closure reduces this effect to 4 vol.-%. At 100 ka, the canister is entirely degraded to iron oxyhydroxides and in the surrounding bentonite, montmorillonite is totally converted to non-swelling Fe-silicates. The original porosity and hydraulic conductivity are preserved, but the swelling pressure is decreased to zero. This altered zone is 45 cm thick and extends to the bentonite zone altered by interaction with the concrete liner. Alteration of the buffer and sealing element adjacent to the concrete liner is estimated to be 20 cm thick (35 vol.-%) at 100 ka. The porosity and hydraulic conductivity of this zone are unchanged, but with zero swelling pressure due to removal of montmorillonite. The mineralogical composition of this zone is characterised by a sequence of calcite, C-(A)-S-H minerals, Ca-zeolites, sepiolite and saponite clays, with C-S-H minerals forming nearest the cement contact, and other minerals such as zeolites and clays forming further away. The amount of bentonite mass altered is increased by a factor of 2.5 and alteration thicknesses are increased by a factor of between 2.5 and 3 for the tunnel diameters considered (∼60 vol.-% total bentonite). The concrete liner is estimated to be totally degraded as a pessimistic estimate at 100 ka, but a zone of ettringite, calcite and tobermorite may exist marking its former presence. The state of the bentonite barrier at 1 Ma after closure is estimated to be similar to that at 100 ka, albeit with more crystalline degradation products of clay and concrete. The interactions of a copper canister with bentonite are restricted to minor amounts of cation exchange in montmorillonite (Cu for Na), resulting in no changes to safety-relevant properties over the lifetime of the repository

[en] This report reviews the geochemical data requirements and their application to the assessment of the performance of a geologic repository for radioactive wastes. It is concluded that information must be acquired on pH, redox reactions, inorganic and organic ligands in groundwater and colloids and processes such as precipitation, adsorption and diffusion and palaeohydrogeologic information in order to predict present hydrogeologic properties of potential repository sites. (UK)

No abstract available

[en] The selective removal of H₂S gas from a normally gaseous mixture containing H₂S and CO₂ is accomplished by contacting the gaseous mixture with an absorbent solution comprising a strongly basic tertiary amino compound having a pK /SUB a/ at 20⁰ C. greater than 8.6 and a boiling point at 760 mm greater than 180⁰ C. whereby H₂S is selectively absorbed from the mixture

[en] This paper reports the relative plutonium extraction rate from 1M nitric acid with thenoyltrifluoroacetone (TTA) dissolved in several different solvents. The marked differences in extraction rate are discussed in terms of the extraction mechanism. Experiments with varied phase mixing conditions, and with varied TTA or nitrate concentrations, have been performed to help clarify the mechanisms and explain the differences in extraction rate. The results show that in several solvents, plutonium is extracted as a mixed nitrate-TTA complex. The suitability of the various solvents for differing applications was also examined. Methyl benzoate is an ideal solvent when preparing counting sources from the solvent phase. Chloroform is preferable as solvent when the plutonium is to be backwashed into nitric acid. (author)