Post closure radioactive release scenarios from geologic salt formation, such as the WIPP (Waste Isolation Pilot Plant) (USA) include hydrologic transport of radionuclides through a chloride saturated aquifer. Consequently, the understanding of actinide solution chemistry in brines is essential for modeling requiring accurate knowledge of the interaction between AnO2 2+ and chloride ions. Complexation constants of two U(VI) chloride species UO2Cl+ and UO2Cl2 0 have been intensively studied for about 40 years using different methods. However, large uncertainties reflect the general difficulty in determining accurate stability constants of weak complexes. In order to model the behavior of U(VI) in brines, we studied the formation of its chloride complexes by UV-Vis spectroscopy as a function of the NaCI concentration at 25 °C. The experiments were performed at constant ionic strength by varying the concentration ratio of NaCI and NaClO4. Deconvolution resulted in single component absorption sepectrum for UO2Cl+ and UO2Cl2 0. The apparent stability constants of UO2Cl+ and UO2Cl2 0 are determined at different ionic strengths. The experimental data are used to parameterize using the SIT approach.

To evaluate the effect of geochemical conditions in the repository on the solubility of uranium under strongly oxidizing conditions, a mathematical model has been developed to determine the solubility, by utilizing a set of non linear algebraic equations to describe the chemical equilibria in the groundwater environment. The model takes into account the predominant precipitation-dissolution reactions, hydrolysis reactions and complexation reactions that may occur under strongly oxidizing conditions. The model also includes the solubilitylimiting solids induced by the presence of carbonate, phosphate, silicate, calcium, and sodium in the groundwater. The thermodynamic equilibrium constants used in the solubility calculations are essentially taken from the NEA Thermochemical Data Base of Uranium, with some modification and some uranium minerals added, such as soddyite, rutherfordite, uranophane, uranyl orthophosphate, and becquerelite. By applying this model, the sensitivities of uranium solubility to variations in the concentrations of various groundwater component species are systematically investigated. The results show that the total analytical concentrations of carbonate, phosphate, silicate, and calcium in deep groundwater play the most important role in determining the solubility of uranium under strongly oxidizing conditions.

Static dissolution experiments were performed with unirradiatcd UO2 in Boom Clay water. The objectives were (1) to measure the solubility of uranium species in Boom Clay water, with UO2 as the solid phase, and (2) to assess the impact of dissolved organic matter and carbonate concentration on this solubility. The tests were supported by calculations with geochemical codes to indicate possibly solubility controlling solid phases. The tests were performed in anoxic and reducing conditions, at 20 and 25°C. The following conclusions could be drawn: (1) Within 2 months in anoxic conditions, the uranium concentrations appear to approach saturation. (2) The “near-saturation” concentrations are between 2.4 and 7.8× 10−7 M. (3) The influence of the carbonate concentration and humic acids on the uranium concentration was apparently small, but the interpretation is hampered by pH and Eh variations. (4) The concentrations tend to be higher than the concentrations found in literature for similar Eh and/or pH conditions; this can probably be explained by small differences in experimental conditions. (5) The measured “near steadystate” uranium concentration in the real clay water agrees relatively well with the solubility calculated for uraninite. (6) Addition of sulfide species reduced the rcdox potential, but not the uranium concentrations, except in real Boom Clay water.

The stability constants for the complexation of hafnium by hydroxide ions is investigated by potentiometric titration over a range of ionic strengths (1m = 0.1 to 6.6 molal). The stability constants are determined from the titration data using the HYPERQUAD suite of programs. The stability constants at infinite dilution are determined using the Specific Ion Interaction Theory from the stability constants determined by titration. The results are shown with the error given as 2 standard deviations.

log β11 (Hf(OH)3+) = 13.84 ± 0.54log β12 (Hf(OH)2 2+) = 25.71 ± 0.39log β13 (Hf(OH)3 +) = 36.24 ± 0.88log β14 (Hf(OH)4 0) = 45.52 ± 0.80

The solubility product of Hf(OH)4 (s) is determined in 0.1 M NaC1O4 by measuring the total hafnium in solution that is in equilibrium with an excess of hafnium hydroxide solid under an argon atmosphere. The total Hf concentration is determined by ICP-AES. The solubility product is determined using the stability constants measured for the H f hydrolysis products in 0.1 M NaClO4. The precipitate examined is confirmed to be a hydroxide by IR spectroscopy. For Hf(OH)4 (s) in 0.1 M NaC1O4, the solubility product is log Ksp (Hf(OH)4 (s)) = −51.8 ± 0.5

The solubility and stability constants determined are used, along with literature values for plutonium solubility and complexation constants, to examine the behavior of hafnium and plutonium under the conditions expected at Yucca Mountain.

Experiments were conducted to determine the ability of surfactant-modification to enhance the ability of natural zeolites to sorb U(6+) and Np(5+). Natural zeolite material, comprised mainly of clinoptilolite and treated with the cationic surfactant exadecyltrimethylammonium-bromi(le (IDTMA), was reacted with U(6+) and Np(5+) solutions open to the atmosphere and having a range of radionuclide concentration, pH, and NaCI concentration. The results indicate surfactantmodification of the zeolite enhances its ability to sorb U(6+), particularly at pHs greater than six where U(6+) sorption on unmodified zeolite is typically low due to formation of anionic U(6+) aqueous carbonate complexes. In contrast, there is little enhancement of Np(5+) sorption onto surfactant-modified zeolite. The presence of chloride anions in solution makes surfactantmodification less effective. The enhanced sorption of U(6+) is interpreted to be due to anion exchange with counterions on the external portion of a surfactant bilayer or admicelles.

The effects of picolinate, an organic ligand used to decontaminate nuclear reactor cooling systems, in leachates generated from shallow-land burial (SLB) of low-level nuclear wastes (LLW) on soil adsorption was determnined. Using batch adsorption tests and varying the concentration of picolinate, the adsorption tendencies of two metals [Ni(II) and U(VI)] and the ligand were measured as a function of solution pH. We found that when total metal concentrations were fixed at 10−5 M, picolinate at ligand-to-metal [L:M] ratios ≥10 did significantly reduce adsorption of Ni but even at a L:M ratio of 100 there was no effect on U(VI) adsorption. These results are compared with data on other metals in the presence of picolinate and for metal adsorption in the presence of EDTA. We conclude that picolinic acid is less of a threat than EDTA in waste leachates to reduce metal adsorption (increase mobility) and that picolinate concentrations must reach or exceed 10−4 M for the most impacted metals (i.e., those that form the very strongest complexes with picolinate). There are no leachate data on these decontamination agents for the common burial technique (disposal of de-watered resins in high integrity containers) that can be used to evaluate potential hazards of these organo-radionuclide complexes.

Although transport calculations are often formulated in terms of mass-based isotropic distribution coefficients, it is the abundance of reactive surface areas of subsurface materials that controls contaminant adsorption. In water-saturated homogeneous systems devoid of advective fluxes (e.g., batch experiments), the available reactive surface area is similar to the total surface area (as measured by conventional methods such as BET gas adsorption). However, in physically and chemically heterogeneous systems with advective fluxes, the effective reactive surface area (i.e., the surface area that a packet of advecting water interacts with) is smaller than the laboratory measured surface area and is a complex function of advective velocity and the correlation structures of the physical and chemical heterogeneities. Theoretical derivations for an important but simple type of heterogeneity (fine-scale horizontal layering) suggest that the effective reactive surface area is an anisotropic property of the medium and is inversely correlated with the anisotropy in hydraulic conductivity. The implications of reactive transport anisotropy include the concept that the retardation factor should be treated as a directional property rather than being treated as a constant. Furthermore, because of the inverse relationship between effective reactive surface area and hydraulic conductivity, batch adsorption experiments tend to overestimate the retention of contaminants relative to intact natural materials.

Laboratory studies were conducted to quantify and understand the processes by which iodide (I−) sorbs to minerals found in subsurface arid sediments. Little or no I− sorbed to montmorillonite (Kd = −0.42 ± 0.08 mL/g), quartz (Kd = 0.04 ± 0.02 mL/g), vermiculite (Kd = 0.56 ± 0.21 mL/g), calcite (Kd = 0.04 ± 0.01 mL/g), goethite (Kd = 0.10 ± 0.03 mL/g), or chlorite (Kd = −0.22 ± 0.06 mL/g). A significant amount of I− sorbed to illite (Kd = 15.14 ± 2.84 mL/g).). Upon treating the iodide-laden illite with dissolved F−, Cl−, Br−, or 127I−, desorption (or isotopic exchange in the case of 127I) removed, respectively, 43 ± 3%, 45 ± 0%, 52 ± 3, and 83 ± 1 % of the I− originally adsorbed to the illite. The fact that such large amounts of I− could be desorbed suggests that the I− was weakly adsorbed, and not chemically bonded to a soft metal, such as mercury or silver, that may have existed in the illite structure as trace impurities. Finally, I− sorption to illite was strongly pH-dependent; the Kd values decreased from 46 to 22 mL/g as the pH values increased from 3.6 to 9.4. Importantly, I− sorbed to illite even under alkaline conditions. Together, these experiments suggest that illite removed I− from the aqueous phase predominantly by reversible physical adsorption to the pH-dependent edge sites. Illites may constitute a substantial proportion of the clay-size fraction of many arid sediments and therefore may play an important role in retarding I− movement in these sediments.

Analytical electron microscopy (AEM) has been employed to characterize the distribution of U(VI) on several mineral substrates, including standard kaolinite samples and weathered rock samples from Koongarra, Australia. Results of this study reveal that pre-existing impurity phases play a significant role in the uptake of U(VI) on standard kaolinite samples. Using a natural sample from the weathered zone of the Koongarra uranium deposit, we have also demonstrated that U(VI) sorption is controlled by iron oxyhydroxides (predominantly goethite) which constitute a small fraction of the sample. Limited sorption of U(VI) was observed on the clay minerals of this substrate. Uptake of U(VI) by goethite increases with total uranium content up to the point of uranium precipitation, after which it decreases dramatically. Uranium precipitation is indicated by the appearance of a uranyl oxyhydroxide phase.

The containment of radionuclides over very long periods of time is based on the interposition of a multi-barrier system between the waste and the biosphere. A performance allocation study is usually conducted for each barrier. Each barrier is then designed, in terms of geometry and composition, to cope with the performance it has been allocated. However geochemical interactions will occur between the different barriers. An alkaline plume will be generated by cement materials, a redox front will be generated by container corrosion and geochemical gradients will be generated by the dissolution of artificial barriers by natural groundwater. Radionuclide retention mechanisms are strongly pH and Eh dependent, therefore the impact of these geochemical transients on retention must be quantitatively evaluated to check the performance of each barrier for realistic in-situ situations. To assess this impact, two types of engineered barriers (clay and cement) for a spent fuel repository are simulated with a coupled hydrogeochemical model. Comparisons between hydraulic heterogeneous (fractured) and simple homogeneous systems are also carried out in terms of waste dissolution.

In a simulated engineered barrier system consisting of a simulated HLW glass doped with Pu, a compacted bentonite and water under reducing conditions, migration tests of Pu from the glass into the bentonite through water were performed. The following results were obtained: (1) The presence of bentonite enhances the glass dissolution while it has no remarkable influence on the concentration of Pu dissolved species in the solution. (2) Only a small part of the leached Pu, which may correspond to the dissolved species, can diffuse into the bentonite with the apparent diffusion coefficient of 3×10−14m2/s. (3) Interactions between the glass and the bentonite have no remarkable influence on the diffusion of Pu.

Distribution coefficients and apparent diffusion coefficients of cesium in some compacted bentonites were determined by the penetration profile method. Cylindrical compacted bentonites with the dry density of 0.8 to 1.6 Mg/m3 were contacted with tracer solutions containing 1000, 100 or 10 ppm of cesium. The apparent diffusion coefficients were obtained from the concentration profiles of cesium in compacted bentonites. The distribution coefficients were obtained concurrently by dividing the intercepts of the profiles by the concentration of the tracer solution. The apparent diffusion coefficients of cesium in compacted bentonite were obtained in the range of 0.42 to 9.6· 10−12 m2/s. The apparent diffusion coefficients in the compacted bentonite contacted with three different concentrations of cesium tended to decrease with increasing dry density of the specimen; but, they had no dependence on cesium concentration within a factor of 3 at the same dry density. The distribution coefficient of cesium for the specimens contacted with three different concentrations of cesium were obtained in the range of 0.3 to 90 L/kg and had little dependence on dry density. The distribution coefficients obtained in the compacted bentonites were dependent on pH of the solution rather than concentration of cesium. These distribution coefficients obtained in the compacted bentonites were 10 to 1000 times smaller than those obtained by batch experiments. The data suggest that not all sorption sites for cesium are available in highly compacted bentonite. It is necessary to consider surface diffusion as a significant migration mechanism of cesium through the compacted bentonites at very high pH condition such as 12.

Different approaches for measuring the interaction between radionuclides and rock matrix are needed to test the compatibility of transport models and retardation experiments. In this work sorption of cesium (134Cs) was studied on unaltered mica gneiss, and on unaltered, moderately altered and strongly altered tonalite. The crushed rock samples were sieved into seven fractions from 0. 1 mm to 3.15 mm. The mass distribution ratio values for each fraction was determined using the static batch method. Cesium sorption onto different minerals was demonstrated using digital image analysis in order to interpret thin section autoradiographs. The autoradiographic method based on irradiation-induced luminescence properties in feldspars was applied in order to estimate the alteration of tonalites.

Cesium sorption on moderately altered and strongly altered tonalite was stronger than on unaltered rocks owing to larger specific surface areas and the composition of alteration minerals. Strongest sorption was found on biotite. Mass distribution ratio, Rd, values of 0.3–1.1 m3·kg−1 for unaltered rocks and 0.9–3.4 m3·kg−1 for altered rocks were determined. Moderate dependence on surface area was found for mica gneiss after I and 3 days sorption time. For strongly altered tonalite only slight dependence on surface area was found. Research-articled -values increased as a function of sorption time due to diffusion into the particles. Higher increase in values for larger diameter particles indicated the availability of internal specific surface areas.

This paper describes a combined experimental and modelling programme of generic sorption studies to increase confidence in the performance assessment for a potential highlevel radioactive waste repository in Japan. The sorption of polonium, actinium and protactinium onto geological materials has been investigated. Sorption of these radioelements onto bentonite, tuff and granodiorite from equilibrated de-ionised water was studied under reducing conditions at room temperature. In addition, the sorption of actinium and protactinium was investigated at 60°C. Thermodynamic chemical modelling was carried out to aid interpretation of the results.

79Se is a long-lived (1.1×106 years) fission product which is chemically and radiologically toxic. Under Eh-pH conditions typical of oxidative alteration of spent nuclear fuel, selenite or selenate are the dominant aqueous species of selenium. Because of the high solubility of metalselenites and metal-selenates and the low adsorption of selenite and selenate aqueous species under alkaline conditions, selenium may be highly mobile. However, 79Se released from altered fuel may be immobilized by incorporation into secondary uranyl phases as low concentration impurities, and this may significantly reduce the mobility of selenium. Analysis and comparison of the known structures of uranyl phases indicate that (SeO3) may substitute for (SiO3OH) in structures with the uranophane anion-topology (α.-uranophane, sklodowskite, boltwoodite) which are expected to be the dominant alteration phases of UO2 in Si-rich groundwater. The structural similarity of guillemninite, Ba[(UO2)3 (SeO3)2O2](H2O) 3 to phurcalite, [(UO2)3(PO4)2O2](H2O)7, suggests that the substitution (SeO3)↔ (PO4) may occur in phurcalite. The close similarity between the sheets in the structures of rutherfordine and [(UO2)(SeO3)] implies that the substitution (SeO3) ↔ (CO3) can occur in rutherfordine. However, the substitutions: (SeO3) ↔ (SiO3OH) in soddyite and (SeO3) ↔ (PO4) in phosphuranylite may disrupt their structural connectivity and are unlikely to occur.

The sorption of radionuclides onto cement materials is a very important parameter when considering the release of radionuclides from radioactive wastes. Once the composition or crystallinity of the constituent minerals of a cement material is changed by alteration in the disposal environment, its sorption ability might be affected. In this study, the effect of hydrothermal alteration on the sorption properties of two cement grouts and Calcium Silicate Hydrogels (CSH-gels) is investigated by using the batch sorption technique. In the case of strontium (a model cation) sorption, the distribution ratio for Ordinary Portland Cement (OPC) and OPC/Blast Furnace Slag blended cement (OPC/BFS) decreased as the alteration temperature increased. This is mainly caused by the decrease of sorption onto CSH-gel which is a constituent of cement materials. In the case of selenium (selenite, a model anion) sorption, the distributionratio decreased as the alteration temperature increased for OPC treated in both distilled water and groundwater, and for OPC/BFS in groundwater. This is attributed to the decomposition of ettringite which sorbs anions. The distribution ratio for OPC/BFS in distilled water increased as the alteration temperature increased, although ettringite decomposed. This is attributed to the formation of monosulphate which also sorbs anions. These results show that the hydrothermal alteration of cement mineral phases in a disposal environment may cause changes which could increase or decrease the sorption of radionuclides onto cements depending on the cement composition and radionuclide speciation.

We examined the ability of a halophilic bacterium (WIPP 1A) isolated from the Waste Isolation Pilot Plant (WIPP) site to accumulate uranium in order to determine the potential for biocolloid facilitated actinide transport. The bacterial cell surface functional groups involved in the complexation of the actinide were determined by titration. Uranium, added as uranyl nitrate, was removed from solution at pH 5 by cells but at pH 7 and 9 very little uranium was removed due to its limited solubility. Although present as soluble species, uranyl citrate at pH 5, 7, and 9, and uranyl carbonate at pH 9 were not removed by the bacterium because they were not bioavailable due to their neutral or negative charge. Addition of uranyl EDTA to brine at pH 5, 7, and 9 resulted in the immediate precipitation of U. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) analysis revealed that uranium was not only associated with the cell surface but also accumulated intracellularly as uranium-enriched granules. Extended X-ray absorption fine structure (EXAFS) analysis of the bacterial cells indicated the bulk sample contained more than one uranium phase. Nevertheless these results show the potential for the formation of actinide bearing bacterial biocolloids that are strictly regulated by the speciation and bioavailability of the actinide.