Granitic Groundwater Research Articles

Granitic groundwater colloids sampling and characterisation: the strategy for artefact elimination

Colloids were separated by submicro-filtration of granitic groundwater samples collected at-line under in-situ thermodynamic conditions after down-hole groundwater sampling and transfer at the well head. The methodology avoids the generation of artefacts produced by pH changes due to CO(2) exchange, yielding potential carbonate precipitation, or by O(2) contamination yielding oxidized insoluble phases. The enhanced pressure and the anoxic conditions are also maintained through the filtering procedure. This procedure was carried out after a period of regular sampling of groundwater pumped to the ground surface and continuous on-line long-term measurements (weeks, months) of chemical and physical parameters in the unbroken sample water both at the ground surface and at depth down-hole. Colloid samples were characterized on the submicro-filtration membrane by scanning electron microscopy. Under deep granite groundwater conditions, natural colloids occur sparsely. The colloid concentration was determined C(col) approximately 1 and approximately 50 microg L(-1) for sizes ranging from 50 to 200 nm or n(col) approximately 3.9 x 10(9) and 47 x 10(9) L(-1) for sizes larger than 50 nm for KFM11A, Forsmark, and KLX17A, Laxemar, Oskarshamn, respectively, Sweden. These colloids are expected to be clay particles with an average size smaller than 200 nm for the Na-Ca-Cl and Na-Cl groundwaters (pH 7.6 and 8.00, ionic strength approximately 10(-1) and approximately 10(-2) mol L(-1), respectively, for KFM11A and KLX17A), the colloid concentrations were comparable with values previously reported in the literature.

LABORATORY TESTS ON THE OCCURRENCE OF FLUORIDE RICH GROUNDWATER OF TONO AREA, JAPAN

High concentration of fluoride ion up to 12 mg/l is observed in deep groundwater at the Tono area, Ja pan. A simple but general method to evaluate the occurrence of high fluoride ion concentration in groundwater has reported in this study. Our method based on the water-rock interaction test using different sizes of granite powders sampled from a boring core drilled at the site. Results indicated that, the weathering of granitic rock might be one of the key components for changing the fluoride ion concentrations. And also the increase of the reaction time led to enhance leaching of the fluoride ion from granite. To deduce the contribution of the co-existence of the chemical parameters, which affected on the change in the fluoride ion concentration, a stochastic model was developed. This model shows significant evaluation of the water-rock interaction test with the chemical data of a low Na-(Ca)-Cl water-type. Results from our stochastic model indicated that the approximation reaction time of the water-rock in the granitic groundwater was about 300 days at this region.

Release of Colloids from Injection Grout Silica Sol

AbstractNon-cementitious grouts have been tested in Olkiluoto for the sealing of fractures with the small hydraulic aperture. A promising non-cementitious inorganic grout material for sealing the fractures of the apertures less than 0.05 mm is commercial colloidal silica called silica sol. The use of colloidal material has to be considered in the long-term safety assessment of a spent nuclear fuel repository. Objective of this work was to determine colloid release from the silica sol gel and stability of silica colloids in different groundwater conditions. To use silica sol as a grout, the injected colloids have to aggregate and form a gel within a predictable time by using a saline solution as an accelerator. Silica sol gel samples were stored in contact with medium salinity and low salinity groundwater simulates. Release of silica colloids and colloid stability was followed by analyzing the colloid concentration, particle size distribution, concentration of reactive silicon, solution pH and zeta potential after one month, half a year and one year. Malvern Zetasizer Nano ZS equipment was used to determine colloidal particle size distributions applying the dynamic light scattering method and zeta potential based on dynamic electrophoretic mobility. The colloidal particle concentration was estimated from Zetasizer measurements applying a standard series. Dissolved reactive silica concentration was determined using the molybdate blue method and total silica concentrations were determined using ICP–MS. The release and stability of silica colloids were found to be dependent significantly on groundwater salinity. Zeta potential values near zero and the increase in particle size at first and then the disappearance of large particles indicated particle flocculation or coagulation and instable colloidal dispersion in a saline groundwater simulate. In low salinity ground water simulate high negative zeta potential values, small particle size and constant size distribution indicate the existence of stable silica colloids. The concentrations of the released colloids were slightly higher than determined in natural granitic ground waters. Under prevailing saline groundwater conditions in Olkiluoto no significant release of colloids from silica sol is expected but the possible influence of low salinity glacial melt waters has to be considered.

Modelling Ni(II) sorption in granitic groundwater

Modelling Ni(II) sorption in granitic groundwater

Leaching of Cesium and Uranium from Spent PWR fuel in the Gel-state Clays

ABSTRACTThe amounts of cesium and uranium released from crushed spent PWR fuel in the gel-state clays with a few ml of supernatant at hot cell temperature under Ar-atmosphere have been measured. The fractions of cesium dissolved from the fuel for 873 days were 0.29 and 0.25% in Boom clay/Boom-clay water and Ca-bentonite/synthetic granitic groundwater, respectively. These cesium fractions were very close to the gap inventory of cesium, which was determined to be around 0.30% in the previous experiment. The fraction of uranium released up to 193 days in the Boom clay media was 0.011% and this fraction has been retained until 873 days. Such this phenomenon was also obtained in the Ca-bentonite media even though the released fraction was higher than that in Boom clay. The increase of less than 0.001% in the dissolved uranium fraction between 193 and 873 days suggests that the long-term leach rate of uranium from spent fuel would be much less than 24 μg·m−2·day−1.

Indicators of Petroleum Hydrocarbon Biodegradation in Anaerobic Granitic Groundwater

The aim of this study was to find indicators of petroleum biodegradation in granitic groundwater. Both pristine and contaminated groundwaters from boreholes around petroleum storage vaults located approximately 40 m below the surface in granite and with storage capacities of up to 120,000 m3 were sampled. Total numbers of microorganisms, “most probable numbers” (MPN) of anaerobic bacteria, and chemical indications of microbial activity were determined in the groundwater. Hydrocarbon contaminants and metabolites were detected using gas chromatography-mass spectrometry (GC-MS). In contaminated groundwater, the total number of microorganisms was 2–4 × 106 ml−1, which was significantly higher than the 6 × 104 ml−1 found in pristine groundwater. This microbial abundance was also reflected in the MPN analysis. Up to 7 × 104 nitrate-, 2 × 103 iron-, and 3 × 104 sulfate-reducing bacteria were detected in contaminated groundwaters. In such groundwaters, depletion of anaerobic electron acceptors and detection of reduced species could be established. We also proposed using a high alkalinity/hardness of water quota (A/H quota) as an indicator of microbial activity. In contaminated groundwaters the A/H quota averaged 2.8, while in pristine groundwater the same was only 1.3. Moreover, the presence of 20 oxidized petroleum hydrocarbons, i.e., putative metabolites of which 9 were strictly intracellular, was detected in the contaminated groundwaters. Phylogenetic neighbor-joining analysis of 16S rRNA genes provided information about the bacterial communities. The bacteria in contaminated groundwater were found to be strikingly similar to bacteria in other hydrocarbon-contaminated environments.

Hydrochemical baseline condition of groundwater at the Mizunami underground research laboratory (MIU)

Hydrochemical conditions up to depths of 1000 m below ground level around the Mizunami Underground Research Laboratory were investigated to construct a “baseline condition model” describing the undisturbed hydrochemical environment prior to excavation of the underground facilities at Mizunami, Gifu, Japan. Groundwater chemistry in this area was classified into a Na–Ca–HCO 3 type of groundwater in the upper part of sedimentary rock sequence and a Na–(Ca)–Cl type of groundwater in the deeper part of the sedimentary rock sequence and basement granite. The residence time of the groundwaters was estimated from their 14C contents to be approximately 9.3 ka in the middle part of the sedimentary rock and older than 50 ka in the deep part of the granite. The evolution processes of these groundwaters were inferred to be water–rock interactions such as weathering of plagioclase, dissolution of marine sulphate/sulphide minerals and carbonate minerals in the Na–Ca–HCO 3 type of groundwater, and mixing between “low-salinity water” in the shallow part and “higher-salinity water” in the deeper part of the granite in the Na–(Ca)–Cl type of groundwater. The source of salinity in the deeper part of the granite was possibly a palaeo-hydrothermal water or a fossil seawater that recharged in the Miocene, subsequently being modified by long-term water–rock interaction. The Cl-depth trend in granitic groundwater changes at a depth of −400 m below sea level. The hydrogeological properties controlling the groundwater flow and/or mixing processes such as advection and diffusion were inferred to be different at this depth in the granite. This hydrochemical conceptual model is indispensable not only when constructing the numerical model for evaluating the hydrochemical disturbance during construction and operation of the MIU facility, but also when confirming a hydrogeological model.

Adsorption of cesium on Czech smectite-rich clays—A comparative study

The adsorption of cesium on various smectite-rich clays was studied as a function of the initial concentration of cesium and in the presence of competitive ions (Na+, K+ and Ca2+) using the radiotracer method, examining the effects of the clay structure and comparing the behaviour of bentonites with that of sedimentary clays. The experiments were performed under laboratory conditions, at 22–25 °C, in ambient atmosphere and using synthetic average granite groundwater as the liquid phase. Reference smectites (STx−1, SAz−1), illite (IMt−1) and kaolinite (KGa−2), three Czech smectite-rich clays (bentonite from Rokle, sedimentary clays from Skalná and Maršov) and three Spanish bentonites from Cabo de Gata were used as the natural sorbents. The adsorption data were fitted by the Freundlich isotherm. Smectites exhibited the highest values of the adsorption capacity and the highest affinity for cesium within the studied concentration range, 10−6 to 10−1 M. The Kd values disclosed two different trends—a relatively constant adsorption rate at Cs concentrations below 10−2 M (typical for smectites–bentonites from Spain), or a continuous decrease in the adsorption rate over the whole concentration range (“non-smectites”, such as illite and kaolinite-sedimentary clays (Skalná, Maršov). Therefore, the higher contents of illite and kaolinite (48 wt.% in Skalná and 25 wt.% in Maršov) significantly influence the cesium adsorption trend and the cation exchange capacities of the bulk material. Surprisingly, the Rokle bentonite, containing more than 60 wt.% of expandable clay structures, shows the “non-smectite” trend. This can be explained by the possible presence of expandable vermiculite in addition to smectite. The cesium adsorption behaviour of vermiculite is considered similar to that of illite. For trace cesium concentrations (10−6 M), the major competitive ion is K+, followed by Ca2+ and finally by Na+. Bentonites are more affected than sedimentary clays containing illite—this is due to the ion exchange mechanism predominating in the adsorption on smectites, compared to specific selective adsorption on illite and mica-type clay minerals.

Kinetics and irreversibility of cesium and uranium sorption onto bentonite colloids in a deep granitic environment

Sorption of radionuclides onto a stable colloidal phase may significantly enhance their transport in groundwater. A key point, to be analysed to assess the relevance of colloids in the safety of a deep geological radioactive waste repository, is the irreversibility of the colloid/radionuclide bond. In this work, sorption and desorption kinetics of cesium and uranium(VI) onto bentonite colloids in a granitic reduced environment was studied by means of batch experiments, carried out in anoxic conditions under N 2 atmosphere. Sorption kinetics was followed during 18 weeks, and sorption isotherms were also carried out to get additional information on sorption mechanisms. The water used in all the experiments was an alkaline, low ionic strength (pH=9.5 and I=1×10 −3M) granitic groundwater from the NAGRA's Grimsel Test Site (GTS), Switzerland, which also presents reduced Eh (−200 mV). In this water, bentonite colloids were shown to be stable during several months. Both cesium and uranium presented a nonlinear sorption behaviour in the range of concentration investigated. In kinetic experiments, the measured log Kd for Cs ([Cs]=1×10 −7 M) was 3.94±0.15, and this value did not show significant variations with time. However, the adsorption of cesium on bentonite colloids involves two reactions, a rapid exchange on planar sites (hours) and a slower component (days) in which cesium diffuses to less available but highly selective sites. This slow process, that can be evidenced only when very low tracer concentrations are used (<1×10 −9 M), is most probably responsible for the fixation of a fraction of the sorbed cesium, and for the partial sorption irreversibility shown in desorption tests. Kd values measured after several desorption experiments increased significantly with the age of the sorption complex. For example, for the sample with 1-day contact time, the second desorption Kd was ∼8600 ml/g whereas the 5 and 8 weeks contact time samples showed second desorption Kd ∼15 000 and ∼30 000 ml/g, respectively. The measured log Kd for U ([U(VI)]=4×10 −7 M) varied from 2.91 to 3.21 (±0.15) during 18 weeks of the kinetic experiment. The main variation of Kd values took place in the first 4 weeks, and then a very slow increasing trend was observed, which could be probably attributed to a partial reduction of U(VI) to U(IV). In desorption tests with uranium, desorption K ds were independent on the initial contact time. Nevertheless, a certain sorption/desorption hysteresis was observed, which is most probably due to the contribution of surface complexation reactions, at the edge sites of clay colloids, to uranium sorption. Hence, U sorption is not completely reversible.

Study of Ni(II) sorption on chlorite - a fracture filling mineral in granites

AbstractChlorite is an Fe(II)-containing phyllosilicate which is often present as a fracture filling mineral in e.g. granitic rocks. It may therefore be significant in influencing redox conditions and sorption processes in granitic groundwaters. The sorption properties of chlorite may therefore be important when modelling the migration of radionuclides under reducing conditions around nuclear waste repositories or in sites contaminated by mining waste.The sorption behaviour of Ni(II) onto a natural chlorite (Karlsborg, Sweden) was investigated using a batch technique. The effects of three different background electrolyte concentrations (0.01 M, 0.1 M and 0.5 M NaClO4), different pH values (ranging from 4 to 11) and different Ni(II) concentrations (10−6 and 10−8 M) were studied under anoxic conditions in a glove-box. Ni(II) solutions were spiked with 63Ni and β-Liquid scintillation counting (LSC) was used to determine the concentration of nickel in the bulk solution, allowing the calculation of solid-water distribution coefficients for the metal ion.The results of the sorption experiments show strong pH dependence at pH &gt; 5, but the sorption is independent of ionic strength. The maximum adsorption is found in the pH range between 7 and 11 with Kd values ≍103 cm3/g. A diffuse double layer model has been used to describe the experimental results.

The kinetics of O2(aq) reduction during oxidative weathering of naturally occurring fracture minerals in groundwater

Aqueous chemistry methods assessed the kinetic reactivity and reduction capacity of fracture-filling minerals from a granitic groundwater environment at the Aspo Hard Rock Laboratory, a field station for research and development into the geological disposal of spent nuclear fuel. Naturally occurring fracture filling reacted with oxygenated test solutions of known composition in recirculating batch reactors.The loss of O2(aq) with time was consistent with second-order reaction kinetics where O2(aq) is consumed through reduction by reaction with structural Fe(II) at the surface of the fracture minerals. Values of the second-order rate constant (k, l mole—1 h—1) varied between experiments within the range 1.7 &lt; log k &lt; 2.9 and values for the total concentration of reducing sites as a measure of the reduction capacity of the mineral (St, mole g—1) varied within the range 8.5 x 10—5 &lt; St &lt; 4.3 x 10—4. Values for the rate constant are somewhat less than those published previously for reaction between O2(aq) and Fe(II) surface complexes; i.e. structural Fe(II) appears to be less reactive than adsorbed Fe(II)(aq) but is significantly more reactive than Fe2+.Values of the rate constant did not depend on release of network ions from the reacting minerals, and did not vary significantly with pH, mineral mass, specific surface area or mineral Fe(II) content. Oxidative weathering of sulphide minerals does not occur to any measurable extent. When corrected to rock/water ratios for granite aquifers, the rate constants correspond to a half-life for O2(aq) on the order of seconds indicating an essentially instantaneous reaction on repository time scales. Comparison of reducing capacity for the fracture fillings and oxidizing capacity for groundwater saturated with O2(aq) indicates that oxidizing fronts within the geological barrier would travel ~4000 times more slowly than the velocity of groundwater flow.

東濃地域における地下水化学と地下微生物の相互作用

The abundance and diversity of groundwater microorganisms were studied in a geochemically defined borehole (TH-6, DH-3) in the Tono area, central Japan. Total cell counts by epifluorescence microscopy were estimated to be approximately 1.6 × 105-5.7 × 106 cells ml -1 and showed little decrease with depth. Cell viability, based on cell membrane integrity, respiration-based metabolism, and esterase activity was estimated to be 0.001% to approximately 100% of the total counts. The distribution of microbial abundance seems to be related to a variety of environmental factors, including fracture numbers, hydrological, and geochemical conditions of the groundwater. The geochemistry of the groundwater suggests that sulfate reducing bacteria play an important role in the sedimentary rock environment. It appears that microbial sulfate reduction occurs at the highest level in the upper part of the Toki Lignite-bearing Formation. This microbial reaction may play an important role in maintaining anaerobic conditions for uranium fixation, that could in turn scavenge soluble and oxidized uranium. On the other hand, in the granite groundwater, iron-oxidizing/reducing bacteria seem to play an important role in iron redox cycling. Iron-oxidizing bacteria may contribute to the formation and deposition of iron colloids in the upper part of Toki granite.

A new leaching test based in a running water system to evaluate long-term water resistance of concretes

The results of leaching tests are one of the criteria used to evaluate the performance of cementitious materials for stabilisation/solidification of hazardous wastes. The action of water is of special importance due to its effect on the integrity of the cement solid phases. Likewise, the degraded material originated by the water action may facilitate the transport of toxic components from the material to the environment. Therefore, there is an interest in determining the material behaviour under the long-term action of water in representative conditions of a real storage scenario, in particular where radioactive waste storage times of 300 years or more are needed. During this time, the stabilised waste should maintain its properties. In the present paper a new leaching method to test the performance of concretes for long-term behaviour under natural water interaction is presented. An open system and a system of continuous running of natural water flowing at a fixed rate have been used. The system has been applied to test high- and ultra-high performance concretes for over a year. It gives different results to those previously reported using demineralised water or acid solutions. The method was developed to represent real scenarios in contact with granitic groundwater.

Microbial production and oxidation of methane in deep subsurface

The goal of this review is to summarize present studies on microbial production and oxidation of methane in the deep subterranean environments. Methane is a long-living gas causing the “greenhouse” effect in the planet's atmosphere. Earlier, the deep “organic carbon poor” subsurface was not considered as a source of “biogenic” methane. Evidence of active methanogenesis and presence of viable methanogens including autotrophic organisms were obtained for some subsurface environments including water-flooded oil-fields, deep sandy aquifers, deep sea hydrothermal vents, the deep sediments and granitic groundwater at depths of 10 to 2000 m below sea level. As a rule, the deep subterranean microbial populations dwell at more or less oligotrophic conditions. Molecular hydrogen has been found in a variety of subsurface environments, where its concentrations were significantly higher than in the tested surface aquatic environments. Chemolithoautotrophic microorganisms from deep aquifers that could grow on hydrogen and carbon dioxide can act as primary producers of organic carbon, initiating heterotrophic food chains in the deep subterranean environments independent of photosynthesis. “Biogenic” methane has been found all over the world. On the basis of documented occurrences, gases in reservoirs and older sediments are similar and have the isotopic character of methane derived from CO 2 reduction. Groundwater representing the methanogenic end member are characterized by a relative depletion of dissolved organic carbon (DOC) in combination with an enrichment in 13C in inorganic carbon, which is consistent with the preferential reduction of 12CO 2 by autotrophic methanogens or acetogens. The isotopic composition of methane formed via CO 2 reduction is controlled by the δ 13C of the original CO 2 substrate. Literature data shows that CH 4 as heavy as −40‰ or −50‰ can be produced by the microbial reduction of isotopically heavy CO 2. Produced methane may be oxidized microbially to carbon dioxide. Microbial methane oxidation is a biogeochemical process that limits the release of methane, a greenhouse gas from anaerobic environments. Anaerobic methane oxidation plays an important role in marine sediments. Similar processes may take place in deep subsurface and thus fuel the deep microbial community. Organisms or consortia responsible for anaerobic methane oxidation have not yet been cultured, although diverse aerobic methanotrophs have been isolated from a variety of underground niches. The presence of aerobic methanotrophs in the anoxic subsurface remains to be explained. The presence of methane in the deep subsurface have been shown all over the world. The flux of gases between the deep subsurface and the atmosphere is driven by the concentration gradient from depth to the atmosphere. However, methane is consumed by methanotrophs on the way of its evolution in oxidized environments and is transformed to organic form, available for further microbial processing. When the impact of subsurface environments to global warming is estimated, it is necessary to take into account the activity of methane-producing Archaea and methane-oxidizing biofilters in groundwater. Microbial production and oxidation of methane is involved in the carbon cycle in the deep subsurface environments.

Anaerobic Corrosion of Carbon Steel and Cast Iron in Artificial Groundwaters: Part 1—Electrochemical Aspects

Abstract In Sweden, high-level radioactive waste will be disposed of in a canister with a copper outer container and a cast iron or carbon steel insert. If the iron insert comes into contact with anoxic geological water, anaerobic corrosion will occur. This paper presents a study of the electrochemical aspects of anaerobic corrosion of carbon steel in artificial Swedish granitic groundwaters. The electrochemical measurements confirm that anaerobic corrosion will lead to the generation of hydrogen, but the rate of corrosion will be anodically limited by the formation of a corrosion product film. The corrosion rate was unaffected by hydrogen pressures up to 100 atm.

A mobile laser-induced breakdown detection system and its application for the in situ-monitoring of colloid migration

Abstract A mobile Laser-Induced Breakdown Detection (LIBD) system is developed for the field study of the aquatic colloid migration. The principle of LIBD is based on the plasma generation at breakdown of colloids in the laser focus region, which is then observed by either photo-acoustic detection or optical monitoring. The method provides information on the number density and average size of colloids. The present instrumentation built up particularly for the field study is applied for the aquatic colloid migration in a natural granite fracture. The study is performed within the Colloid and Radionuclide Retention project (CRR) in order to investigate the role of colloids on the radionuclide migration. For this purpose, bentonite colloids dispersed in granite groundwater are injected into a rock fracture zone with a dipole distance of 5 m. After passing through the fracture, the groundwater is guided into a flow-through detection cell of LIBD for the quantification of colloids. Another experiment is performed with the same colloids traced by tetra- and tri-valent metal ions, Th(IV), Hf(IV) and Tb(III). The recovery of colloids as found by LIDB appears to be about 55±5% of the injected colloid mass concentration, which is corroborated by ICP-MS analysis of Al present in bentonite colloids. An average size of recovered colloids is slightly decreased from the value of before injection and hence the relative number density increased in the course of the rock fracture migration. Recovered fractions of colloid-borne trace elements in the extracted groundwater are 78±8% for both tetra-valent elements and 33±3% for trivalent element of the originally traced concentrations. Plausible explanations are given for the different recoveries of heavy metal ions.

Release of boron and cesium or uranium from simulated borosilicate waste glasses through a compacted Ca-bentonite layer

The long-term release behavior of some elements from simulated borosilicate waste glasses (S-, K- and A-glass) in contact with a domestic compacted Ca-bentonite block and synthetic granitic groundwater at 80°C under argon atmosphere has been studied by dynamic leach tests since 1997 at KAERI. S- and K-glass differ mainly in their aluminum content, and A-glass contains 19.35 wt% UO 2 instead of fission product elements. Up to the present, the mass loss is almost the same as the normalized boron loss. This means that boron is an indicator on the dissolution of borosilicate waste glass. The leach rates of boron from K- and S-glasses after 861 days were approximately 3.1×10 −2 and 3.0×10 −2 g/ m 2 day , respectively. However, the release rates of cesium through the bentonite block from K- and S-glasses were about 1/10th of the release rate of boron, which were almost the same around 2.5×10 −3 g/ m 2 day . This may be due to their adsorption on the bentonite. The leach rate of boron from the A-glass was about 5.4×10 −2, but the leach rate of uranium from the A-glass specimen was quite low, below 4×10 −7 g/ m 2 day . The low concentration of uranium in the leachates suggests that it hardly moves in a compacted bentonite block. By the EPMA, a yellowish uranium compound was deposited on the surface of the bentonite in contact with the A-glass specimen. The species of this phase should be identified to understand the release mechanism of uranium.

Laboratory migration experiments with radionuclides and natural colloids in a granite fracture

Natural colloids in groundwater could facilitate radionuclide transport, provided the colloids are mobile, are present in sufficient concentrations and can adsorb radionuclides. This paper describes the results of a laboratory migration study carried out with combinations of radionuclides and natural colloids within a fracture in a large granite block to experimentally determine the impact of colloids on radionuclide transport. The 85Sr used in this study is an example of a moderately sorbing radionuclide, while the 241Am is typical of a strongly sorbed radionuclide with very low solubility. The natural colloids used in this study were isolated from granite groundwater from Atomic Energy of Canada (AECL) Underground Research Laboratory (URL), and consisted of mostly 1–10 nm organic colloids, along with lesser amounts of 10–450 nm colloids (organics and aluminosilicates). The measured coefficients for radionuclide sorption onto these colloids were between 3×10 2 and 1×10 3 ml/g for 85Sr, and between 7×10 4 and 7×10 5 mg/l for 241Am. The 85Sr sorption on the natural colloids appeared to be reversible. Migration experiments in the granite block were carried out by establishing a flow field between two boreholes (out of a total of nine) intersecting a main horizontal fracture. These experiments showed that dissolved 85Sr behaved as a moderately sorbing tracer, while dissolved 241Am was completely adsorbed by the fracture surfaces and showed no evidence of transport. However, when natural colloids were injected together with dissolved 241Am, a small amount of 241Am transport was observed, demonstrating the ability of natural colloids to facilitate the transport of radionuclides with low solubility. Natural colloids had only a minor effect on the transport of 85Sr. In a separate experiment to test the effect of higher colloid concentrations on 85Sr migration, synthetic colloids were produced from Avonlea bentonite. The introduction of a relatively high concentration of bentonite colloids actually reduced 85Sr transport because, compared to natural colloids, the bentonite colloids were less mobile and they sorbed 85Sr more strongly.

Influence of temperature elevation on the sealing performance of a potential buffer material for a high-level radioactive waste repository

The sealing performance of buffer material in a high-level waste repository depends largely upon the hydraulic conductivity, the swelling pressure, and the dissolution of organic carbon in the buffer material. Temperature effects on these properties were evaluated. The hydraulic conductivity and the swelling pressure of compacted bentonite increase with increasing temperature, but the effect of temperature elevation is not large. The dissolution of organic carbon in bentonite also increases with increasing temperature, but the resultant aqueous concentrations of organic carbon in bentonite suspensions are less than those of deep groundwater in granite. Therefore, the organic carbon dissolved from the bentonite will not cause a significant increase in the organic carbon content of deep groundwater in the repository environment. Overall, temperature effects on the sealing performance of buffer material in a waste repository is not important, if the maximum temperature is maintained below 100°C.

Effect of Spent Fuel Burnup and Composition on Alteration of the U(Pu)O2 Matrix

ABSTRACTFor a potential performance assessment of direct disposal of spent fuel in a nuclear waste repository, the chemical reactions between the fuel and possible intruding water must be understood and the resulting radionuclide release must be quantified.Leaching experiments were performed with five spent fuel samples from French power reactors (four UO2 fuel samples with burnup ratings of 22, 37, 47 and 60 GWd·tHM−1 and a MOX fuel sample irradiated to 47 GWd·tHM−1) to determine the release kinetics of the matrix containing most (over 95%) of the radionuclides. The experiments were carried out with granitic groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25°C) in a hot cell.After 1000 or 2000 days of leaching, the Sr/U congruence ratios for all the UO2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO2 fuel matrix. The daily strontium release factor was approximately 1 à 10−7 d−1 for UO2 fuel, and five to six times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings.