Various ion exchange materials have been proposed for the removal of Cs from high level waste streams produced during the reprocessing of fuel rods. Cs can be released from loaded traditional exchange resins by elution and then the resin can be reused. However large quantities of secondary wastes are generated. Another class of “single use” exchangers is directly incorporated in the loaded state into a solid waste form (e.g. borosilicate glass logs). A third alternative is electroactive ion-exchange materials, where the uptake and elution of Cs are controlled by an applied potential. This approach has several advantages over traditional reusable ion-exchange resins including much reduced secondary waste, higher Cs selectivity, and higher durability.

XAS experiments were conducted at the Fe K-edge and Cs Lm-edge on a series of electrochemically produced nickel ferrocyanide films to determine the effects of deposition conditions and subsequent alkali exchange on structural and chemical aspects of the films. The deposition conditions include methods described in the literature and PNNL proprietary procedures. Although the performance and the durability of the films do vary with processing conditions, Fe K-edge EXAFS results indicate that all deposition conditions result in the. formation of the cubic phase. Initial results from Cs Lm-edge EXAFS analysis suggest that the Cs ion is present as a hydrated species.

The adsorption of soluble Sn(TV) species on goethite (FeOOH), a naturally occurring mineral, was investigated in 0.01 M NaCl solution at ambient temperature. The sequential spike method was used to obtain the distribution coefficient (Kd). Kd values of higher than 1×107 ml/g at neutral pH, and 3×105 ml/g at pH 11 were obtained. It is observed that Kd values decrease with increasing pH. In the desorption experiment, the sequential extraction method was applied. The extractants such as 0.1 M NaOH, Tamm's acid oxalate and Coffin's reagent were used. The result indicates that the adsorption consists of both reversible and irreversible reactions, and that the most of fractions in adsorption of Sn on goethite correspond to the irreversible adsorption or a stable fixation. Moreover, it is suggested that the reversible adsorption converts to the irreversible adsorption with time.

Most of the studies on sorption processes take into account only a ihcrmodynamic approach, and only lew studies have paid attention to ihc structural aspect (i.e. structure of ihc postulated surface complex). Experiments were conducted to study UO22+ sorption onto zirconium and thorium phosphate surfaces. Moreover, in order to identify the nature of the sorption sites and the structure of the surface complex, various spectroscopie techniques such as EXAFS, XPS and laser induced fluorescence have been used. Two different types of sorption sites on the thorium phosphate but only one on the zirconium phosphate have been detected.

Colloid facilitated transport is still an issue in radioactive waste management. Sophisticated phenomenological transport models are available, but progress is required to fully understand mechanisms and parameters. This study lead in this direction. Investigation of the marl groundwater colloids in steady-state conditions at Wellenberg, shows that their concentration is independent of the water flow rate. Their generation is caused by the re-suspension of the rock clay fraction only. The re-suspension process is presently being studied under flow transient conditions. Extension of our measurements to other safety relevant systems as well as a literature survey show that the colloid concentration under steady-state conditions is correlated to the concentration of alkali elements and earth alkali elements. The higher their respective concentration, the fewer colloids are occurring. Considerations on colloid contamination models are also included. The paper emphasises various colloid transport mechanisms including colloid generation from the irreversibly contaminated aquifer.

Last year we developed a two-dimensional deterministic heterogeneous matrix diffusion model, which is capable of utilising porosity information originating from real drill-core samples. The results of numerical infiltration experiment we had performed with the model displayed substantial spatial variations in the penetration depth. Because it is practically impossible to verify experimentally those two-dimensional penetration profiles we had computed, this time we decided to try modelling of measured leaching curves. Unfortunately we have not succeeded in acquiring such curves for the exact same samples, which we have used in numerical leaching experiments. Nevertheless it can be seen, that the shape of leaching curves computed with the heterogeneous model is clearly closer to the shape of measured curves, than the shape of curves provided by the standard model. These differences can be utilised as a basis for an approximate numerical method of assessing the geometric factor, which has traditionally been a purely empirical parameter.

The results of the new numerical experiments agree with our older results from last year: the heterogeneity of the rock matrix has highly significant impact on the diffusion. However, when interpreting the results, one must not neglect numerous limitations of the model, and hence, one should not attempt to overgeneralise the conclusions.

In a repository for spent nuclear fuel, gas generated by corrosion of the iron in the canister may form small bubbles that will escape and rise to the ground surface. Colloidal particles may attach to the surface of the bubbles and be carried by them. If the colloids are supplied by the montmorillonite clay of the buffer material surrounding the canister, the clay can be carried away. Nuclides sorbed in the clay can be carried with the bubbles. We have estimated the carrying capacity of the gas of the clay particles and the escape rate of nuclides carried by the gas bubbles. The latter is also compared to the escape rate by the conventional escape mechanisms from the near field. We have further estimated the detachment of the nuclides from the clay and their sorption onto the fracture surfaces of the rock as well as their uptake by diffusion into the rock matrix along the bubble transport paths.

The present paper is speculative and uses some hypothetical assumptions. Although the processes that are modelled are known to exist there is not enough known of several of them to quantify them accurately. The carrying capacity of the gas used in the calculations is an upper bound and probably very much exaggerated. Even so, the consequences are minor for the release of radionuclides.

A new method for the physical characterization of rock matrices for use in site investigations of nuclear waste repositories has been developed. The method can provide information needed in the assessment of the performance of the geosphere working as a natural barrier retarding the migration of radionuclides by diffusion into the rock matrix. Most conventional methods for the physical characterization of rocks give only bulk information. The combination of mercury po-rosimetry and computer tomography can give 3-D data on mineral-specific porosity distributions with additional pore size information. Additionally, limits for mineral-specific internal surface areas can be estimated, which is essential for the assessment of water-rock interaction and reactive interaction with radionuclides (sorption). Results of measurements on granitic rock (granodiorite) from the Baltic shield are discussed and integrated with results by complementary methods.

Static through-diffusion experiments were performed to study the diffusion of alkali- and alkaline earth-metals in fine-grained granite and medium-grained Äspö-diorite. Tritiated water was used as an inert reference tracer. Radionuclides of the alkali- and alkaline earth-metals (mono- and divalent elements which are not influenced by hydrolysis in the pH-range studied) were used as tracers, i.e. 22Na+, 45Ca2+ and Sr The effective diffusivity and the rock capacity factor were calculated by fitting the breakthrough curve to the one-dimensional solution of the diffusion equation. Sorption coefficients, Kd, that were derived from the rock capacity factor (diffusion experiments) were compared with Kd determined in batch experiments using crushed material of different size fractions.

The results show that the tracers were retarded in the same order as was expected from the measured batch Kd. Furthermore, the largest size fraction was the most representative when comparing batch Kd with Kd evaluated from the diffusion experiments. The observed effective diffusivities tended to decrease with increasing cell lengths, indicating that the “transport” porosity decreases with increasing sample lengths used in the diffusion experiments.

The interactions between sulfate reducing anaerobic bacteria and plutonium, with or without bentonite present, were investigated using distribution coefficients {Kd (ml/g)} as an index of the radionuclide behaviour. Plutonium Kds for living bacteria varied within a large range, from 1,804 to 112,952, depending on the pH, while the Kds ranged from 1,180 to 5,931 for dead bacteria. In general, living bacteria had higher plutonium Kds than dead bacteria. Furthermore, the higher Kd values of 39,677 to 106,915 for living bacteria were obtained for a pH range between 6.83 and 8.25, while no visible pH effect was observed for dead bacteria. These Kd values were obtained using tracers for both 236Pu and 239Pu, which can check the experimental procedures and mass balance.

Another comparison was conducted for plutonium Kd values of mixtures of living bacteria with bentonite and sterilized bacteria with bentonite. The range of Kd values for the non-sterilized bacteria with bentonite were 1,194 to 83,648 while Kd values for the sterilized bacteria with bentonite were from 624 to 17,236. Again, the Kd values for the living bacteria with bentonite were higher than those of sterilized bacteria with bentonite. In other words, the presence of living anaerobic bacteria with bentonite increased, by roughly 50 times, the Kd values of 239Pu when compared to the mixture of dead bacteria with bentonite. The plutonium Kd values for bentonite alone, both non-sterilized and sterilized, were within a constant range of around 10,000 even though some of the data are not yet available.

The bentonite used for this experiment was a product of Japan and the sulfate reducing anaerobic bacteria was previously used for the treatment of a pulp and paper wastewater. The results indicate that the effects of anaerobic bacteria within the engineered barrier system (in this case bentonite) will play a significant role in the behaviour of plutonium in geologic repositories.

The geologic media near Yucca mountain site consist of fractured welded tuffs along with less fractured unwelded tuff. Numerical simulation of flow and transport in such media poses a number of challenging problems, due mainly to the heterogeneities and disorder in the media. In addition, because of different dominant transport mechanisms in different regions of the media, investigations need to be carried out at different time-scales. “Time-marching” will pose a considerable problem in analyzing such multi-scale transient problems. We develop a field-scale network model of fractures and study transport of radionuclides through geologic media as a function of disorder and correlated fracture-permeabilities.

For performance assessments of geological disposal of high-level radioactive waste, activation energies for the diffusion of strontium ions and the basal spacings of compacted sodium montmorillonite in the water-saturated state were determined.

Basal spacings determined by XRD indicated changes in the interlamellar space from a three-water layer hydrate state to a two-water layer hydrate state as the dry density of the montmorillonite increased from 1.0 to 1.8 Mg m-3. Activation energies from 17.3 to 30.8 kJ mol-1 for the apparent diffusion coefficients of strontium ions were obtained. The lower activation energies than for diffusion of strontium ions in free water were determined for montmorillonite specimens of lower dry density (1.2 Mg m-3 and below), while the higher activation energies were at higher dry densities (1.4 Mg m-3 and above).

These findings cannot be explained by changes in only the geometric parameters, which the pore water diffusion model is based upon. Possible explanations for the dry density dependence of the activation energy are the changes of the temperature dependence of the distribution coefficients and/or of the diffusion process with increasing dry density

Chlorine-36, including the natural cosmogenic component and the component produced during atmospheric nuclear testing in the 1950's and 1960's (bomb pulse), is being used as an isotopie tracer for groundwater infiltration studies at Yucca Mountain, a potential nuclear waste repository. Rock samples have been collected systematically in the Exploratory Studies Facility (ESF), and samples were also collected from fractures, faults, and breccia zones. Isotopie ratios indicative of bomb-pulse components in the water (36Cl/Cl values > 1250 × 10-15), signifying less than 40-yr travel times from the surface, have been detected at a few locations within the Topopah Spring Tuff, the candidate host rock for the repository. The specific features associated with the high 36Cl/Cl values are predominantly cooling joints and syngenetic breccias, but most of the sites are in the general vicinity of faults. The non-bomb pulse samples have 36Cl/Cl values interpreted to indicate groundwater travel times of at least a few thousand to possibly several hundred thousand years. Preliminary numerical solute-travel experiments using the FEHM (Finite Element Heat and Mass transfer) code demonstrate consistency between these interpreted ages and the observed 36Cl/Cl values but do not validate the interpretations.

In the vicinity of a high-level waste repository, corrosion of carbon steel overpacks will create a reducing environment. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as technetium.

The apparent diffusion coefficients of technetium were measured in compacted bentonites (Kunigel VI® and Kunipia F®, JAPAN) in contact with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients measured were 10-12 to 10-13 m2/s under oxidizing conditions and 10-12 to 10-13 m2/s under reducing conditions. There were significant effects of redox condition, dry density (0.2 to 2.3 g / cm3) and montmorillonite content (50% for Kunigel VI or 100% for Kunipia F) on the apparent diffusion coefficients. Montmorillonite density could be a good index to explain density dependence of the diffusion coefficients under both reducing and oxidizing conditions.

A new method for estimating the mass transport by using the stochastic values (the arithmetic mean, the standard deviation and the skewness) of permeability is presented. Generally, detail of permeability distribution cannot be obtained except for moments of the distribution. Also, measurement results of permeability for the rock matrix including cracks or fast flowpaths do not always follow the log-normal distribution frequently applied. In such a situation, we must evaluate the characteristic permeabilities for the whole or some regions of the disposal site including the accessible environment.

The authors have investigated the characteristic permeability on the basis of some probability density functions of permeability, applying the Monte Carlo method and FEM. It was found that its value does not depend on type of probability density function of permeability, but on the arithmetic mean, the standard deviation and the skewness of permeability [1].

This paper describes the use of the stochastic values of permeability for estimating the rate of radioactivity release to the accessible environment, applying the advection-dispersion model to two-dimensional, heterogeneous media. When a discrete probability density function (referred to as ‘the Bernoulli trials’) and the lognormal distribution have common values for the arithmetic mean, the standard deviation and the skewness of permeability, the calculated transport rates (described as the pseudo impulse responses) show good agreements for Peclet number around 10 and the dimensionless standard deviation around 1. Further, it is found that the transport rates apparently depends not only on the arithmetic mean and the standard deviation, but also on the skewness of permeability. When the value of skewness dose not follow the lognormal distribution which has only two independent parameters (the mean and the standard deviation), we can replicate the three moments estimated from an observed distribution of permeability, by using the Bernoulli trials having three independent parameters.

A long-term waste degradation experiment has been performed with actual low-level radioactive wastes (LLRW) at the Chalk River Laboratories (CRL), to support the licensing and modelling efforts for near-surface disposal. The wastes consist of paper, mop heads, paper towels, used clothing, etc. The wastes were compacted into bales and sealed into separate steel containers, which were connected to leachate collection systems for sampling. The leachates collected had a composition typical of landfill leachates. The major inorganic ions were Na, Ca, Cl, and Fe, and the ionic strength was ∼0.05 M. The relative distribution of inorganic ions in the leachates was remarkably similar between bales. Volatile fatty acids (VFA) were the major species of dissolved organic carbon (DOC, total DOC up to 7000 mg/L). A typical composition of leachates is proposed, which can be used in geochemical and source term modelling.

This work gives a detailed description of the important aspects of a long-term Low-Level Radioactive Waste (LLRW) degradation experiment, performed at Chalk River Laboratories (CRL). This experiment utilized actual LLRW. The wastes consist of unconditioned compacted refuse (paper, mop heads, paper towels, used clothing, etc), which represents the bulk of the waste volume intended for near-surface disposal at CRL. Waste material was collected and compacted to make a total of 11 bales for this experiment. Each bale was then placed and sealed in separate steel containers which were connected to sampling lines. After a dry monitoring period, water was added to promote leaching and decomposition of the wastes. The leachate sampled had a composition similar to landfill leachates. Some applications of this experiment, used to support the safety case of near-surface disposal, are briefly discussed in this paper, e.g., the production of colloidal material, the nature and role of dissolved organics of microbial origin, etc.

Epifluorescence microscopy was utilized to enumerate halophilic bacterial populations in two studies involving inoculated, actual radioactive waste/brine mixtures and pure brine solutions. The studies include an initial set of experiments designed to elucidate potential transformations of actinide-containing wastes under salt-repository conditions, including microbially mediated changes.

The first study included periodic enumeration of bacterial populations of a mixed inoculum initially added to a collection of test containers. The contents of the test containers are the different types of actual radioactive waste that could potentially be stored in nuclear waste repositories in a salt environment. The transuranic waste was generated from materials used in actinide laboratory research. The results show that cell numbers decreased with time. Sorption of the bacteria to solid surfaces in the test system is discussed as a possible mechanism for the decrease in cell numbers.

The second study was designed to determine radiological and/or chemical effects of 239Pu, 243Am, 237Np, 232Th and 238U on the growth of pure and mixed anaerobic, denitrifying bacterial cultures in brine media. Pu, Am, and Np isotopes at concentrations of ≤1×10–5M, ≤5×10–6M and ≤5×10-4 M respectively, and Th and U isotopes at concentrations of ≤4×10-3 M were tested in these media. The results indicate that high actinide concentrations affected both the bacterial growth rate and morphology. However, relatively minor effects from Am were observed at all tested concentrations with the pure culture.

Performance assessment of a potential repository at Yucca Mountain includes flow and transport modeling of the unsaturated zone as the critical predictive component, and involves a series of model calculations that provide predictions of the migration of important radionuclides in the inventory to the water table. The modeling requires the relevant properties of both the natural environment and the engineered systems. The Unsaturated Flow Apparatus, UFA, was used to directly measure the unsaturated and saturated transport properties of whole rock tuff cores and candidate barrier materials to provide real input parameters to the models. These properties included hydraulic conductivity, matric potential, air permeability, and diffusion coefficient, all of which are strong functions of the volumetric water content. Results show that for all recharges above 0.1 mm/yr, fractures will be partially saturated and conducting at that recharge rate. Whenever the thermal conditions relax enough to allow rewetting of the host rock, there will be dripping from the drift ceiling. Unsaturated transport of colloids through fractured cores of Topopah Spring and Prow Pass tuffs was also investigated and found to depend primarily upon colloid charge, and not size, for the rock cores investigated.

In this study, the transport behavior of colloids through compacted bentonite and sand-bentonite mixtures was investigated. Colloidal gold was used to simulate mobile colloids because it was well characterized and its dispersivity was controlled. The bentonite used was a sodium bentonite. The sand-bentonite mixtures were prepared by mixing up to 50wt.% silica sand with the bentonite. The bentonite and the sand-bentonite mixtures were compacted to dry densities of 1000 and 1800 kg/m3 and then saturated with distilled water. The sand-bentonite mixture was also saturated with synthetic sea-water. Column experiments were performed to investigate colloidal transport. Further, colloidal particles stabilities in high ionic strength water such as bentonite porewater or saline groundwater were interpreted based on the repulsion potential from the double layer force and the attraction potential from the van der Waals force.

The results indicated that the colloidal particles were effectively filtered by both the compacted bentonite and the sand-bentonite mixtures. This study indicated that the effect of colloids on radionuclide transport in compacted bentonite is negligible for the safety assessment of high level radioactive waste (HLW) disposal.

The effects of gas permeation on the hydraulic permeability of bentonite/sand mixtures were studied experimentally by conducting permeability tests with various fluids and by microscopic observations. In these tests, hydraulic permeabilities were measured before and after helium gas was applied to permeate the mixtures.

Although gas formed preferential migration paths through the mixtures, the imperviousness of the bentonite/sand mixtures to the tested fluids never deteriorated because the paths became filled by swelling bentonite on re-saturation.