For safety assessment of the geological disposal of the high level radioactive waste, it is necessary to study corrosion of the overpack materials and migration of the corrosion products in the compacted bentonite. In the present study, average corrosion rates of iron foil and apparent diffusion coefficients of the corrosion products were determined using a neutron- activated iron foil. The average corrosion rates were on the order of 10-6m/y, while apparent diffusion coefficients were in the range from 10-12 to 10-14m2/s. No tendency to decrease in the corrosion rates with increasing corrosion time was observed. This suggests that the iron foil corrodes under reducing condition. Asymmetric concentration profiles of iron corrosion products were obtained in some experiments. It can be considered that either cathodic or anodic reaction would dominantly occur on one side of the iron foil surfaces and that each reaction would change the pH in pore water of bentonite specimens adjacent to the iron foil surfaces.

Interactions of sodium bentonite with distilled water and two types of synthetic groundwater were studied by batch experiments. In the experiments, clay and pure minerals were reacted at room temperature under aerobic and anaerobic condition as a function of time and liquid/solid ratio. The clay and pure minerals used in the experiments were Kunigel-VI (crude Na-bentonite), Kunipia F(purified Na-bentonite), purified Na-smectite(purified from Kunipia F), calcite and pyrite as accessory minerals. The chemical composition in the liquid phase was analyzed through centrifugation and ultra filtration. Alteration of the distribution of exchangeable cation in the bentonite was analyzed by NH4Ac and XRD. The results indicated that the interaction between bentonite(Kunigel-Vl) and groundwater under aerobic condition was described by ion exchange reaction of smectite, dissolution of calcite and oxidation of pyrite.

From these experimental studies, the model of the interaction of groundwater with bentonite proposed by Wanner was modified. The comparison between calculation and experimental results showed good agreement and indicated that this model could be adopted to predict porewater chemistry of bentonite for performance assessment of geological isolation system of high level waste.

The radiation sensitivity of naturally occurring microorganisms in buffer materials vas investigated as veil as the sensitivity of Bacillus subtilis spores and Acinetobacter radioresistens in a buffer matrix. The D10 values obtained in our radiation experiments varied from 0.34 to 1.68 kGy and it vas calculated that the surface of a nuclear fuel vaste container vould be sterilized in 9 to 33 d after emplacement, depending on the type of container, and the initial bioburden. This suggests that formation of biofilms and microbially influenced corrosion vould not be of concern for some time after emplacement. The results also indicated that sterilization throughout a 25 cm thick buffer layer is unlikely and that repopulation of the container surface after some time is a possibility, depending on the mobility of microbes in compacted buffer material.

Transport and retardation of slightly sorbing sodium was studied in Syyry area SY-KR7 mica gneiss and in altered porous tonalite. Experiments were performed using dynamic fracture and crushed rock column methods and the static batch method. Flow conditions in the columns were determined using tritiated water and chloride as non-sorbing tracers. 14C-PMMA method was used to study the pore structure of matrices and the surface areas were determined by B.E.T. method. Sodium was retarded strongly in altered tonalite owing to homogeneously porous matrix structure and the composition of alteration minerals. An agreement between retardation values in batch and crushed rock column experiments as well as in fracture column experiments was good.

Tracer transport experiments involving colloids that showed little tendency to attach to rock surfaces and a nonsorbing solute (iodide) were conducted in three different well-characterized natural fractures in tuff. The objective was to investigate the potential for using nonattaching colloids in tracer experiments to (1) obtain a measure of the dispersion in a fractured system in the absence of diffusion into the matrix or into stagnant zones along the fracture walls, and (2) provide insights into predicting colloid transport in fractures under conditions of minimal deposition.

In the experiments, the colloids always arrived earlier in the effluent than the iodide, which we believe is evidence of (1) hydrodynamic chromatography and/or (2) the fact that the colloids experience a smaller effective volume in the fracture because they diffuse too slowly to enter low-velocity regions (dead zones) along the rough fracture walls. The iodide also approached the inlet concentration in the effluent more slowly than the colloids, with the concentration at a given elution volume being greater at higher flow rates. By contrast, the rate of approach of the colloid concentration to the inlet concentration did not vary with flow rate. We attribute this behavior to matrix diffusion of the iodide, with the colloids being too large/nondiffusive to experience this phenomenon. Dispersion of all tracers was greatest in the fracture of widest average aperture and least in the fracture of narrowest aperture, which is consistent with Taylor dispersion theory.

The tracer experiments were modeled/interpreted using a three-step approach that involved (1) estimating the aperture distribution in each fracture using surface profiling techniques, (2) predicting the flow field in the fractures using a localized parallel-plate approximation, and (3) predicting tracer transport in the fractures using particle-tracking techniques. Although considered preliminary at this time, the model results were in qualitative agreement with the experiments.

The effect of nonlinear Freundlich sorption isotherm on the transport of sorptive solute in a system of fracture and porous matrix was investigated through numerical simulation. To solve a set of partial differential equations of solute transport with nonlinear term, use of the Laplace transform Galerkin (LTG) technique was investigated. It was shown that the LTG method could be applied successfully to solve quasi-linearized transport equation. Sensitivity analysis showed that nonlinear sorption in porous matrix with order less than 1 increased the skewness of the breakthrough curve. The fitting of experimental effluent data using a transport model with nonlinear isotherms was also conducted.

Various geophysical and hydrologic tests were conducted in a cluster of nine wells to image the hydrologic connections of a fractured rock mass. Results of intra-borehole flow surveys and cross-hole radar and seismic tomography surveys correlated very well, and indicated that there is a major feature at a depth of 30m. Systematic injection tests were conducted in all nine wells. Three to four intervals in each well were isolated using pneumatic packers. Each interval was equipped with a high resolution pressure transducer. Some 130 injection tests were conducted, and more than 4,100 cross-hole transient pressure measurements were obtained. A computer algorithm was developed to analyze such massive interference data systematically. As a result of the analysis, an image of the fracture connections emerged which is consistent with the geophysical data.

This paper presents the results of a discrete fracture network modelling for the non-sorbing tracer migration experiments, which were conducted in the fractured rock at the Kamaishi mine. A newly developed in-plane heterogeneity model, which can address the channelling effects within a fracture, and the matrix diffusion model, which can simulate the retardation time due to the matrix diffusion adjacent to fracture were applied in this study. As a result, the matrix diffusion model better reproduces the general shape of breakthrough curves. The transport aperture At was approximately one order of magnitude larger than the hydraulic aperture Ac derived from the cubic law.

A new model concept for describing flow and transport in fractured rock has been tested on some field tracer experiments. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. For times of interest for deep geological repositories for nuclear waste it has been found that the volume of the channels, i.e. the flow porosity of the rock, has no practical influence on the Residence Time Distribution of the nuclides. It is totally dominated by the matrix diffusion effects. The specific Flow Wetted Surface, FWS, of the channels therefore needs to be known. A method to evaluate the FWS from hydraulic measurements in boreholes is presented.

Tracer tests at the Swedish ÄSPÖ rock laboratory have been predicted using this model and data obtained in the way outlined in the paper. The main tracer test was predicted reasonably well without using any adjustable parameters. Some other tracers also used were not detected in the tests as they were expected to be. We discuss some of the possible reasons for this in relation to geological observations made in the expected pathways for these tracers. The sensitivity of results to uncertainties in parameter values is also discussed.

Temperature and relative humidity are primary environmental factors affecting waste package corrosion rates for the potential repository in the unsaturated zone at Yucca Mountain, Nevada. Under ambient conditions, the repository environment is quite humid. If relative humidity is low enough (<70%), corrosion will be minimal. Under humid conditions, corrosion is reduced if the temperature is low (<60°C). Using the V-TOUGH code, we model thermo-hydrological flow to investigate the effect of repository heat on temperature and relative humidity in the repository for a wide range of thermal loads. These calculations indicate that repository heat may substantially reduce relative humidity on the waste package, over hundreds of years for low thermal loads and over tens of thousands of years for high thermal loads. Temperatures associated with a given relative humidity decrease with increasing thermal load. Thermal load distributions can be optimized to yield a more uniform reduction in relative humidity during the boiling period.

The radioactive decay heat from nuclear waste packages may, depending on the thermal load, create coupled thermal-mechanical-hydrological-chemical (TMHC) processes in the near-field environment of a repository. A group of tests on a large block (LBT) are planned to provide a timely opportunity to test and calibrate some of the TMHC model concepts. The LBT is advantageous for testing and verifying model concepts because the boundary conditions are controlled, and the block can be characterized before and after the experiment. A block of Topopah Spring tuff of about 3 × 3 × 4.5 m was sawed and isolated at Fran Ridge, Nevada Test Site. Small blocks of the rock adjacent to the large block were collected for laboratory testing of some individual thermal-mechanical, hydrological, and chemical processes. A constant load of about 4 MPa will be applied to the top and sides of the large block. The sides will be sealed with moisture and thermal barriers. The large block will be heated by heaters within and guard heaters on the sides so that a dry-out zone and a condensate zone will exist simultaneously. Temperature, moisture content, pore pressure, chemical composition, stress, and displacement will be measured throughout the block during the heating and cool-down phases. The results from the experiments on small blocks and the tests on the large block will provide a better understanding of some concepts of the coupled TMHC processes. The progress of the project is presented in this paper.

Constraints on groundwater flow and groundwater travel time (GWTT) at the proposed HLW repository site at Yucca Mountain, Nevada, U.S.A., can be provided through thermodynamic modeling of relations between gas, liquid, and solid phases, and mass transport modeling. In the unsaturated zone at Yucca Mountain the observed distribution of 14C can be reasonably represented by a mass transport and mass transfer model representing diffusion of 14C02 in the gas phase from the ground surface and equilibrium between aqueous and gas phases, in the absence of net advective flow. This mechanism could also account for aqueous phase 14C activities in the unsaturated zone. Thermodynamic interpretations indicate that groundwaters extracted from boreholes in the water saturated tuffaceous aquifer at Yucca Mountain are undersaturated with respect to calcite, providing evidence that groundwater flow bypasses zones of rock containing calcite and therefore must be channelized. This conclusion is supported by published isotopic data for calcites and groundwaters.

The long-term programme of in-situ radionuclide migration experiments in the underground test site at Grimsel (GTS) involves the development and testing of radionuclide transport models with their associated databases. The field experiments are carried out in a water-bearing shear zone in crystalline rock utilising a suite of tracers of differing geochemical behaviour. A rigorous model testing procedure has been developed for the GTS radionuclide migration experiment. This paper describes application of this testing procedure to a solute transport code developed by PNC.

A new gas flow technique is introduced such that experiments on very long samples are possible. This new technique together with increased accuracy of the measurements, allows the observation of power law tails in the break-through curves. Dispersion in these experiments can be controlled in great detail, and therefore the power law tails can be used to determine very accurately the parameters relevant in matrix diffusion. Results for rock and metal samples are shown, and they are fitted with model calculations which include both dispersion and matrix diffusion. The introduced technique, which is designed for ordinary drill cores, is suitable for scanning a large number of samples in a very short time.

Concentrations of U series nuclides (238U, 234U, 230Th, 226Ra and 231Pa) across secondary U accumulations in rock matrix around water-carrying fractures were examined to find records of radionuclide movement. The accumulations, loaded in rock pores, were separated without dissolving the host rock. The concentration profiles observed, form patterns which point to both “old” and “young” U accumulations and, moreover, post accumulation movement of certain radionuclides. In the case of the young U accumulation, shown to have occurred between 10,000 and 33,000 years ago, the apparent diffusivity of Da=6.10-15 m2/s for U could be calculated.

A semi-analytical model for percolation problems, called the Dynamical Network Model, is developed. Because of its semi-analytic nature, the model is more efficient than the existing numerical techniques especially for three dimensional problems and, thus, can be used to a wide range of percolation-like problems in performance assessment of radioactive waste disposal.

We present a new method for conditioning realizations of the logarithm Y of transmissivity on head data. Each realization is conditioned by subtracting a linear combination of certain basis functions, each corresponding to a measurement, obtained by a process based on co-kriging. The required cross covariance between head and Y can be numerically computed very efficiently using the adjoint method. The coefficients in the linear combination are chosen to try to rninimise the differences between computed and measured heads. The most efficient methods for such non-linear optimisation problems are those that exploit information about the second derivatives of the function being minimised. We show how these second derivatives can be computed very efficiendy using the adjoint method for certain classes of finite-element schemes. We illustrate our method by application to the WIPP-2 test case of the international INTRAVAL project. We show that the match between computed and measured heads is greatly improved. We compare our method with approaches that use pilot points. Our method is exact if the variability of the Y field and the deviations of the measured heads from the values computed for the mean of the Y field are both small, whereas pilot-point methods are only approximate. However, our method may be more expensive computationally. Finally, we discuss the extension of our method to conditioning on transient head data and to dealing with boundary conditions.

Inflow measurements at Stripa and in other underground openings in Sweden, as well as observations elsewhere in mines and tunnels, reveal that there is generally an extremely broad distribution of groundwater flux in fractured rock. Non-sorbing and sorbing tracer tests typically show similar variability in groundwater travel time (GWTT) and tracer transport.

In the U.S. Nuclear Waste Program, Nuclear Regulatory Commission regulations require the GWTT from the disturbed zone to the accessible environment to exceed 1000 years. The regulations seem to envision a rather uniform and narrow distribution of travel time, with perhaps a few identifiable “fast pathways” contained within the rock mass surrounding a potential repository. The premise is that most of these features could be mapped during site characterization, and that regions of the potential repository host rock containing such features could be avoided during waste emplacement.

However, both field experience and theoretical studies in recent years provide strong evidence that groundwater flux, GWTT, and aqueous transport of dissolved substances exhibit extremely heterogeneous behavior, even in intact porous media and in fractured rock regions between major features. These phenomena are all dominated by the spatial distribution of permeability within the rock mass of interest. The permeability distribution is often approximately log-normal, with a natural log standard deviation, σ. For unfractured porous rock, σ typically ranges from about 0.6 to about 1.2 for field-scale investigations, and for fractured permeable media, it frequently exceeds 2. Values of σ smaller than 0.6 may be observed in small field-scale projects when the macroscopic flow regime is essentially linear within very uniform sediments and in laboratory displacement experitments.

With some additional assumptions, a log-normal permeability distribution implies that groundwater flux, GWTT, and the transport of radionuclides from a potential repository are also log-normal. To first order, the appropriate value of σ describing these distributions is the same as the value for the permeability distribution. This allows σ to be estimated from a large number of hydraulic or pneumatic packer tests within the fractured rock mass of interest.

We define a groundwater transport function (GWTF) for the rate of radioactivity release to the accessible environment (AE) at time t resulting from the release of a pulse of unit activity at time 0. The GWTF depends on the mean groundwater travel time, tw, and σ, as well as the retardation factor and decay constant. As σ increases from 0 (a hypothetical completely homogeneous system), the radioactivity breakthrough at early time increases from 0 to 100%. This behavior is consistent with our intuitive notions of “fast transport pathways” in heterogeneous systems, and σ is thus seen to be a parameter for quantifying the effects of heterogeneity.

Convolution of the GWTF with a time-dependent release function for the Engineered Barrier System (EBS) is easily performed numerically, resulting in the rate of release to the AE as a function of time, which can then be integrated numerically to calculate the cumulative release as a function of time. The convolution approach clearly separates the effects of uncertainty and heterogeneity on repository performance and is extremely useful for sensitivity analyses. An example calculation shows the combinations of σ and tw required for compliance with total system release standards.

Since the effect of heterogeneity is captured by a single parameter in a deterministic calculation, uncertainty can be investigated separately by Monte Carlo sampling from distributions of such parameters as σ, tw and source term strength, allowing (in the future) specific and scientifically meaningful goals to be defined for both site characterization and design.

Finally, we emphasize that this approach, in its present form, does not include thermal effects. These effects may dominate both the EBS failure rate and hydrogeochemical behavior, including radionuclide transport, for most of the compliance period and beyond. It cannot be used directly to support any particular thermal loading strategy.

An innovative EBS design of sand-gravel layers for waste disposal under partially saturated conditions is described. Advantages of this EBS are 1) as a hydraulic barrier to advective inflow from the surrounding rock, and 2) as a diffusion barrier to the transport of dissolved or colloidal radionuclides. This sand-gravel barrier design for unsaturated sites is analogous to EBS designs using compacted bentonite backfills/buffers in saturated repository sites. Percolation box experiments, field tests, and natural analogues supporting the advantageous performance of this flow-diversion design are described and estimates for source-term release are calculated.

In assessments of the safety of final repositories for nuclear waste situated in crystalline basement rock it is usually postulated that the transfer of radionuclides to the biosphere can take place only through transport by water. However, in order for such an assumption to be valid, it must be verified that any geogas that is present will not affect the transport - at least not to any significant degree. (The word geogas refers to the occurrence in the crystalline basement rock of substances which become gaseous at normal pressures and temperatures.)

Geogas in crystalline rock consists of species such as nitrogen, argon, helium, hydrogen, methane, carbon monoxide, carbon dioxide, hydrogen sulfide, and oxygen. The gas originates from the atmosphere, chemical reactions in the rock, the decay of radioactive elements in the rock, and degassing from the mantle of the earth. In most observed cases, geogas is dissolved in the groundwater. However, at elevated pressures and at low temperatures, methane may combine with water to form a solid phase commonly called methane-ice.

The transfer of geogas through the rock and to the surface takes place through flow in fractures. Firstly, dissolved geogas migrates due to the flow of the groundwater, and secondly, pockets of gas may form and eventually be released in the form of bursts. In the second case, the gas might act as a carrier for heavy elements through four different mechanisms: 1) formation of volatile compounds, 2) formation of surface active complexes, 3) flotation, and 4) formation of aerosols.

When a potential site for waste disposal is being evaluated, studies of geogas should form part of such a characterization programme. Favourable conditions for the formation of free gas may develop as a result of the heating of the rock by radioactive decay in the waste. It is also conceivable that methane-ice might form in the backfill of a repository in connection with a glaciation. The decomposition behaviour of such methane-ice appears to be largely unknown. Positive aspects may include the possibility of utilizing geogas flow for the non-destructive monitoring of a site after closure of the repository.