UK Nirex Ltd is seeking to develop a deep underground repository for the disposal of solid low- and intermediate-level radioactive wastes in the UK. The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the post-closure performance assessment of the repository. One of the investigations carried out by AEA Technology under the NSARP is the investigation of the formation of water-soluble complexants from the degradation of the solid organic polymers and cellulosic materials present in such wastes. The influence of these complexants on the solubility and sorption of a number of radionuclides is also being investigated by a combination of experimental measurements and thermodynamic modelling. Under the alkaline, anaerobic conditions representative of the near field of the repository, cellulose has previously been shown to degrade to yield soluble products which enhance the solubilities and reduce the sorption of several radionuclides. As part of this investigation, cellulose was chemically degraded under conditions similar to those expected in the near field and the leachate analysed. Separation of the products by HPLC gave direct identification of 2-C-(hydroxymethyl)-3-deoxy-D-erythro-pentonic (isosaccharinic) acid as one of the degradation products. More tentative assignments were made for glycolic, formic, lactic and acetic acids. A comparison was made with the degradation products suggested in the literature. The possible mechanisms and extent of cellulose degradation are discussed, as well as the factors affecting its degradation in the near field. The sorption of plutonium on to cement from a leachate produced under alkaline, anaerobic conditions is reported.

Corrosion products resulting from the oxidation of steel containers and iron-containing waste can alter the local geochemical environment in radioactive waste disposal vaults and trenches. In addition, corrosion products can enhance the retention of radionuclides by providing a highly sorptive media. Initial hydrous corrosion products are poorly crystalline with large surface area, high sorption capacity, and large Kds. Over time, corrosion products recrystallize by step-wise dehydration to phases with significantly smaller surface areas. The decreased surface area results in decreased sorption and the potential for remobilization of previously sorbed radionuclides. The impact of time-dependent variation in sorption on radionuclide transport is a complex function of the rate of recrystallization and the radionuclide half-life.

A mathematical model has been developed, which coupling mass transport with corrosion product recrystallization and equilibrium sorption of radionuclides. The model is applied release of radionuclides with a range of half-lives (year to 100,000s of years) from radioactive waste disposal facilities to illustrate the significance of corrosion product recrystallization on release.

UK Nirex Ltd is proposing to develop a deep underground repository for the disposal of solid low- and intermediate-level radioactive wastes in the UK, and is currently investigating Sellafield in Cumbria as a possible site. The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the post-closure performance assessment of the repository. One of the investigations carried out by AEA Technology under the NSARP is the study of the sorption of radionuclides onto the backfill. Sorption is one of the key parameters determining the rate of release of radionuclides from the repository. The radionuclides present in the waste include 94Nb, 126Sn, 129I and 36Cl. This paper reports the measurement of the sorption of niobium, tin, iodine and chlorine onto the Nirex reference vault backfill at 20±5°C by the batch sorption method. The temperature dependence of tin sorption was investigated by carrying out additional experiments at 77±5°C. Experiments on niobium solubility and sorption were carried out under saline conditions to reflect the nature of the groundwater at Sellafield. Sorption coefficients (Rd) for niobium are within the range measured in other work, whereas for tin the results are a factor of ten lower than literature values. For iodine and chlorine, the results are consistent with the literature; they also show an increase in Rd with decreasing initial inventory. The Rd values for niobium and tin are significantly higher than those for chlorine and iodine.

The distribution coefficients (Kd) were measured for Pu, Am and Se on sodium-bentonite in bentonite-equilibrated distilled water in a pH range 4~ 10. Sodium-bentonite includes sodium-montmorillonite and ferrous minerals such as pyrite. The experiments were carried out by using the batch method under aerobic conditions for Pu, Am and under anaerobic conditions for Se. The distribution coefficients for Pu and Am were in the range 100 ~ 102m3/kg on sodium-bentonite in the above pH range. The sorption of Pu and Am was modeled by ion exchange of these cationic aqueous species with interlayer cations of sodium-bentonite. The distribution coefficients of Se on sodium-bentonite were interpreted by using a surface complexation model. The model was based on the assumption that the sorption behavior was dominated by ferric-oxyhydroxide, goethite, coating on the surface of pyrite. Sorption behavior of Am in compacted bentonite were also discussed by using cation exchange model.

Ion-exchange experiments were conducted at 25 °C between the zeolite mineral clinoptilolite and aqueous solutions of varying equivalent ratios of Na+ and Sr2+ and total concentrations of 0.005, 0.05, and 0.5 N. The experiments were designed to investigate the effects of changes in total solution concentration and in the relative concentrations of exchangeable cations on the following ion-exchange equilibrium:

Sr2+ + 2NaZ ⇄ SrZ2 + 2Na+

Using the isotherm data at 0.05 N solution concentration, a thermodynamic model for the ion-exchange reaction was derived using a Margules formulation for the activity coefficients of zeolite components and the Pitzer ion-interaction approach for activity coefficients of aqueous ions. The results of the forward experiments showed that the ion-exchange isotherm strongly depends on the total solution concentration. Additional experiments demonstrated that the above ion-exchange reaction is reversible. The derived equilibrium constant, K, and Gibbs energy of ion-exchange, ΔG°, are equal to 0.321±0.021 and 2,820±170 J/mol, respectively.

Using thermodynamic parameters derived from the 0.05 N isotherm experiment, the model was used to predict isotherm values at 0.005 and 0.5 N, which showed excellent agreement with measured data. Because the thermodynamic model used in this study can be easily extended to ternary and more complicated mixtures, it may be useful for modeling ion-exchange equilibria in multicomponent geochemical systems.

An analytical solution covering the entire range of adsorption properties of rock has been derived for the migration of radiionuclide in a porous rock matrix. The analysis takes into account the advective transport, hydrodynamic dispersion, adsorption between solid phase and liquid phase, and the radioactive decay. For adsorption of nuclide within the rock, the effects of no adsorption, linear nonequilibrium adsorption, and linear equilibrium adsorption are integrated into a generic transient analytical solution. The results indicate that the assumption of equilibrium adsorption can result in underestimation of the concentration profile in the early stages of migration. However, both the equilibrium and nonequilibrium profiles eventually approach the same value. It is also noted that for the case of nonequilibrium adsorption, plateaus appear in the concentration profile of the breakthrough curves. The effects of different adsorption rates are also analyzed.

The influence of microbial activity on the transport and migration behaviour of redox-sensitive radionuclides (e.g. Technetium (Tc) and Selenium (Se)) in groundwater environment were investigated within an interdisciplinarary research program. Soil material and sediments were investigated with batch- and recircu-lation column experiments. The number of microorganisms in the water was determined by counting the colony forming units (CFU) on R2A agar at regular periods.

The results of sterile and non sterile recirculation column experiments showed within 80 days a high fixation of the radionuclides in the unsterile samples (80% of Tc, 50% of Se compared with the initial concentration Ao), and no fixation in the sterile samples. There was no difference in the redox values.

It could be shown that the immobilisation of the redox-sensitive elements Tc and Se are strongly determined by microbial activity, depending on the different sediments and their microbial live stock. There are indications that the microorganisms found in the non sterile columns are allochthonous bacteria which were dragged into the experimental systems and samples during laboratory work and sampling. The composition of the microbial flora was shown to be very heterogenous and different in each individual column and within the same column at different times. Thus these columns might have been contaminated with microorganisms during sampling and handling of the experimental systems under non sterile conditions, as done generally in most laboratories.

Prediction of radionuclide release rates for high-level waste requires estimates of the rates of waste form alteration and formation of secondary minerals inside the failed canister. Unsaturated repository sites may promote development of a variety of chemical environments related to two phase (liquid/ vapor) transport and temperature gradients caused by radiogenic decay. A mass balance (shell balance) approach is used to estimate the effects of dripping water, evaporation, and condensation on the waste canister and the presence of saline water inside the failed waste canister. The simplified calculations predict large variability of water chemistry over spatial scales of a few centimeters. The effects of the predicted aqueous chemistry on waste form alteration, secondary mineral formation, and radionuclide solubility are examined.

A series of experiments for a simplified system has been performed to evaluate a near-field model of radioactive waste repository developed by the University of California, Berkeley. In the experiments, iodine as nonsorbing species and cesium as sorbing species were employed. This paper presents the initial 200-day results of these experiments. Good agreement is found between the model and the experimental results. The results of this work will give basic ideas on the design of the near field of a repository.

Source-term codes to predict the release of radionuclides from nuclear waste packages have been developed and implemented worldwide. A survey and initial comparison of the attributes and capabilities of 13 international source-term codes was recently completed. This preliminary analysis focused on comparison of transport factors/processes and solution methods. This initial comparison is a necessary first step in a properly-conceived, systematic benchmarking of source-term codes. Advantages of such a comparison include assurance of the mathematical correctness of implemented models, comparison and quantification of variances introduced by different types of simplifications, and identification and quantification of the impact of near-field processes.

An international Workshop was conducted to discuss the present state of modelling of near-field performance of high-level waste repositories, with emphasis on the conceptual and mathematical models for source-term calculations. The Workshop was based on an international survey and review of this topic by the authors. Technical position papers developed by the participants are summarized here. A Proceedings of the Workshop, including the full position papers and the survey and review document, are to be published by Nuclear Energy Agency by the end of 1993.

This paper reviews potential sources of colloids and enhanced adsorption of radionuclides that may stem from materials introduced into a repository setting. Three major sources of colloids are examined: metals, cements, and organics. The sensitivity of colloids to chemical species, pH, time, temperature, radiolysis, redox state, gradients of the aforementioned variables, and microbial activity is shown. We consider these influences on colloid formation and sorption with respect to introduced materials. We also discuss areas that have not been addressed but may have consequences in a repository setting.

In the safety analyses of radioactive waste disposal, it is important and indispensable to analyze coupled problems of ground water flow, thermal transport and/or migration of nuclides. The three-Dimensional coupled problems is solved by boundary element method in this paper. The results of this method are compared with those experiments of JAERI and STRIPA SWEDEN on the thermal problem, and with those analyses of analytical and FEM results on the migration problem. In this formulation, natural convection is considered by Boussinesq approximation. An example of coupled ground water flow and migration of nuclides with decay chain U2 3 4 → Th2 3 0→ Ra2 2 6 is also tried.

A new application for surface complexation modeling is described. These models, which describe chemical equilibria among aqueous and adsorbed species, have typically been used for predicting groundwater transport of contaminants by modeling the natural adsorbents as various metal oxides. Our experiments suggest that this type of modeling can also explain stainless steel surface contamination and decontamination mechanisms.

Stainless steel transportation casks, when submerged in a spent fuel storage pool at nuclear power stations, can become contaminated with radionuclides such as 137Cs, 134Cs, and 60Co. Subsequent release or desorption of these contaminants under varying environmental conditions occasionally results in the phenomenon known as “cask weeping.” We have postulated that contaminants in the storage pool adsorb onto the hydrous metal oxide surface of the passivated stainless steel and are subsequently released (by conversion from a fixed to a removable form) during transportation, due to varying environmental factors, such as humidity, road salt, dirt, and acid rain. It is well known that 304 stainless steel has a chromium enriched passive surface layer; thus its adsorption behavior should be similar to that of a mixed chromium / iron oxide.

To help us interpret our studies of reversible binding of dissolved metals on stainless steel surfaces, we have studied the adsorption of Co+2 on Cr2O3. The data are interpreted using electrostatic surface complexation models. The FITEQL computer program was used to obtain the model binding constants and site densities from the experimental data. The MINTEQA2 computer speciation model was used, with the fitted constants, in an attempt to validate this approach.

Thermally induced flow of liquid water and water vapor at the potential repository site at Yucca Mountain, Nevada, will extend hundreds of meters away from the repository edge. The resultant transfer of heat and mass will sufficiently perturb the ambient conditions such that a variety of mineralogical and chemical reactions will occur that may modify hydrological properties. The consequences of this “coupling” of geochemical and hydrological processes will vary through time, and will occur to different degrees in four regimes (T < Tboiling; T = Tboiling; T > T boiling; cooling) that will develop within the repository block. The dominant processes in the regimes differ, and reflect the local balance between: 1) kinetics and equilibrium; 2) dissolution and precipitation; 3) evaporation and boiling; and 4) fluid flow in matrix and fractures. Simulations were conducted of the evolution of these regimes, using laboratory derived kinetics and thermodynamic data, and site specific mineralogical and hydrological properties. These simulations identify regions where chemical and mineralogical equilibrium is likely to be achieved, and where net changes in hydrological properties will be concentrated. Tests of the results of these simulations have been initiated using field data from the Taupo Volcanic Zone, New Zealand. A preliminary series of calculations suggest that relative changes in porosity of as much as ± 20% to 30% may be possible for rocks with an initial porosity of 10%.

A coupled geochemical transport program PHREEQM2D has been tested in order to examine intrusion of seawater into fracture zones which can be connected to tunnels, such as nuclear-waste repositories on the sea shores. The code consists of a flow and transport program HST2D and a chemical equilibrium program PHREEQE. It uses a mixing cell approach, where chemistry and transport are solved separately. Ion exchange is calculated by means of a simple surface complexation model.

In the present artificial exercise, replacement of groundwater by seawater in a connecting porous fracture zone was modelled without comparing the results to observed data. Dilution, mixing, ion exchange, and dissolution/precipitation reactions were examined.

The results of modeling showed that the sulphate front of seawater moved some distance towards the tunnel during the simulations. The characteristic solutes of dilute groundwater were assumed to enter the tunnel in a short period of time. Amounts of sulphates decreased due to dilution and mixing. The chemical contents of waters were also affected by dissolution, precipitation, and ion exchange. Calcite dissolved and oxyhydroxides precipitated in oxidizing water. The strongest exchangeable ions in water, Ca and Na, were retarded due to ion exchange in the mixing zones. This caused an increase in dissolution of calcite in the mixing zones. Calcite would, however, start to precipitate in the mixing zone when the ion exchange capacity of rock would be consumed.

Some important factors, such as retardation on oxide surfaces (adsorption models) and microbial activity (organic complexation models) have not been tested in the present exercise. Otherwise, the PHREEQM2D showed its applicability to modelling of chemical equilibrium, dilution, mixing, ion exchange, and elution/precipitation in discrete porous zones in transient conditions. It could be used to model temperature-enchanced, coupled geohyd-Γochemical, processes in porous zones consisting of densely fractured or crushed crystalline rock.

The present status of the recently introduced gas method equipment for migration studies of fractured and porous media is briefly reviewed together with advances in the experimental techniques. The conditions under which matrix diffusion can be observed in both gas flow and water flow experiments are discussed in some detail. Results for a gas flow experiment are shown, and explained with a numerical model which incorporates the effects of hydrodynamic dispersion and matrix diffusion. The necessary parameters for a corresponding water flow experiment are also briefly discussed.

The estimation of diffusive transport of gases through tuffs is important in order to assess whether Yucca Mountain is an environmentally safe and acceptable repository for high-level radioactive waste. The purpose of this study is to provide effective diffusivity data which may be used to calculate the amount of diffusion of carbon dioxide through Yucca Mountain tuff layers to the environment. The effective diffusivity of carbon dioxide was measured for tuff samples from five different layers of the USW-G4 drillhole, which is located in the central part of the proposed repository horizon. The tuff samples studied are from the following layers and depths: Vapor Phase (83.6 m), Upper Lithophysal (130.1 m), Middle Nonlithophysal (224.5 m), Lower Lithophysal (243.9 m, 266.9 m, 297.3 m and 335.2 m), and Lower Nonlithophysal (356.3 m and 371.0 m). A steady-state, counter-diffusion method, using gas chromatographic analysis, was used to determine the effective diffusivity of carbon dioxide through tuff samples. Effective diffusivity increased with temperature between 25°C and 125°C for all layers. The effect of temperature on diffusivity correlated well using second-order polynomials. The porosity, specific surface area and mean pore radius were also determined. In general, diffusivity increased with porosity, specific surface area and mean pore radius, but did not correlate well. From the measured data , it appears that the diffusion takes place primarily in the combined Knudsen and bulk diffusion regimes.

This paper addresses our ability to predict the chemical consequences of the presence of introduced materials, many of them man-made, in a radioactive waste repository. The chemical modeling ability required to describe this environment over a 10,000 year period is unique and unprecedented. It requires knowledge of parameters that have never been measured, many of them with respect to introduced materials. This paper discusses considerations that are required to establish the potential significance of introduced materials, especially those that could compromise the lifetime of the waste packages or affect the transport of radionuclides from breached containers. The paper presents issues related to the stability of individual compounds, the potential alteration of predicted natural chemical reactions, the potential moderation of those effects by natural zeolites, and the potential for interactions at elevated temperatures between rock, water, water vapor, radiation, waste package, and introduced materials.

It is thought that a significant amount of diesel fuel and other hydrocarbon-rich phases may remain inside the candidate nuclear waste repository at Yucca Mountain after construction and subsequent emplacement of radioactive waste. Although the proposed repository horizon is above the water table, the remnant hydrocarbon phases may react with hydrothermal solutions generated by high temperature conditions that will prevail for a period of time in the repository. The preliminary experimental results of this study show that diesel fuel hydrous pyrolysis is minimal at 200°C and 70 bars. The composition of the diesel fuel remained constant throughout the experiment and the concentration of carboxylic acids in the aqueous phases was only slightly above the detection limit (1–2 ppm) of the analytical technique.