Aquatic Chemistry and Thermodynamics of Actinides and Fission Products Relevant to Nuclear Waste Disposal

PROPOSAL N°: FIS5-1999-00157

CONTRACT N°: PROJECT COORDINATOR: Forschungszentrum Karlsruhe GmbH (FZK)

CONTRACTORS:

DURATION: 36 month

PROJECT SUMMARY

The main objective of this project is to improve the scientific basis for performance and safety assessment of various waste management strategies. The goal is to provide fundamental knowledge about the behaviour of actinides and fission products in solution and solid phases. The research programme is focused on understanding the phenomena that are of key importance for modelling of the chemical processes that i) control the integrity of waste canisters; ii) control the dissolution of the waste form (source term); and iii) determine the rate and extent of radionuclide transport in dissolved or particle form from the repository to the biosphere. This project follows out of the findings of a concerted action entitled "Joint European Thermodynamic Database for Environmental Modelling - JETDEM", published in the Nuclear Science and Technology series under EUR 1913 1EN.

The project is structured into three work packages:

Thermodynamics of aquatic actinides

To eliminate the most serious deficiencies in the thermodynamic database for the actinides in aqueous systems, the following chemical reactions will be studied: complexation of tetravalent ions, formation of ternary complexes, complexation with phosphate, and their redox behaviour. To tackle these problems, classical wet-chemistry methods will be combined with state-of-the-art laser spectroscopic tools. In addition semi-empirical and more direct theory-based methods will be developed for predictions of chemical data for the actinide system.

Thermodynamics of the solid-water interface reactions

The underlying goal of the experimental/modelling programme is to understand the processes controlling the uptake and release of safety relevant radionuclides on materials and minerals important in radioactive waste management. The work is directed toward understanding of sorption mechanisms and the development of models to predict sorption in real systems.

Thermodynamic properties of secondary solid phases

The formation of secondary solid phases is considered to be one of the key processes controlling the mobility of radionuclides in the environment. Classical wet-chemical experiments will be coupled to modern microscopic and spectroscopic techniques, to link empirical macroscopic parameters to well-defined molecular-level processes.

TABLE OF CONTENTS

1 Objectives

The objective of the project is to improve the quality of performance and safety assessment of radioactive waste disposal by providing fundamental process understanding and data for the geochemical behaviour of long-lived fission products and actinide ions.

State-of-the-art approaches describe the release of radionuclides from the waste to the biosphere by a source-term, the mobility of groundwater and the radionuclide transport relative to the groundwater movement. Hydrogeological models describe advection, dispersion and diffusion. Chemical behaviour is currently considered in two ways: The source term is generally based on solubility measurements. The retardation relative to groundwater flow is based on distribution coefficients, i.e. Kd values. Measured source terms and Kd values lump terms of different geochemical processes and thus these data are valid only for the given geochemical conditions. Their use in performance and safety assessment lacks of any theoretical justification. In order to overcome these problems, predictive geochemical equilibrium modelling is under development. Use of geochemical equilibrium modelling requires a complete list of all relevant chemical reactions and a database of their thermodynamic constants.

Limitations of the equilibrium approach are, that all relevant reactions need to be known together with their thermodynamic parameters. This, however, is not the case. Lack of understanding of the key processes that control contaminant behaviour also consistently handicaps our efforts to define the source term, and to differentiate between various types of uptake such as adsorption, coprecipitation, recrystalisation and solid-state diffusion. This limits our ability to predict both remobilisation and immobilisation. These approaches ignore the open system conditions of natural aquifer systems and the resulting kinetic constraints. A lack of mechanistic understanding prevents description of the time-dependent nature of dissolution, transport and uptake properties. Without theory based information about the processes that release the contaminant to the groundwater, we cannot predict the mass transport and its evolution in time.

The project will make steps toward advancing the state-of-the-art of safety assessment. It will contribute toward the establishment of a more rigorous scientific basis for the chemical aspects of safety assessment, thus strengthening our potential for making confident predictions. The project will provide some of the relevant missing thermodynamic data; it will elucidate mechanisms for several of the reactions that are known and it will define more of the molecular-level processes that control uptake both on mineral surfaces and within their bulk structures.

The innovative character of the project lies partly in the application and further development of sophisticated analytical methods. The questions in the field of radioactive waste disposal are not new, but answers to them are lacking because of limitations in technology and thus scientific understanding. Resolution limits have prevented direct observation at the Ångström scale, thus requiring that molecular-level processes be elucidated from macroscopic data, which averages over several possible concurrent reactions. Likewise, analyses of solutions at the low concentrations that are realistic for environmental systems have been impossible. However, in the last decade, new spectroscopic methods have been developed for direct speciation of actinides in trace level concentrations and structural information on the molecular level. Another innovative characteristic of the project lies in its strategy and approach.

For development of basic understanding of relevant processes, a limited number of well defined systems will be investigated. In parallel, more complex system, which are more relevant for the direct application by performance assessment will be studied. Based on fundamental process understanding, development of scientifically justified simplified approaches for application to safety assessment is encompassed. In order to optimise the use of resources, the project focuses on three issues, reflected in the three work packages described below.

WP1: Thermodynamics of aquatic actinides: Experiments to improve the database of unknown or uncertain data

A large body of chemical information is available, but gaps are apparent in some areas of fundamental importance for nuclear waste management. The objectives of this WP are to:

WP2: Thermodynamics of solid-water interface reactions (sorption phenomena)

The objectives of this WP is to elucidate the processes controlling the uptake and release of safety relevant radionuclides in contact with materials and minerals important in radioactive waste management. The work is directed towards understanding sorption mechanisms and the development of models, which can be incorporated into computer codes to predict sorption in real systems. The aim is not to understand each system in every detail but rather to demonstrate a sufficiently complete understanding of the sorption processes guided by the needs of safety assessment.

WP3: Thermodynamics of secondary solid phase formation

The formation of secondary solid phases is considered to be one of the key processes for the immobilisation of radionuclides in the environment. The mechanisms by which a radionuclide is bound to a mineral are of fundamental importance and at present, are not well understood. The objectives of this WP are to develop basic process understanding and data for the impact of secondary solid phase generation on the retention of radionuclides. Processes will be described by thermodynamic solid-solution models.

Expected achievements of the project are reduction of uncertainty in performance and safety assessment of radioactive waste disposal, leading to increased confidence within the scientific community, amongst decision makers and a broad public.

2 Project work-plan

2.1 Introduction

Thermodynamic data and the underlying chemical models are the basis for predicting the performance of a nuclear waste repository. Therefore, in the past, considerable effort was made to determine thermodynamic data for actinides and fission products relevant to repository systems. Primary experimental data were evaluated and compiled in database systems. Although in many areas considerable progress has been made, there are still areas where our knowledge is insufficient and the available data have high uncertainty, are partly inconsistent and reaction mechanisms are not understood. These key areas where further research is necessary were identified by a concerted action within the 4th Framework Programme entitled "Joint European Thermodynamic Database for Environmental Modelling - JETDEM", published in the Nuclear Science and Technology series under EUR 1913 1EN.

Based on these key issues, the project has been divided into three work-packages:
WP 1: Thermodynamics of aquatic actinides: Experiments to improve unknown or uncertain data
WP 2: Thermodynamics of solid-water interface reactions (sorption phenomena)
WP 3: Thermodynamics of secondary solid phase formation

The work-packages within this project are mainly addressed to experimental work to elucidate reaction mechanisms and determining thermodynamic data. In addition, we envisage the development of theory-based methods for the prediction of actinide system data. We will strengthen the co-operation between computational and experimental chemists that already exists, and which has given successful results. The theory work will be financed from other sources.

The expected main results from this project will be an improved molecular-level understanding of the fundamental processes that control the solubility and retardation of the actinide elements. This will lead to increased certainty in thermodynamic data, and thus an improved ability to predict behaviour in systems where experiments are impossible or impractical.

In addition to reports required by the Commission, deliverables are annual technical progress reports and original peer-reviewed publications. The technical progress reports will contain detailed description of results, including process understanding, data and models. Individual achievements, such as data-sets and models are not formulated as separate deliverables out of the overall context.

2.2 Project planning and time table

The three work-packages will run in parallel throughout the project. They will start at the beginning of the project and will cover nearly the total duration of the project. Each work-package is subdivided into several tasks. Because of restricted resources, it is not possible to work on all the task over the whole period, as indicated in the diagram by a broken line.

2.3 Graphical presentation of the projects components

The project´s components are presented graphically on page 9. The topics of the tasks are partly interrelated in a form where some results are a prerequisite for another task. The individual topics are complementary in character and will be performed in parallel. Interaction between the groups as well as frequent reporting and discussion at meetings will ensure synergy and sharing of information as it becomes available.

2.4 Detailed project description

2.4.1 WP 1: Thermodynamics of aquatic actinides: Experiments to improve the database of unknown or uncertain data

In recent years, considerable progress has been made in improving our knowledge of the aquatic chemistry of tri-, penta- and hexavalent actinides. As a result, thermodynamic data necessary for performance assessment (PA) are available, i.e. solubility products and complexation constants with potential inorganic ligands (excluding phosphate). However, the thermodynamic database for tetravalent actinides is rather scarce and those values that have been derived have high uncertainty. In many cases, available data are also conflicting. Experimental difficulties are related to the high charge of these ions, resulting in sparing or low solubility, formation of amorphous or poorly characterisable solid phases, and their strong tendency to form colloids and to sorb to surfaces.

To overcome these problems, our project proposes to combine classical methods (e.g. solubility) with new spectroscopic techniques and with theoretical approaches. Laser based methods will be used for direct quantification and characterisation of actinide solution species, for the detection of extremely low colloid concentrations, and thus to determine solubility. This work will be performed in direct interaction with theoretical chemists, to develop computational tools for the estimation of stoichiometry and equilibrium constants in complex systems. Semi-empirical correlations will be used to predict complexation constants for reactions that are not directly accessible such as those for ternary complexes.

The work-package is split into separate tasks and focuses on reactions for which data are insufficient or lacking.

Task 1.1: Hydrolysis and complexation of tetravalent actinides and ternary complexation

Hydrolysis and complexation with carbonate and other ligands will be studied by direct spectroscopic speciation methods, as well as by solubility at equilibrium with solid phases. FZR will focus on U(IV) and Np(IV), whereas FZK will study Th(IV), U(IV), Np(IV) and Pu(IV). The partial overlap between the work performed at FZK and FZR is justified by the experimental difficulties involved in this task. The reactions will be studied first at constant ionic strength. Based on the success of the experimental methods, the studies will be extended by FZK to other higher ionic strength to derive Pitzer parameters. KTH will perform experimental studies on the formation of ternary complexes and provide a continuos link with theoreticians.

Solid/liquid equilibria. Most literature data have been derived from solubility measurements as a function of pH and ligand concentration. However, separation of solid and liquid phases may be obscured by colloid formation. Therefore, precipitation reactions will be studied using a very sensitive method for the detection of colloids, which are formed when saturation is reached. Laser-induced breakdown detection (LIBD), which enables detection of 10 nm colloids in the lower ppt range, was developed by FZK and has been applied for the hydrolysis reaction of Th(IV) and Pu(IV). The solubility limits found by LIBD are lower than many published data. To minimize the effect of colloid generation by local oversaturation during the addition of base to the solution, pH adjustment and control are performed by a coulometric method. In addition, classical solubility measurements will be performed as a function of pH, ligand concentration and ionic strength. Solid phases will be characterized by XRD and XAFS. From experimental data, solubility products for a given solid phase, hydrolysis and complexation constants, as well as activity coefficients for solution species will be derived using the Pitzer approach to describe ion-specific interactions.

Spectroscopic speciation of actinide ions. Quantification of species concentration by spectroscopic methods allows a direct determination of thermodynamic data. Depending on the kind of spectroscopy, additional information on structure and binding may be obtained for individual species. For the non-fluorescent tetravalent actinides, laser-induced photoacoustic spectroscopy (LPAS) has been applied by FZK; the spectroscopic speciation of Np(IV) down to 10-6 mol/L has been achieved which is about two orders of magnitude better than conventional absorption spectroscopy. Similar concentration limits are expected for U(IV) and Pu(IV). Because the spectroscopic characteristics of these ions are different, they will be studied in parallel by FZK (Np, Pu) and FZR (U, Np). The detection limit presently achievable is not sufficient for studying tetravalent actinide solutions in the neutral pH range in the absence of a complexing agent. Further development of LPAS with respect to sensitivity and reproducibility will be a task within this project.

Ternary complexation of actinides. There is a general lack of thermodynamic data for alkaline systems, which means that the chemical basis is unsatisfactory for performance assessment in waste systems containing large amounts of cement. Many of the experimental data for speciation of actinides in systems containing organic hydroxy-acids are flawed, because appropriate consideration has not been given to all deprotonation reactions. These are important already in the pH-range 3-4 for hexavalent actinides, around 6 for trivalent ions, and the status is unknown in systems containing tetravalent actinides. These reactions may affect speciation in systems containing degradation products of organic material. In order to avoid excessive numbers of experimental studies, the main focus will be on the development of semi-empirical methods for predicting the properties of ternary systems. The sub-task studies include:

Computational actinide chemistry and empirical estimation methods. Ab initio and density functional methods (DFT) will be used for the estimation of chemical properties of actinide complexes. In a first step, various theoretical approaches will be compared with experimental data, e.g. the variations of the chemical properties for a given oxidation state, within the actinide series. The results of this comparison will be used to select and develop the theoretical methods, a task that will be accomplished by a group of theoretical chemists, financed from sources outside this project. Solvent effects are very important for predictions of equilibrium constants and for the understanding of reaction dynamics and calibration requires extensive theoretical development that also has to be made within a theory oriented project.

Correlation of thermodynamic data of various kinds, e.g. ionic radius and effective charge, with the physical properties of actinide ions have been used to find underlying principles or to extrapolate unknown data. Based on recent progress in the determination of complexation constants of tri- and pentavalent actinides, a new semi-empirical approach was developed by FZK. The first results for the application of the model to binary mononuclear complexes of actinide and also transition metal ions with inorganic ligands, in particular carbonate, are very promising. The model will be further developed by FZK and applied to the prediction of unknown actinide complexation constants with inorganic ligands, such as mononuclear carbonate, sulphate, fluoride, chloride and hydroxo complexes. Prediction will also be made on limiting complexes and the stability of possible ternary complexes, for example, in the system An(IV)-OH-CO3. In co-operation with theoretical chemists, an attempt will be made to find a theoretical justification for this approach.

Task 1.2: Complexation with minor ligands

Uranium (VI) phosphates, notably members of the autunite series (M2+(UO2)2(PO4)2.nH2O), together with U(VI) silicates are the most common secondary alteration products of pitchblende ores. Although tetravalent forms are much less important in nature, recent evidence suggests the occurrence of stable, mixed valence minerals offering potential as actinide immobilisation matrices. Improved solubility data are required for the majority of end-member compositions. Further, it is highly likely that trace actinides derived from a spent fuel repository will be incorporated as solid solutions in such phases. Therefore, solid phosphates constitute a primary uranium sink and therefore a secondary source of this element. However, the concentrations and conditions at which uranium and trace impurities are immobilised in or released from these solid phases are not known.

Within this task, UPC will develop an experimental program to study the solubility of U(VI) solid phosphates by means of solution and solid (XPS, XRD; EXAFS) techniques under the conditions of interest in natural waters, in the pH range from 6 to 10 and at temperatures ranging from 25 to 60 °C. The ionic media selected to undertake this experimental work will reproduce the conditions of low salinity granitic groundwaters and synthetic U(VI) and U(IV) phosphate phases will be prepared in the laboratory and characterised prior to the solubility experiments.

EP will supplement the experimental work on synthetic U compounds by microanalysis (EPMA, LAMP-ICPMS) of thorium and REE zonation in natural uranyl minerals. This will furnish evidence on the structure, lattice configuration, degree of incorporation and, in the case of Ce and Eu, oxidation state of the impurities from which the likely long-term behaviour of the transuranics may be inferred.

Task 1.3: Redox behaviour of actinides

The chemistry of the actinides varies strongly with their oxidation state, and redox transformations are therefore an essential factor controlling actinide mobility. A characteristic feature of the safety relevant redox processes is the presence of large amounts of solid reducing phases such as UO2 and iron in the near field region and FeS2 in the geosphere; these can reduce oxidants generated by radiolysis and actinides in high oxidation states. A great deal of work has been done to model spent nuclear fuel systems. In spite of this, very little is known about the kinetics of these processes. Such understanding is a prerequisite for extrapolating experimental data in time and space and thereby reducing the uncertainty in current PA models.

The focus of the proposed project is to gather scientific evidence for the relevant redox-reactions at the molecular level. Information of this type is scarce or non-existent at present. It is necessary for scenario development, the formation of conceptual models and the determination of quantitative parameters that are required for PA models. The efforts of KTH and UPC on laboratory systems will be focused on the following items:

  1. Rates and mechanisms of redox reactions involving U(IV), U(VI) and transuranium elements in carbonate containing homogeneous solutions.
  2. Rates and mechanisms of redox reactions between U(VI) and transuranium elements, on solid uranium dioxide, UO2, used as a model for spent nuclear fuel.
  3. Rates and mechanisms of redox processes involving H2, O2 and H2O2 on UO2 in order to simulate the interactions between spent nuclear fuel and radiolysis products.

In this context, the UPC will contribute to the study of the O2 and H2O2 mediated oxidative dissolution ot UO2 in the presence and absence of carbonate. This will be made by a combination of solution chemical methods using emf titrations and surface spectroscopic techniques to differentiate the oxidation state of U (XPS).

2.4.2 WP 2: Thermodynamics of solid-water interface reactions (sorption phenomena)

The retention of radionuclides in the geosphere, e.g. in the near- and far field of a repository for nuclear waste, is governed by their chemical reactions at the mineral-water interface. In PA models, the sorption processes are usually described in a phenomenological approach using a measured distribution coefficient between the solid and aqueous phases. The Kd-method ignores the actual species present in solution and at the surface. Thus the applicability of the derived data is restricted to the given conditions and may not be extrapolated to a different geochemical environment. Recently, the interaction at the mineral-water interface has been interpreted and quantified using quasi-thermodynamic models, such as the surface complexation model. These intend to account for the chemical interaction of metal ions with surface complexation sites and the electrostatic potential arising from surface charge. However, only arbitrary surface complexes are derived by fitting sorption data as a function of the metal ion concentration, pH and ionic strength, without any direct possibility to distinguish or validate the postulated species. The surface complexation model has been mostly applied for sorption of divalent cations, but much less for tri- and tetravalent metal ions, especially for lanthanide and actinide ions.

The objectives of this work-package are:

It will not be possible, nor is it necessary, to understand each relevant solid-liquid system in every detail. On the basis of a fundamental understanding of the sorption processes, a simplified description used for PA studies can be justified and defended. However, a clear methodology for process simplification and data reduction is required.

Task 2.1 Quantification of the reactions (sorption isotherms)

Determination of radionuclide distribution between solid and liquid phase by batch experiments, using radioanalytical or isotope-sensitive methods (ICP-MS), will provide basic data and will be performed by FZK, FZR, PSI and CIEMAT. Variation of radionuclide concentration (isotherms), of pH (pH edges) and of ionic strength will yield most important input data for e.g. surface complexation modelling. Characterisation of mineral surfaces is a prerequisite for a successful sorption study and will be performed using various methods by each of the groups. The following mineral/water systems will be studied:

Metal oxides: FZK will study the sorption of tri- (Am, Cm) and tetravalent (Th) actinides and chemical homologues (Eu(III)) on simple, well-defined pure metal oxides (silica, alumina, titania). The aim of this study is to derive basic sorption data (dependency of the sorption isotherm on pH, metal ion concentration and ionic strength) as a complement to the direct spectroscopic speciation that will be performed in Task 2.2. These systems have been selected to provide basic understanding of the reactions at the mineral-water interface and to derive experimental results that will be used for the development of appropriate thermodynamic models.

Iron oxides, formed in the near- and far field of a repository by canister corrosion or aerobic weathering of the Fe-bearing minerals may have a strong effect on the retention of radionuclides. Despite the large number of sorption data available, a thorough knowledge about the effect of the canister corrosion products on the sorption of radionuclides is still required, and needs further research efforts. CIEMAT will focus on U(VI) and Pu(IV)) to perform sorption and kinetic experiments on Fe- (oxy)hydroxides (goethite and magnetite) as the most representative solid phases formed by canister corrosion under the physicochemical conditions expected in a HLW repository.

Clay minerals: A sorption study of Np(V) and U(VI) on purified, conditioned and well-characterized illite will be performed by PSI. The acid/base characteristics of the homo-ionic Na-illite will be investigated using batch titrations at various ionic strengths. Sorption edges at varying ionic strength and sorption isotherms at constant ionic strength with fixed pH will be determined for both Np(V) and U(VI). Experience has shown that these sorption measurements represent the minimum number of data sets required for a reliable modelling exercise.

Rock forming mineral phases: The uptake of U(VI) on rock forming mineral phases will be studied by FZR. Crystalline minerals such as calcite and layer silicates (biotite) will be examined. The minerals proposed to study are relevant in environmental systems. They form a major part of the rocks in areas heavily influenced by former uranium mining and milling activities, or are used in various technologies proposed for future radioactive waste repositories, particularly as shielding layers, covers or backfill. For some proposed repository locations, they are present in the host rock. In addition, earlier investigations have shown that many of these minerals interact significantly with heavy metals and radionuclides.

The minerals will be characterised properly by methods such as AFM, SEM / EDS, PIXE and polarizing microscopy of thin sections. Sorption studies will be conducted at two ionic strengths (0.01 and 0.1 mol/L) using potentiometry and electrochemical methods. Results will be combined with observations from analytical tools such as ICP-MS, ICP-AES, or IC.

Validation and development of sorption models: Surface complexation models will be refined based on the spectroscopic investigations of sorption mechanisms and the identified surface-sorbed actinide species. Important input data to be used from the data collected include the pH, ionic strength and concentration dependence of actinide sorption isotherms. The available surface sorption models: ion exchange, surface complexation and surface precipitation will be applied to the data, parameterised and modified as necessary. Currently available codes as FITEQL and codes developed by the participants will be used. Sorption data for clay minerals will be interpreted using a combination of cation exchange and surface complexation. The code MINSORB, will be used which allows speciation, cation exchange and surface complexation reactions to be calculated simultaneously.

In addition to the modelling of sorption reactions on pure mineral phases, investigations will test if data for the sorption properties of real-world rocks can be described by a linear combination of the properties of the constituing minerals. Most real-world solids relevant for performance assessment studies are rocks made up of several minerals in varying compositions. However, there are indications that only one or two minerals dominate overall sorption behaviour. Hence, it is worthwhile testing whether an additivity effect of sorption on rocks is generally observed, and if so, to identify those phases dominating the sorption. Based on preliminary results, FZR will study phyllite, while PSI will focus on argillaceous rocks.

Task 2.2 Spectroscopic studies on surface interaction mechanisms for actinides

The surface complexation model is widely used for a thermodynamic description of adsorption, mainly however, for mono- and divalent metal ions. The electrostatic potential is more or less treated as a fitting parameter, so the results depend strongly on the electric double layer model used. The deviation from Langmuir isotherms found experimentally is often explained by the existence of strong and weak sites. What is their physical meaning?

The answer to these and other questions may be deduced by direct speciation of the individual surface complexes. Various spectroscopic methods will be used by the individual institutes to characterize the coordination and bonding of the metal ion at the mineral/water interface. These studies will complement the classical wet-chemical approach, covering a similar range of chemical parameters, or will be conducted on selected samples. The following methods will be applied:

Time-resolved Fluorescence Spectroscopy (TRLFS) is a highly sensitive and selective speciation method for Cm(III) at trace level concentrations (nmol/L). It was developed and applied by FZK to study complexation reactions with various ligands and sorption reactions onto silica, alumina etc. TRLFS allows quantification of individual dissolved and sorbed Cm(III) species and also yields structural information, such as the number of coordinated water molecules. Interface reactions of Cm(III) sorbed onto silica, alumina titania, ferrihydrite and clay minerals (montmorillonite, kaolinite and illite will be studied by FZK using systematic variation of metal ion and sorbent concentration, pH and ionic strength. FZR will use TRLFS, LPAS and UV/Vis spectrometry for speciation of U(VI) and Cm(III) sorbed onto the rock-forming minerals mentioned in Task 2.1.

Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). This technique will provide direct structural information about the coordination of the metal ion. It may be applied to any dissolved or sorbed radionuclide used in this study. The lowest concentration for EXAFS measurements on actinides is approximately 500 ppm on solids and 5·10-4 mol/L in aqueous solutions. Measurements on active samples will be conducted at the Rossendorf beamline (ROBL) at the ESRF, Grenoble.

EXAFS measurements on illite reacted with Np(V) and U(VI) will be conducted under varied conditions by PSI; parameters include: reaction time, pH, ionic strength, and initial radionuclide concentration. Emphasis will be focused on sorption behaviour at low metal surface loadings. EXAFS and XANES of U(IV), U(VI) and Np(IV) sorbed onto tecto-silicates, ferrihydrite, quartz, calcite and silica gel will be performed by FZR. Polarized EXAFS will be used to probe preferentially aligned atoms along the polarization direction, such as for sorption on layered silicates by FZK, FZR and PSI. Surface-sensitive measurements will be done by grazing-incident EXAFS on planar surfaces (FZK).

Atomic Force Microscopy (AFM) will be applied by KU to investigate site specific interactions and interpretet adsorption processes by probing the surface under varying conditions of ionic strength, pH, solution concentration, saturation state and time. X-ray photoelectron spectroscopy (XPS) will be used to determine the bonding properties of uranium at the sample surface (FZR) and for Eu uptake by calcite (KU).

2.4.3 WP 3: Thermodynamics of secondary solid phase formation

In addition to the interaction of radionuclides at the mineral-water interface (WP 2), incorporation into the bulk mineral structure represents an important mechanism for radionuclide retention. Uptake at an interface would be expected to have reasonably fast reaction kinetics, whereas radionuclides tightly bound in lattice sites within a mineral's structure would be released slowly or be bound irreversibly. In this case, description by a Kd or a surface complexation model is inadequate; the incorporation process is better described by a thermodynamic solid-solution model. Such an approach should be included in the chemical modules of safety assessment codes, but the behaviour of the actinide elements in secondary phases is poorly understood.

Task 3.1: Formation of solid solutions of radionuclides with calcite

Carbonates are among the most important secondary alteration products that form during the degradation of cement in radioactive waste repositories. In addition, calcite is an important accessory constitutent of bentonite backfill materials and it is an ubiquitous solid phase in the geosphere. Previous studies have shown that many trace elements interact strongly with calcite surfaces and are thus efficiently removed from the aqueous phase. The results indicate that even in solutions under or at saturation with calcite, trace metals can at least in part be bound irreversibly to calcite. This is normally explained by recrystallization with simultaneous formation of a solid solution at the mineral surface. Isotopic exchange tests carried out with Ca-45 revealed that calcite was continuously recrystallizing during the experiments. A theoretical description of such results in terms of solid solution formation is missing. For a better understanding of the thermodynamics and kinetics of the incorporation of actinides into calcite, a wide variety of experimental methods, from classical wet chemistry to modern spectroscopic and nanoscopic techniques will be applied. These studies will be performed in close collaboration by FZR, PSI and KU.

Recrystallisation experiments are preferred to true coprecipitation experiments, which are carried out in strongly oversaturated solutions. They minimise disturbing kinetic effects because partition coefficients depend on precipitation rates. By minimizing mineral growth rates, recrystallisation experiments provide favourable conditions for the establishment of thermodynamic equilibrium conditions. An important prerequisite is the ability to discriminate among several uptake processes. Specifically, one must be able to differentiate between sorption on a mineral surface and incorporation into a crystal lattice. With the advent of modern surface analytical techniques, it is in principle possible, though still challenging, to follow the fate of trace elements during their interaction with mineral surfaces.

X-ray photoelectron spectroscopy (XPS), a technique probing the top few nanometers of mineral surfaces, will be applied to detect changes in the surface concentration of sorbed trace metals as a function of time and solution composition.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS), with its high resolution in both space and mass, will provide chemical maps of surface composition, making it possible to detect inhomogeneous distribution patterns such as the association of trace components to micro-topographic features.

Time-resolved laser fluorescence spectroscopy (TRLFS) will be used to study the mechanism of the Eu(III) and Cm(III) uptake. This highly sensitive method enables the hydration status of the Cm species to be studied and hence to distinguish between surface sorption and incorporation into the lattice. This technique will also provide information on the dynamics of the Cm(III) uptake. Structural information such as bond length and coordination number for solid solutions of Am(III) and homologue lanthanides will be collected using X-ray absorption spectroscopy (XAS) at the ESRF in Grenoble.

Atomic Force Microscopy (AFM) will provide essential complementary information on the physical features of crystal surfaces at the sub-nanometer scale and contribute toward formulating a mechanistic model for site selectivity differences.

Modelling of secondary phase formation: The experimental work will yield a large amount of data on the uptake mechanism of radionuclides in calcite, a major secondary product in cementitious repositories and rock forming minerals. These data will require interpretation and analysis with appropriate thermodynamic models. In addition to the determination of empirical partition coefficients, the data will be treated by PSI and FZR with the help of state-of-the-art speciation and solid-solution models.

The aim is to identify a generally valid systematic pattern in the thermodynamic parameters governing the uptake of radionuclides into the calcite structure and to determine activity coefficient models for radionuclides in the solid. If successful, this approach will allow us to make general extrapolations and to predict the behaviour of radionuclides during interaction with calcite. Another major benefit will be the improved general understanding, in terms of thermodynamic theory, of the incorporation processes controlling radionuclide concentrations in both the solid and liquid phases. This understanding is fundamental for including coprecipitation processes in safety assessment models.

Task 3.2: Formation of solid solutions of radionuclides with phosphate minerals

In nature, the "light" rare earth elements (REE) commonly occur as the phosphate mineral monazite, while the "heavy" REE and Yttrium occur as the phosphate mineral xenotime, which has a similar composition, but a different coordination for the cation. Although these minerals are normally formed hydrothermally, there are some indications of their formation at ambient or diagenetic temperatures and fractionation of the heavy and light REE can be extreme. The behaviour of REE in several natural systems has been explained in terms of their incorporation in the major phosphate minerals present in the media, such as apatite. The most common substitutions observed in the crystal structures of REE minerals are of the type 2Ca2+ = REE3+ + Na+. This is the exchange observed in apatite; (Ca,REE,Na)5(PO4)3(F,OH) and the solid solution range of this type is quite wide. In general a strong enrichment of light REE is observed in apatite.

Solubility data on REE phosphates in the literature is scarce and their interpretation is hampered by a lack of reliable data on aqueous REE-phosphate complexation. Therefore, the study of REE phosphate solubility is of crucial importance. It has three main aims:

The experimental programme will be performed by UPC and EP. The former will concentrate on synthetic phases whereas EP will seek to verify models of REE incorporation by analysis of fractionation patterns in natural samples, Subject to resources, FZR will attempt doping studies on Pu and/or Cm to assess extension of the findings to the transuranics. The study will begin with the synthesis of REE phosphates and solubility studies of the synthetic samples will be conducted under the experimental conditions expected in granitic environments. Experiments will consider the variation in the REE/Ca ratio in the solid in order to determine the influence of this parameter on solid phase thermodynamics. Also the influence of pH and phosphate concentration will be investigated.

The formation of discrete secondary U minerals (Task 1.3) is most likely in the immediate vicinity of the repository where concentrations are highest. At greater distances, minerals containing accessory U or Th are expected to become more important. Where these form by isomorphous replacement of constituent elements (e.g Zr, Y, REE) the U/Th content could be appreciable; natural monazites and zirconalites may contain more than 20 wt%. Thus, information is needed on minerals that are potential hosts for the transuranics and, particularly on factors that determine stability. It is also important to establish the mode of actinide incorporation, both from experiments on synthetic phases and from microanalysis of natural minerals. This applies whether substitution occurs at the percent or more commonly, at ppm levels. This work will be carried out by EP using electron probe, XRD and AFM analysis of relevant phases. The results obtained will provide direct input to PA studies.

3 Scientific and Technological Prospects

The participants in this consortium are all familiar with or actively participating in various national nuclear waste programmes, most of which are more technology oriented than the present project. The more fundamental results of the present project will be an important complement to the national efforts. Because the results are going to be published in the open literature, the participants will benefit from peer-review and discussions within the international scientific community. The consortium will use the possibility of patenting inventions/discoveries made within the programme. The joint venture will result in reduced cost for the research required, and a much more efficient use of human resources than in research conducted only within national programmes. Intellectual property and licensing will follow the rules set by EU and the participating organisations.

A workshop will be organised around 6-9 months after commencement of the project to discuss dissemination strategies with representatives of waste management organisations. The objective of the workshop is to define the need for and strategy for implementation of scientific process understanding in performance assessment. The outcome of the workshop will be a report where researchers and persons responsible for implementation of scientific process understanding in PA discuss the level of required scientific process understanding and procedures for its implementation in PA.

Scientific results that have reached maturity will be disseminated at different scientific conferences. Emphasis will be on the international conference Migration'01, which will be held in 2001 in Bregenz, and will be organised by FZK, FZR and PSI. Such results from the second half of the project can be expected to be disseminated at the conference Migration'03, presumably taking place after finalisation of the project. Organisation of an EURESCO conference in 2002 on the subject of the project is under consideration.

From the project an improved understanding of fundamental processes governing the mobilisation and immobilisation of radionuclides will result. As a consequence the reliability of known data and those generated within this project will be increased. The published data will enter into thermodynamic databases (NEA and national thermodynamic data basis) that will be used by end-users, i.e. performance assessors in waste-management organisations and licensing authorities. Consequently, within this project there will no specific activities to disseminate the results directly to the end-users. For this reason, no technology implementation plan (TIP) will be defined.

4 Project Management.

4.1 Project management and decision making

The project will be co-ordinated by FZK/INE. Lead contractors responsible for various work packages will be: FZK for WP1, PSI for WP2 and FZR for WP3. They will also be responsible for liaison with other related work packages. The lead contractors will form the management committee. The management committee will assist the co-ordinator in establishing technical progress reports, the final report, and semi-annual and annual reports to the Commission.

There will be one project manager, responsible for administrative and scientific co-ordination. The manager, together with one principal investigator from each participating institution, form the scientific managing board. Communication within the consortium takes place using electronic media and a common consortium home-page.

4.2 Project meetings

There will be meetings of the scientific board two times per year. One of these meetings will offer a forum for the scientists from the collaborating teams to present and discuss their results. The members of the management committee will also organise separate work package meetings, as required. All meeting reports will be disseminated to all partners via the coordinator. The participating individuals/institutions know one another well and have a long and successful collaboration history.

4.3 Project reporting

The schedule for reporting and cost statements is listed in Table B4. In addition to semi-annual and annual progress reports, technical progress reports will be disseminated on a yearly basis. These technical progress reports contain an executive summary by the co-ordinator and annexes with individual contributions of each project partner. Detailed results of the project are also disseminated to a broad scientific and technical community through the final report. Publication of technical progress reports ensures that detailed results are made available also during the progress of the project. For the purpose of project management, these detailed reports ensure adequate documentation of progress throughout the project.

5 The Consortium

The consortium consists of eight principle contractors, including the co-ordinator (1):

  1. Institut für Nukleare Entsorgungstechnik, Forschungszentrum Karlsruhe GmbH (FZK), Germany
  2. Institut für Radiochemie, Forschungszentrum Rossendorf (FZR), Germany
  3. Waste Management Laboratory, Paul Scherrer Institut (PSI), Switzerland
  4. Dep. of Inorganic Chemistry, Royal Institute of Technology (KTH), Sweden
  5. Waste Management Laboratory, Centre Tecnologic de Manresa, Universitat Politècnica Catalunya (UPC), Spain
  6. Interface Geochemistry, Geologisk Institut, Kobenhavn Universitet (KU), Denmark
  7. Enterpris Ltd, (EP), United Kingdom
  8. Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Spain

One common characteristic among the members of the consortium, is their dedication to waste management research. Most of the participants have been involved in national nuclear waste programmes since their beginning and have co-operated frequently in the past. Each of the institutions is well-equipped for standard methods, insuring overlap and cross-checks for the traditional analyses. At the same time, each of the partners has state-of-the-art equipment and expertise in its application that is unique and complimentary, resulting in coverage of the broad range required for successful completion of the project.

The role of the individual members are as follows:

FZK will be responsible for project management; and will also be lead contractor for WP 1 (Thermodynamics of aquatic actinides: Experiments to improve unknown or uncertain data). FZK has expertise in the chemistry of transuranium elements and in laser spectroscopy. It will study complexation reactions (Task 1.1) and interface reactions (WP 2) of actinide ions by spectroscopic speciation methods. FZK will development and apply semi-empirical correlation methods for the predictions of thermodynamic data (Task 1.1).

FZR will be lead contractor for WP 3 (Thermodynamics of secondary solid phase formation) and will also contribute to the aquatic chemistry of tetravalent actinides (Task 1.1) and to the interface reactions (WP 2). FZR has expertise in the field of EXAFS and XANES and will be responsible for synchrotron based research at the Rossendorf beam line at ESRL in Grenoble.

PSI will be lead contractor for WP 2 (Thermodynamics of solid-water interface reactions (sorption phenomena)). PSI will contribute to Task 3.1 on the formation of solid solutions with calcite.

KTH will manage the scheduled co-operation with the experimental programme and a net-work of theoretical chemists (Task 1.1). KHT will also be responsible for the studies on redox reactions (Task 1.4).

UPC will be responsible on the study of solubility and dissolution processes of phosphate solid phases in Task 1.2 and Task 3.2. and will participate on the study of the U(VI)/UO2 redox reactions in Task 1.3.

KU will be responsible for investigations of element uptake by calcite and the definition of processes that control liquid and solid phase composition (Task 2.2 and 3.1).

EP will participate in the study of phosphate complexation (Task 1.2) and the uptake and release of radionuclides by secondary phosphate phases (Task 3.2).

CIEMAT will be responsible for the sorption experiments in Task 2.1 on iron corrosion products.


Back to ACTAF homepage