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Over the last 20 years, increasing reliance has been placed on numerical modelling techniques to predict the behaviour of hazardous contaminants in the natural environment. Chief among these are the development of methods to simulate groundwater flow and to calculate the speciation of potential contaminants using relevant chemical thermodynamic data. The chemical equilibrium modelling for the speciation of mobile contaminants is, for obvious reasons, critically dependent on the thermodynamic data employed. However, many of the data necessary for reliable simulations are either still unavailable or known only with large uncertainties. Furthermore, significant discrepancies have been found to exist amongst the thermodynamic data sets in use at leading European laboratories.
The aim of the current Concerted Action is to establish the mechanism by which a definitive thermodynamic database for radiological and environmental assessment may be built. The Concerted Action will review ongoing thermodynamic database activities (aqueous and pure substance data bases) in order to define a clear programme where future work is necessary to perform and where our knowledge is sufficient.
Key objectives include:
The proposed work is a necessary expansion of activities carried out under the previous phase of MIRAGE, notably the CHEMVAL Projects (Contracts FI2W-CT92-0122 and FI2W-CT90-0065).
Chemical equilibrium modelling is a powerful tool which may be used to guide each stage of the nuclear fuel cycle, from uranium extraction through to final disposal of radioactive wastes. Over recent years, the intractability of many waste disposal situations to direct experimentation has led to increasing reliance on numerical modelling in order to determine wasteform degradation, source term concentrations and contaminant transport behaviour through the geological barrier. Ultimately, the bioavailability of each contaminant is also determined by its speciation within a given geochemical environment. However, this has received less attention in the national radioactive waste programmes of many countries to date.
Predictions obtained by modelling are critically dependent on the thermodynamic data employed and the need for a well verified, common database is clear, particularly in the case of radiological assessment work. However, up to now the ever-expanding number of computer programs available for performing equilibrium simulations has been matched by an equal number of data compilations, each differing from the next depending on perceived requirements, alternative methods of data treatment and, in essence, the compiler's judgement. The scale of the problem became apparent at an early stage in the CHEMVAL Project conducted under the umbrella of the EC-coordinated project MIRAGE (Migration of Radionuclides through the Geosphere); a multidisciplinary forum encompassing both laboratory investigations of radionuclide migration and field scale studies. Here concerted efforts were made to account for differences in equilibrium simulations and, thereby, increase the level of confidence which could be ascribed to thermodynamic calculation methods. Consequently, the decision to provide a common database for participants received unanimous support as this gave a template against which results produced using other databases could be compared.
At the beginning of the MIRAGE Project, the Radioactive Waste Management Committee of the OECD/NEA commissioned a critical review of thermodynamic data pertaining to the actinides, uranium, neptunium, plutonium and americium together with the fission product, technetium. The project commenced in 1985 and, to date, has produced definitive reviews of uranium and americium thermodynamics.
A consensus has now emerged that progress made within the MIRAGE Project should be built upon leading to the production of a comprehensive thermodynamic database under the aegis of the European Communities. The current Concerted Action, represents a crucial step in bringing together the experience gained in the above and other projects leading, ultimately, to standardised and internationally accepted guidelines.
Most compilations of thermodynamic data are based on earlier selections by other workers, with the result that estimates of data reliability are highly subjective. An exception is the review of actinide data by the OECD/NEA in which published values for the complexes and solids considered were traced to the original experimental determination and critically assessed. This work represents the benchmark in terms of scientific rigour and, consequently, key personnel have been invited to participate in the current project.
The stated aim of CHEMVAL database activities was also to produce an internally consistent data set referenced to source literature. Data were selected according to pre-defined criteria but, given the requirement to distribute a workable compilation with limited resources, an exhaustive review for all 47 elements included was clearly not possible. Rather, the reliability of the information contained in the database was continually assessed through formal test procedures employing laboratory and, where feasible, field measurements. Six versions of the database have been released to date, each coinciding with a programme of code verification/model validation.
Both of the above compilations are oriented towards applications in radiological assessment and, in the case of the latter, simulation of geochemical systems. This is reflected in the emphasis placed on the actinides. Thus, not all elements of potential radiological or toxicological significance are represented.
A large number of other databases are to be found in the literature. Few of these have been developed specifically for radiological or environmental assessment purposes and, importantly, none meet the stringent criteria necessary for such an application, principally:
Such a demanding list of requirements can only be achieved through Concerted Action on a multinational level.
The aim of this work is to establish a structure which will allow a definitive thermodynamic database for radiological and environmental risk assessment to be constructed. The latter represents a substantial effort and may form the basis to define where further research work is needed for the production of an EC Database. Emphasis will be placed on developing a practical, working tool for use by researchers throughout the EC countries.
A two-year timescale is envisaged in order to establish a Steering Committee, agree the necessary working arrangements and to plan review, and data management programmes. A schedule is provided in the form of a GANTT chart (Figure 2).
Specifically, the following tasks will be undertaken.
The partners will form a provisional Steering Committee for the EC Database Project. They, in conjunction with the EC representatives, will recommend additional scientists, where necessary, with expertise in inter alia waste matrix properties (cement, glass), mineralogy, groundwater/ surface water chemistry and the behaviour of trace elements in biological systems. The original committee members will be Professor J. I. Kim (Forschungszentrum Karlsruhe - INE; Chair), Dr. Th. Fangh„nel (Forschungszentrum Karlsruhe - INE; Coordination), Dr. J. Bruno (QuantiSci), Professor F. P. Glasser (Aberdeen University), Professor I. Grenthe (Kungl Tekniska H^gskolan), Professor S. K. Saxena (University of Uppsala), Professor H. Nitsche (Forschungszentrum Rossendorf) and Dr. L. Regan (RMC Environmental). This Steering Committee will meet for four Workshops during the course of the Project.
The principal thermodynamic compilations available from different organisations world-wide will be collated by RMC-E and screened by the Steering Committee in an update of an earlier study. More specialised reviews, such as those of individual elements conducted by the OECD/NEA, will also be screened. This task will necessitate conversion of the respective listings to a common format and will address methods for activity corrections in detail. The latter is essential when evaluating published data for consistency, for their transformation to standard state conditions and also when using standard state thermodynamic data for the description of complex chemical systems under non-standard state conditions.
Critical review and data handling will consider the approach adopted by the OECD/NEA in previous studies. Procedures currently in place within MIRAGE allowing access to the database and ease of distribution among participating countries are, nevertheless, likely to be retained. However, the content of the database will need to be expanded. Members of the Steering Committee will meet to identify areas for enhancement and to agree on priorities.
Substantial progress has been made under the previous project (FI2W-CT92-0122) in expanding a database designed for radiological assessment purposes to include chemo-toxic elements, for example, arsenic, cadmium, nickel and copper. Additionally, the database was extended by inclusion of enthalpy and heat capacity data for calculations at elevated temperatures and adoption of the SIT (Specific Interaction Theory) method for activity coefficient correction in saline solutions.
In addition to further review work, to optimise use of existing information, methods will be developed for the estimation of unknown equilibrium constants using established theory. This will use the conceptual framework of aqueous coordination chemistry to suggest stoichiometry and equilibrium constants for species where no laboratory information is available but there are indications experimentally, or from existing theories, that interactions might take place. Guidelines for data estimation will be prepared in the light of previous experience in studies of this type.
Important gaps exist within the current knowledge base which can only be overcome through targeted laboratory measurements of fundamental constants. A programme of work will be recommended for database enhancement reflecting the perceived priorities of both the radio- and chemo-toxic waste disposal sectors. Implementation of this programme through commissioned studies at appropriate EC laboratories may follow the present project subject to timescale and budgetary constraints but such considerations are beyond the scope of this Concerted Action.
The advent of advanced analytical techniques (e.g. LPAS) furnishes an opportunity to validate the results of equilibrium speciation-solubility calculations for well constrained systems. A series of experiments will be designed (but not commissioned) building on the progress made under EC-projects (e.g. Contracts FI2W-0083 "Effect of humic substances on the migration of radionuclides. complexation of actinides with humic substances in a natural aquatic system", FI2W-0084 "Colloid migration in groundwaters: geochemical interactions of radionuclides with natural colloids). A formalised test protocol will be developed and employed in conjunction with complementary code verification/model validation initiatives within the Fourth Framework Programme.
The avowed intention of the project is to disseminate the results of research as widely as possible according to the deliverables set out in Section 3. A report will be produced after each Workshop and at the completion of this two year project setting out the status of EC thermodynamic database activities and providing detailed guidelines for addressing the requirements of both radio- and chemo-toxic waste disposal programmes. Every effort will be made to liaise with the Coordinators of projects within the "Nuclear Fission Safety" programme, all of whom require access to the most up-to-date thermodynamic information available.
The project consists of a series of well-defined and essentially discrete Work Packages (Figure 2). Progress will be monitored at each of the four Steering Group meetings/Workshops during the two years of the project.
Deliverables will comprise:
The two-year Concerted Action is designed to build on work carried out within a previous database project (Contract FI2W-0122) to allow a more exhaustive study compatible with the broader requirements of the Nuclear Fission Safety Programme. FZK-INE will act as Chair for the Steering Group and coordinate the Concerted Action, taking responsibility for organisation of meetings and submission of deliverables to the EC. Four Plenary Meetings will be held. Interim reports consisting of a summary of the contribution of each partner will be delivered to the EC. For this purpose the project coordinator will collect reports from each individual partner.
The contributions of each partner to the Project are summarised below.
FZK-INE will coordinate the project and ensure efficient transfer of information amongst the partners and with the EC. Under the direction of Professor J.I. Kim, who will chair the Steering Committee, FZK-INE will take primary responsibility for data review activities (Task 4). This role will encompass defining the methodology for the Project (Task 3) and assisting the remaining partners in the preparation of project reports. FZK-INE has developed a range of state of the art analytical techniques and, based on this experience, will outline a programme of experimental work for validating the results of equilibrium speciation-solubility calculations on selected systems (Task 6).
RMC-E will take responsibility for data collation/dissemination (Task 2) and make specific contributions in the areas of database guidelines and data estimation methods (Task 4) and in data verification, including issues dealing with experimental design and non-ideality (Task 6). In particular, RMC-E will investigate the implications of the effect of biological activities on chemical speciation and thermodynamics. In addition to this, RMC-E will be involved in definition of the project methodology, data collection, data screening and database definition. RMC Environmental 's involvement in the project will be led by Dr. L. Regan.
FZR will contribute to Work Packages 4 and 5 of the project, though as with the other participants, key personnel will be involved in all discussions and decisions made by the Steering Committee. The role of FZR will be crucial in defining priorities for database enhancement, an area where the project manager, Professor H. Nitsche, has a great deal of experience from both a North American and European perspective.
KTH will occupy a pivotal role in the project by enabling the expertise gained in related programmes to be integrated, for the first time, with research conducted under the ambit of the EC. The KTH group is headed by Professor I. Grenthe who will provide guidelines for handling and review of thermodynamic data. In particular, he will advise on methods for the estimation of activity corrections, an essential pre-requisite when evaluating published data for consistency and for transforming measurements to standard state conditions. Professor Grenthe will also contribute to the development of methods for the estimation of equilibrium constants where no direct experimental information exists.
Professor F. Glasser and his group at Aberdeen University have, for many years, studied the immobilisation of hazardous wastes in cementitious materials. The complex mineralogy of cements, with the majority of phases not attaining true thermodynamic equilibrium has given this group a unique insight into the likely behaviour of immobilisation matrices over protracted timescales. Professor Glasser will provide specific contributions to Task 4 (development of data estimation methods) and Task 6 (non-ideal systems).
QuantiSci will contribute to Task 2, 4 and 6, in particular providing advice on appropriate methods for estimating thermodynamic constants where experimental data are lacking. Advice will also be given on the applicability of equilibrium thermodynamic methods when representing water/rock interactions, as reaction kinetics in such systems are often slow and characterised by the dissolution and precipitation of non-stoichiometric phases. The implications of the project, in the context of performing quantitative risk assessments, will be evaluated. QuantiSci 's involvement in the project will be led by Dr. J. Bruno.
The University of Uppsala will contribute to Task 2, 4 and 6, concentrating on the thermodynamic properties of mineral phases. Advice will be given on the applicability of thermodynamic methods and data for natural minerals over a range of temperatures and pressures, noting that relatively few phases will attain true equilibrium in many environmental situations. Uppsala's involvement in the project will be led by Professor S. K. Saxena.