The use of computational thermodynamics can be applied to multicomponent and multiphase systems to identify the likely species formed for a defined system relevant to nuclear safety assessments and also in the optimisation of chemical processes, such as fuel reprocessing. The central problem in the calculation of thermodynamic equilibria is to obtain a representation of the Gibbs energy of all the components of the system as a function of temperature, pressure and composition. Having derived representations for the Gibbs energies of all the phases in a system, the equilibrium state can then be determined by either minimising the total Gibbs energy or solving a set of nonlinear equations using a suitable chemical equilibrium code. The calculations depend on: (i) reliable fundamental data, obtained by critical assessment and experiments; (ii) reliable models to represent the thermodynamic functions for the phases and (iii) reliable computer codes to perform the calculations. The thermodynamic data for given substances or reactions are typically stored in stand-alone databases which can be accessed by these specialised computer codes, or in databases which are integrated with specific codes.

Chemical kinetics techniques have to be employed when equilibrium conditions are not satisfied or when perturbations such as radiation and fluctuating heat sources need to be taken into account. These techniques involve the solution of the mass, energy and momentum conservation equations using specialised computer codes capable of solving simultaneous nonlinear equations in addition to the chemical rate equations applicable to the particular problem. Databases can be used to store the rate constants for a series of chemical reactions. An example of where such modelling is important in terms of nuclear applications is in the modelling of the water chemistry in a Boiling Water Reactor (BWR). The coolant water takes a complex flow path through the reactor components such as the core, jet pumps and re-circulation pipework. The corrosive species O2 and H2O2 can be generated through the action of radiation on the water and the production of these species can be modelled by solving a series of rate equations for points around the reactor circuit.

At its fourth meeting of May 1993, the NEA Nuclear Science Committee recommended to form a Task force to review the chemical thermodynamic data needs throughout the nuclear fuel cycle in order to determine which areas might benefit from international collaboration. During 1994, a survey of existing databases for chemical thermodynamic and kinetic data relevant to nuclear systems was initiated by the Nuclear Energy Agency (NEA). The aim of the survey was to review the contents and quality of thermodynamic and kinetic data available for scientific and engineering applications throughout the nuclear fuel cycle in order that important gaps in the data could be identified. Questionnaires were distributed to custodians of databases in OECD member countries as well as some end users of thermodynamic data. Sixteen responses were received and these have shown that there are a number of different thermodynamic databases which are in use and these cover a wide range of applications. However, in contrast the response also showed that not as many kinetic databases are in use, although some are commercially available.

The types of thermodynamic databases can be broadly separated into three groups. Firstly, there are pure substance databases which are typically general in application, contain data for thousands of species and can be used in calculations for a wide temperature range, but do not consider solution phases. Secondly, there are databases available which can be used to model solution phases relevant to metal-metal, metal-oxide and oxide-oxide systems. These databases contain data for pure substances (the stoichiometric phases in the system) as well as non-ideal interaction terms for different components of the solution phases which enable complex phase equilibria calculations to be performed including the calculation of phase diagrams. The solution databases are very specific in their applications and are usually focused on a relatively small number of basic components. Finally, there are a number of databases available for aqueous systems. In general the aqueous data are restricted in temperature to 298.15 K and tend to be derived from the same source, with small differences in the databases arising from alternative methods used to calculate the reference states. The data can be in the form of stand-alone databases or those which are integrated within specific codes. In order to model aqueous systems, data for both aqueous species and pure substances are usually required. Therefore, aqueous databases often contain data for solid and gaseous phases as well as aqueous ions and complexes. When using aqueous and pure substance databases together, care must be taken to use a common reference state, otherwise incorrect results will be obtained. Applications of aqueous databases include, for example, the optimisation of techniques for fuel reprocessing and the assessment of leaching behaviour from waste sites.

Section 2 of this report reviews the responses obtained from the questionnaires and describes the thermodynamic databases that are currently being used. Some additional databases not highlighted in the responses from the questionnaires, but which are relevant, in the opinion of the author, are also described. The databases are listed in alphabetical order. Only one of the responses to the questionnaires refers to the use of a kinetic database and Section 3 of this report discusses where the application of kinetic data may be important and the problems associated with assembling a kinetic database. Some kinetic databases which are commercially available are highlighted also in this Section. The main issues arising from Sections 2 and 3 are summarised and discussed in general in Section 4. Tables 1 and 2 show an index of computer programs which are available from the OECD/NEA for the modelling of chemical analysis, radiation chemistry and geochemical systems; some of these programs are described in this report.

In the case of a pure substance, for example gaseous species and stoichiometric compounds, the Gibbs energy can usually be obtained from standard compilations of thermochemical data (e.g. JANAF (1), Barin (2), and the NBS Tables (3)). The following sub-sections describe the pure substance databases which can be used for nuclear applications.

2.1.1. BARIN

The tables of thermodynamic data compiled by Barin (2) contain values of the thermochemical functions of 2372 pure substances (including 91 elements and the electron e- (gas)) which are tabulated at temperature intervals of 100 K. A large number of ternary oxides are provided, including molybdates, zirconates, hydroxides and some inter-metallic compounds. The substances also include about 100 organic compounds. The format of the tables correspond to the conventions which are also used in JANAF and the US Bureau of Mines and the following thermochemical functions are tabulated: Cp, S, -(G-H0298)/T, H, G, H-H0298, DHf, DGf and log Kf. The formation reactions refer to the reference states of the elements, which are given in a separate table. Barin uses critically evaluated data (CODATA (4) and JANAF (1)) where possible. For other substances references are made to the US Bureau of Mines Bulletins (5-7), periodical literature and other compilations. In some cases, Barin has estimated data, but this is clearly denoted in the text. Also, Barin does not justify the selection of data where there are conflicting sources.

2.1.2. CODATA

During the period 1970-1980, agreement was achieved on the key thermochemical values through the Committee on Data for Science and Technology for the International Council of Scientific Unions (CODATA-ICSU). These recommendations were published in a series of CODATA Bulletins. During the period 1980-1983, some of these recommendations were refined and these were re-issued as one publication (4). In late 1986, CODATA-ICSU also published recommended values for the fundamental physical constants (8). It is understood that more work is about to be started on Ba and Sr under the auspices of CODATA (9).

2.1.3. GURVICH et al

The compilation of tables prepared by Gurvich et al (10) (also referred as IVTAN - the Institute for High Temperatures of the USSR Academy of Sciences) contain thermodynamic values for ~1620 condensed and gaseous substances formed by 56 elements. Data for the oxides, hydrides and halides of these elements, as well as some of the compounds formed with sulphur, nitrogen and carbon, including salts of oxyacids, a large number of radicals and ionised gases are included. The tables of thermodynamic properties of all the substances are given in their standard state. For a number of solid substances, the properties of their non-equilibrium modifications are also considered. For many gases, data are given which allow deviations of their properties at high pressures from the properties in their ideal state to be taken into account. The data for the gases are quoted over the temperature range 100 to 6000 K, with some data extended up to 20000 K. All of the experimental data have been recalculated on the basis of critically selected thermodynamic constants, and as a result, total internal consistency is quoted for all the tabulated thermodynamic functions and the accepted values of the thermochemical constants. The whole system of data is based on the key values for thermodynamic constants developed and recommended by CODATA-ICSU. The values tabulated are for Cp, S, H-H0298, G-H0298 and either log K0 for the gases, or the logarithm of vapour pressure for the condensed substances.

The Gibbs energies for solution phases are more complicated to express than for pure substances. An important parameter for a solution phase is the Gibbs energy of mixing, Gmix, which is the change in Gibbs energy accompanying the formation of the solution from its constituents. In the case of an ideal solution phase Gmix is given by the configurational entropy change on forming the solution. In a non-ideal solution, however, Gmix will be given by the ideal configurational entropy change plus an excess Gibbs energy term, Gex. The modelling of multicomponent systems is mainly concerned with deriving suitable representations of Gex which reproduce the experimental phase relationships of a particular system. In the development of the representations for the components of the chemical model, much use is made of all the experimental thermodynamic data to obtain the various parameters that best describe the solution phases. The process involves the assessment of all the available data for all the combinations of the system components. In this manner the unknown excess Gibbs energy terms for the solution phases can be estimated. Although, in general, only binary and ternary data are available, experience has shown that a multicomponent solution phase can be adequately modelled using the representations for the lower order systems.

2.2.1. F*A*C*T Oxide database (Ecole Polytechnic, Canada)

Pelton et al have developed an extensive critically evaluated database for the thermodynamic properties of an eleven component oxide system comprising SiO2-Al2O3-CaO-MgO-FeO-MnO-Na2O-K2O-TiO2-T2O3-ZrO2 (31). All of the binary and ternary sub-systems have been fully evaluated and optimised in order to obtain coefficients of equations for the thermodynamic properties as a function of composition and temperature. In this case, the quasi-chemical model has been used to model the liquid phase. The database is accessible on the F*A*C*T (Facility for the Analysis of Chemical Thermodynamics) system (32).

The F*A*C*T code also supports a pure substance database and Pelton has made a critical assessment of some chloride data, on the commission of the American Ceramic Society, which may be relevant to pyrochemical processes in the nuclear fuel cycle ⁽³³⁾.

2.2.2. GEOKEM95 (Uppsala University, Sweden)

The Department of Theoretical Geochemistry at Uppsala University in Sweden have developed a database called GEOKEM95 (34) for silicates and oxides which is mainly concerned with data for the solid phase. The database contains data for 120 species for the oxide system CaO-MgO-FeO-Al2O3-SiO2 and the multicomponent fluid system (C-H-O-N-S). Further work is required in developing the solid solution models and the excess energy data. The database is internally consistent and is based on a critical assessment of calorimetry measurements and phase equilibrium experiments. The data are stored in the form of DHf and S0, Cp(T), thermal expansion and bulk modulus, and the database can be used in the temperature range 300 to 3000 K and at 0 to 1 megabars of pressure. The database is updated regularly, but no single modification is possible without re-checking all the phases in the system. The database is freely available on request.

2.2.3. MCCI (Thermodata)

In 1991 a collaborative programme was established between Thermodata and the Commissariat a l'Energie Atomique (CEA/France) and AEA Technology and the National Physical Laboratory (NPL/UK) on the development of a thermodynamic solution database involving nuclear materials for molten core-concrete interactions for hypothetical severe accidents in PWRs. Agreement was obtained between thermodynamic specialists on the choice of unary data and the models used in the phase diagram calculations. As part of this collaboration, the basis of a nine component model was developed comprising the oxides UO2-ZrO2-SiO2-CaO-Al2O3-MgO-BaO-La2O3-SrO. Gaseous species are also included in the database in order to investigate the vaporisation of the components.

Thermodata have continued to collaborate with the French CEA and have extended the database called MCCI (35) to include data for the following oxides UO2-ZrO2-SiO2-CaO-Al2O3-MgO-FeO-Fe2O3-BaO-La2O3-SrO. At the end of 1996, the database will include data for all the condensed and gaseous phases in the complex metal-oxide system: O / Ag, Al, Ba, Ca, Cd, Fe, In, La, Mg, Ru, Si, Sr, U, Zr. The database is internally consistent and data are obtained by critical assessment from the literature and the optimisation of thermodynamic data to fit experimental phase diagrams. The MCCI database is available commercially and can be used in conjunction with the GEMINI ⁽¹⁸⁾ code as well as other Gibbs energy minimisation packages.

2.2.4. NUCLOX (AEA Technology)

AEA Technology have also continued to develop the oxide solution thermodynamic database called NUCLOX (36) which now contains a fully integrated 12 component system UO2-ZrO2-SiO2-CaO-Al2O3-MgO-BaO-La2O3-SrO-(Fe-O)-CeO2-Ce2O3. The data are stored as a polynomial expression of the Gibbs energy with reference to the enthalpies of the constituent elements in their standard state at 298.15 K. The database can be used in conjunction with the SGTE Pure Substance database in order to include gas phase species in calculations. The database is internally consistent and data are obtained by critical assessment from the literature and the optimisation of thermodynamic data to fit experimental phase diagrams. The NUCLOX database is available commercially and can be used in conjunction with MTDATA and THERMOCALC.

2.2.5. Solution Database (SGTE)

The SGTE Solution database (37) is focused on metal alloy systems and contains data for multicomponent non-ideal solution phases. There are assessed data for over 230 binary and higher order combinations of the derivative components with data for the liquid, solid-solution and stoichiometric phases. Expressions for the excess Gibbs energy for solution phases are stored for specific combinations of the derivative components using various representations including polynomial descriptions, associate models and sub-lattice models. In some cases, additional terms are included to describe the pressure or magnetic contributions to the Gibbs energy. These descriptions can then be combined for calculations in higher order systems. As with the Pure Substance database, the solution database is compiled from a panel of experts by critically assessing data from the literature and internal consistency is checked. A single database manager, different to that for the SGTE Pure Substance database, is responsible for making changes to the database to ensure that only the same versions are circulated. The Solution database is available commercially and can be used in conjunction with MTDATA, THERMOCALC, CHEMSAGE and GEMINI.

2.2.6. THERMALLOY (Thermodata)

Thermodata also market a solution database called THERMALLOY (38) which is focused on metal alloy systems and contains data for multicomponent non-ideal solution phases. There are assessed data for over 300 binary systems. The solution database is compiled from an internal group of experts by critically assessing data from the literature and optimising the thermodynamic data to fit with experiments. The CALPHAD/SGTE (39) approach to representing the Gibbs energy and estimating data where appropriate are adopted as a standard. The database can be used to calculate phase diagrams and thermodynamic properties such as chemical activities. The Solution database is available commercially and can be used in conjunction with GEMINI or other Gibbs energy minimisation codes.

2.2.7. Other solution databases

AEA Technology have developed two other solution databases which are relevant to nuclear applications. The first is a metal metal-oxide database for (U-UO3)-(Zr-ZrO2)-(Si-SiO2) (40) which also includes data which is consistent with the NUCLOX database referred above. The database is fully integrated to enable calculations to be performed for the uranium-zirconium-silicon system. The data are stored in the same format as the NUCLOX database. The second database is for the Ag-In-Cd (41) system which is relevant to modelling the behaviour of control rods in Pressurised Water Reactors. This fully integrated database is also in the same format as the NUCLOX database and contains data for pure substances and solution phases enabling the experimental phase diagrams to be reproduced. Both of these databases are available commercially and can be used in conjunction with MTDATA and THERMOCALC.

The National Physical Laboratory have also developed the NPL SALTS database (42) which contains data for more than 90 binary systems formed primarily between alkali halides. The data include representations of the Gibbs energies of the unary phases as well as for the multicomponent solution phases. This type of database could be used in modelling, for example, advanced reprocessing methods.

Traditionally the chemistry of water based processes is understood from a basic knowledge of relatively simple chemical reactions and from practical experience. The ability to model quantitatively speciation and the interaction of the aqueous phase with solid phases and/or a vapour is a key step towards the understanding and, in some cases, control of processes. This could lead to an increase in the efficiency of industrial processes, in particular complex applications using concentrated solutions at elevated temperatures. Methods based on thermodynamics can be very useful for understanding the various processes, for example dissolution and precipitation, oxidation and reduction, acid-base and coordinative interactions, which govern the chemical composition of aqueous chemical systems. Typically, these models require the computation of equilibrium compositions for systems containing numerous species distributed among an aqueous phase, a gas phase, and several solid phases. For this reason, when modelling applications for aqueous systems, data are required for both aqueous species and for pure substance compounds. Most aqueous databases include a small sub-set of pure substance data to enable such calculations to be performed. An important consideration in the use of the data is the reference or standard states for the elements. Although the data in a database should be self consistent, in general, care should be taken in the mixing of data from different databases. This is particularly important if critically assessed data are combined with estimated data for which there is a limited experimental basis and it is also important when considering data for pure substances which may not have the same reference state as the aqueous species.

Software which incorporates a comprehensive database suitable for a wide range of conditions for aqueous systems is not available at present. A substantial body of thermodynamic data exists for common substances in dilute aqueous solutions for ambient temperatures, but the study of many processes would also require data for less common substances perhaps in hot and/or concentrated solutions for which data are sparse or non-existent. There is therefore a need to compile model parameters for the prediction of the behaviour of multicomponent aqueous inorganic solutions relevant to a wider range of conditions. In general the available experimental data are for values at 298.15 K. The lack of high temperature data has resulted in the development of methods to estimate such data for species or equilibria. The most widely used method has been that of Criss and Cobble ⁽⁴³⁾ and requires the entropy of the species at 298.15 K. The entropy at any temperature could then be estimated using parameters for broad classes of ions. Other estimation methods use the heat capacity at 298.15 K and derive a simple temperature dependence. In ionic aqueous systems, the behaviour of a component is strongly influenced by the presence of other species in solution. For very dilute solutions, the interaction between species is minimal and the activity of a component, A, can be represented by the concentration, XA, and hence the activity coefficient, A, is 1. The departure of A from unity is a measure of the departure from ideal behaviour and hence is a measure of the interaction between species in solution. Various methods are used to model non-ideal behaviour in aqueous systems.

Examples of databases for aqueous systems are described in the following sub-sections. Section 2.3.1 describes the databases which are stand-alone and can be used in conjunction with a number of computer codes. Some of the codes which use integral databases are described in Section 3.2.2. It should be noted that in some cases the data in the databases are derived from the same source, for example the National Bureau of Standards Tables of chemical thermodynamic properties (3) .

2.3.1. Stand-alone databases

2.3.1.1. AQDATA and HOTAQ (National Physical Laboratory, UK)

The AQDATA (44) and HOTAQ (45) databases have been compiled by the National Physical Laboratory for use with MTDATA. AQDATA contains data for over 450 aqueous species including elements, oxides, hydroxides, halides, sulphates, sulphites, nitrates, nitrites, phosphates, ionic and neutral complexes. The data are derived from the NBS Tables but only include aqueous species for which heat capacity, entropy and enthalpy of formation data at 298.15 K are tabulated. The HOTAQ database contains data for iron, chromium and nickel with carbon and sulphur in dilute aqueous solution up to 573 K. This database contains both aqueous and mineral species and has been developed primarily to study corrosion processes. In using these data with MTDATA the system is treated as ideal unless the options to treat ionic interactions is invoked and the appropriate parameters provided. The MTDATA code uses the Pitzer model ⁽⁴⁶⁾ to perform calculations for such non-ideal systems. Using MTDATA a data file can be produced that combines species from the AQDATA database and the SGTE Pure Substance database in order to model precipitation reactions although the Pure Substance database does not include hydrated species. The AQDATA and HOTAQ databases are available commercially for use with the MTDATA code.

2.3.1.2. CHEMVAL (CEC funded programme)

The CHEMVAL database has been developed by a number of parties with funding from the CEC (47); the project coordinators were W S Atkins. It has been recognised that the use of a number of different databases can often lead to different results for a common problem. To overcome this, one of the main objectives of the CHEMVAL project was to construct a comprehensive and commonly accessible thermodynamic database covering the majority of elements thought to have radiological significance or an influence on groundwater speciation. A further objective of the programme has been to increase confidence in the chemical speciation modelling of aqueous systems. For example, the CHEMVAL database was intended to be used for systems which are not ideal, where the concentration of ions covers an extensive range and the temperature can vary between 0 and 100°C. This requires the development of models and interaction data applicable for a wide range of conditions including the use of the Pitzer equations for the activity coefficients of the aqueous species; corrections for non-ideality are then made in the computer codes with which the database is used.

The latest version of the database (Version 6) (48) contains ~650 aqueous species and ~350 pure solid species with particular emphasis on actinide chemistry and heavy metal chemistry in natural solutions, but it is lacking in data for soil organics and some heavy metals. The log b, Ksp and DHj values for a given reaction are stored and the data are obtained by critical assessment of the literature. The database can be used with codes such as PHREEQC (49), MINEQL (50) and EQ3/6 (51) and is freely available within the public domain.

2.3.1.3. HATCHES (AEA Technology / Nirex)

The thermodynamic database HATCHES (HArwell/Nirex Thermodynamic Database for CHemical Equilibrium Studies) (30) has been compiled for use in radiochemical modelling work in conjunction with the codes PHREEQE (52), EQ3/6 (51) and MTDATA (15). The database includes data for the actinides U, Th, Pu, Np, Am and Cm and for several fission products including Cs, Ru, Tc and Sr; in total there are 1176 aqueous species and 772 solid phases.

The database contains formation constants for the aqueous species and solubility products for all the solid phases and is divided into three sections:

• Elements;
• Species;
• Minerals.

The thermodynamic constants (stability constants and solubility products) included in HATCHES have been selected as the best data available at the present time and the database is continuously being updated as improved data becomes available. In a number of cases, where there was evidence for the existence of a species but no good experimentally derived constants were found, estimated data have been included. Throughout the compilation of HATCHES every effort has been made to produce a self-consistent database. Each entry includes a full description of the source of the data, together with details of any calculations that have been performed on the data, (e.g. correction to zero ionic strength). In addition, each entry includes all the associated input information required by the PHREEQE and EQ3/6 codes (e.g. charge, stoichiometry, operational valency), together with any comments concerning the validity of the data. At present the database has been largely compiled for use at 25C. In a limited number of cases the enthalpy data required for calculations at other temperatures are included (e.g. uranium dataset) but it has not been extensively used at Harwell.

Complete validation of the HATCHES data for all types of problems would be an impossible task, however limited validation is achieved each time a dataset is used to successfully model experimental results. The database is available from AEA Technology or the NEA.

2.3.1.4. NAGRA Thermochemical Database

NAGRA (National Cooperative for the Disposal of Radioactive Waste) use a chemical thermodynamic database for geochemical modelling (53). The data are separated into two types. The core data are well characterised aqueous species, minerals and gases of elements commonly found in significant quantities in natural waters. The supplemental data are for elements that are found in natural waters, but not as major components, or that are of interest principally for the safety assessment of nuclear waste facilities. The supplemental data are subject to revision if new and better data become available. However, the core values are frozen and will not be changed; this is to ensure the compatibility of results obtained at different times. The core substances include the common aqueous species and minerals of fluoride, chloride, bromide, iodide, sulphate, bisulphide, nitrate, ammonia, phosphate, As(III) and As(IV), carbonate, borate, magnesium, calcium, strontium, barium, lithium, sodium and potassium. The data for these substances have been subject to a rigorous peer review before being selected and frozen. The data for the supplemental components have been less rigorously reviewed. The supplemental components include those for silica, iron, manganese, uranium, palladium, nickel, selenium and some organic complexes. Sources of the supplemental data include WATEQ4F, HATCHES (Version 3.0), the March 1991 version of the PSI MINEQL database and the April 1991 version of the PSI PHREEQE database. A compilation of equilibrium constants for the association of cations with various organic ligands was obtained from Hummel (54).

2.3.1.5. Bureau of standards Tables

Recommended data are provided for chemical thermodynamic properties of gaseous, liquid and crystalline substances, for solutions in water, and for mixed aqueous and organic solutions (3). Where available, values are given for the enthalpy of formation, Gibbs energy of formation, entropy and heat capacity at 298.15 K for a standard state pressure of 105 pascal. The sources of the data are original research papers, private communications with workers involved in chemical thermodynamics and data acquired from other evaluators, for example JANAF Tables ⁽¹⁾.

2.3.1.6. NIST/DIPPR Properties of Aqueous solutions database

This database (12) includes the capability to calculate, tabulate and graphically display activity and osmotic coefficients for aqueous electrolyte solutions. Approximately 350 solutes are represented. Temperature dependent properties are given for some species and activities and osmotic coefficients for mixed electrolyte solutions are also provided. This database is available commercially from NIST.

2.3.1.7. PATH.ARC.DAT (Alberta Research Council, Canada)

This database (29) contains thermodynamic data for ~150 condensed phase minerals, ~ 5 gases, liquids, and ~150 aqueous ions and complexes and also includes kinetic data for the dissolution and precipitation of ~30 minerals. The emphasis of the database is on aqueous chemistry. The data are obtained from the literature and the values stored include DG, DH, S0, Cp(T) and kinetic rate constants for specific reactions. From this information, the activity coefficients for the aqueous species are estimated. The database can be used for the temperature range 0 to 300°C and it uses the extended Debye-Hückel model for aqueous ions. The database is in ASCII format and can be used in conjunction with any program and it is available commercially.

2.3.1.8. PNC-TDB (PNC)

PNC-TDB (Version 0) has been developed by the Power Reactor and Nuclear Fuel Development Corporation (PNC), Japan ⁽⁵⁵⁾. The database includes thermodynamic data for pure compounds, gases, liquids and aqueous ions and complexes comprising 44 elements, 7 gases and 465 reactions. Particular emphasis is on the interaction between actinide/fission products with key minerals in rocks and bentonite. Data have been obtained from a number of sources including USGS (geochemical elements), NEA database (U, Tc), Battelle (Am), AEA Technology (Sc, Zr, Ra, Pu, Th, Pa, Np) and the EQ3/6 database (Sn). Most of the data are with reference to 25°C and stored as log K and DH⁰_r, in the same format used by the PHREEQE series of codes, but there are also some limited log K(T) data provided. Checks are made for internal consistency and a summary of the documentation is available in English (while the main document is written in Japanese). The database, which is freely available from the database manager, is currently being up-dated and a new release is scheduled for 1997.

2.3.1.9. SOLMINEQ (US Geological Survey)

The SOLMINEQ tables can be used in conjunction with the SOLMINEQ code. The most recent version, SOLMINEQ-95 (56), contains data for more than 200 pure minerals and gases and more than 200 organic and inorganic aqueous species and complexes. The data are in the format of log K values for dissolution reactions and complexation constants which have been obtained from the literature. The data have in some cases been extrapolated for use at higher temperatures and pressures and the database can be used over the temperature range 0-350°C for high pressures and salinity. The data are checked by the database managers and a detailed report is available.

2.3.1.10. Pure Substance databases

In most modelling problems of aquatic systems, data for both aqueous species and for pure substances are needed. Some of the Pure Substance Databases referred in Section 2.1 also contain data for some aqueous species. One such example is the NEA-TDB ⁽¹¹⁾ which contains data for pure compounds, gases, liquids and aqueous ions and complexes, another is the ALKALI database ⁽²⁶⁾. CODATA values ⁽⁸⁾ are also important, since they are frequently used as standard reference states. However, it must be stressed that pure substance databases cannot be used with aqueous databases without first establishing that common reference states are used.

2.3.2. Databases integrated within computer codes

2.3.2.1. CHEQMATE

The CHEQMATE (CHemical EQuilibrium with Migration and Transport Equations) program (57) has been developed to model the evolution of spatially inhomogeneous aqueous chemical systems. Such systems are characterised by simultaneous chemical-equilibria and ion-migration processes. The program has principally been used to study the evolution of the chemical environment in and around a nuclear waste repository. A repository will consist of a series of engineered barriers of different materials designed to retard the release of radionuclides. The chemical environment will evolve as these barriers degrade and interact chemically, and as aqueous species migrate within the system.

CHEQMATE is a computer code which models one-dimensional diffusion and electromigration of ionic species with chemical equilibration. The chemical part of the code is based on the geochemical program PHREEQE. This calculates the equilibrium water chemistry for a particular chemical inventory and associated minerals and uses a database of thermodynamic data. CHEQMATE predicts the evolution of the aqueous chemistry and mineral inventory in time and space. It includes an automatic mineral-accounting procedure, so that solid phases may be added or removed from the system as precipitation or dissolution occurs.

Unlike a number of other coupled codes under development which solve the ionic-migration and chemical-equilibria equations in a single step, the two parts of CHEQMATE are iteratively coupled, and local chemical equilibrium is maintained as the transport processes evolve. This approach has several advantages; the main improvement is that it allows a more complex description of the chemical system than is possible with many other codes. A second improvement is that the program runs in a much shorter time, which clearly facilitates the calculation of complex systems. This is also particularly advantageous when the code is used in smaller problems, in which there is some uncertainty in the physical parameters in the system; sensitivity tests may be performed over ranges of these parameter values without incurring very large expense. CHEQMATE is also extremely flexible and can be easily applied to many different evolving chemical systems.

2.3.2.2. EQ3 AND EQ6

The EQ3/6 software package (51) for computing chemical equilibrium problems in aqueous geochemistry includes two principal source codes, EQ3 and EQ6, and is available OECD/NEA Data Bank, France and Lawrence Livermore National Laboratory, USA. Data files support model calculations in the temperature interval 0-350C, either at pressures on the 1 atm.-steam saturation curve or at a constant pressure of 500 bars. The activity coefficient approximations for aqueous species are not suitable to describe aqueous solutions of concentration greater than about 1 molal ionic strength. There have been a succession of versions of the database; the latest issue is Version 22.

EQ3 computes from input analytical data the distribution of chemical species (ions, neutral species, ion-pairs, and complexes) in an aqueous solution. The program uses the Newton-Raphson method to solve the governing equations of chemical equilibrium for the specified conditions; convergence is aided by optimising starting estimates and under-relaxation techniques. This calculation produces a model of the fluid, which specifies the concentration and thermodynamic activity of each chemical species occurring in the chemical system and included in the database. The program includes five supporting data files containing both standard state and activity coefficient-related data. Three support the use of the Davies equations for the activity coefficients and two the use of Pitzer's equations. The program then calculates the saturation state of the fluid with respect to all relevant mineral phases in the database. The aqueous solution model calculated by EQ3 is used as a starting point for mass transfer computations by EQ6.

EQ6 can compute several kinds of mass transfer models. If the initial model fluid is supersaturated with respect to any mineral phases, the program first 'equilibrates' it by calculating a new model of modified fluid plus precipitates. This new model satisfies the original mass constraints. EQ6 then computes reaction progress models of compositional evolution and mass transfer in a closed or open (flow-through) system containing this aqueous solution (with or without any initial precipitates). Reaction progress may describe changes in temperature and pressure, irreversible reaction of the fluid with reactants (rocks, minerals, gases), or both of these simultaneously. The calculation predicts in detail the changes in fluid composition, the identity, appearance, and disappearance of secondary minerals, and the values of reaction progress at which the fluid saturates with reactants.

2.3.2.3. ESP

ESP is a comprehensive software program (58) for the simulation of complex chemical systems containing complex aqueous electrolytes, organic systems, vapours and solids both for steady state and dynamic conditions. The software was developed and validated by a consortium of international companies led by OLI Systems Inc. and is based on the 'ProChem' code, which has been used to simulate complex chemical systems.

The program simulates both electrolyte and non-electrolyte based systems by solving mathematical models which accurately represent these systems. The models are sets of nonlinear algebraic equations containing the appropriate thermodynamic parameters and are automatically written by the software. ESP is based upon a thermodynamic framework for aqueous systems that uses activity coefficient correlations and ion-ion interactions as well as other properties which are based upon work carried out by Bromley, Meissner, Pitzer and Zemaitis and combined in the Helgeson framework. Through careful empirical extension of activity coefficient correlations along with estimation and extrapolation techniques, ProChem is generally applicable in aqueous conditions over the range of 0 to 300°C, 0 to 200 atmospheres and 0 to 40 molal ionic strength, and for some organic compounds and vapours to much higher temperatures (1500°C).

The ESP database contains over 3000 inorganic and organic compounds of environmental interest; for each compound there are approximately 35 different thermodynamic and physical property parameters recorded in the database. The software can search for a compound by species name, formula, structure or via the periodic table. All the data is fully referenced and has been thoroughly assessed before being used in the database. A database management system is available that allows the creation of user databases which may include previously undocumented species data as well as data from the main database. The management system also enables the user to produce reports, check the references and update data. The database also has the facility for the estimation and extrapolation of relevant thermodynamic data, when the relevant data is not documented or the range of conditions for the case being studied are outside the range of the documented data. This is extremely important in environmental processes where a wide variety of compounds can exist.

ESP contains a number of unit operations which can simulate single stage and multistage steady state chemical phenomena. In these unit operations the user specifies the conditions (i.e. temperature, pressure, composition etc.) for which he wants to solve the algebraic nonlinear equations, which in turn represent an accurate model of the chemistry involved. The ESP system can predict phase separation and intraphase speciation in chemical systems and can consider, for example, equilibria between aqueous liquid, organic liquid, vapour and solids as well as reaction kinetics.

The ESP system also contains a subsystem called Dynachem which can simulate the behaviour of process plant, and control systems together with complex chemistry under time varying conditions. The process models can be run under real-time/interactive modes which allow the user to watch the process and enable him to stop the process at any moment in time, change some process parameter then restart the process. The software has the capability of simulating numerous process unit operations including amongst others chemical reactors, mixing vessels, fractionating/stripping columns, pumps, piping and heat exchangers.

2.3.2.4. MINEQL/PSI (Paul Scherrer Institute, Switzerland)

The computer code MINEQL (50) was adapted and extended to assess the solubility and speciation of radioactive waste nuclides in groundwaters under conditions which are expected to exist in the surroundings of planned underground repositories. With the use of an additional database which includes standard reaction enthalpies and heat capacities at 25°C, the relevant equilibrium constants at 25°C can be converted to other temperatures using Ulich's formulae. The activity coefficients for dissolved species are modelled with a temperature dependent function of the Davies approximation type.

The database at the Paul Scherrer Institute (59) includes data for 1807 aqueous species/complexes, 415 solid phase pure compounds and 7 gases with particular emphasis in the areas of radionuclide chemistry and organic complexation. It is intended for temperatures in the range 0 to 60°C, but about 20% of the data have not been temperature corrected. The data are compiled from the literature and for a particular reaction, the log K value, DH0r and DCp0 of reaction is stored. As far as possible a standard of zero ionic strength is used. The database is compatible for use with the codes MINEQL and PHREEQE and is freely available on request.

2.3.2.5. MULTEQ

MULTEQ is an interactive computer programme (60) which calculates the composition, pH and the concentration of an aqueous solution as that solution is concentrated at a user programmed temperature. It is designed to be used by water chemists for nuclear pressurised water reactors (PWRs) concerned about solution pH and concentration in PWR steam generators.

MULTEQ handles four classes of species; primary, combinations in solution, combinations limited to the vapour phase, and precipitates. Primary species may be charged or neutral and may combine to form combination species or precipitates. The user specifies the composition of the system by keying in the total concentration of each primary species that is present. Combination species in solution arise from the reaction of two or more primary species or other combination species and are characterised by a charge, mass balance contributions, and an equilibrium ratio between constituents. Neutral species (primary or combination) may partition into the vapour phase. Some species may be limited to the vapour phase. These are species which have appreciable concentration in the vapour phase, but are considered to have concentrations in the liquid phase which are small enough to ignore in that phase. Precipitates are combination species which are characterised by solubility products relating the components and mass balance contributions. Precipitates and vapour phase species always have neutral charge. The concentration of each species at equilibrium is determined by solving the set of nonlinear equations simultaneously.

A feature of MULTEQ is that the species database is separated from the program code itself. The species file is read each time the program is executed. A representative species data set is supplied with the program. However, this data set may be altered without any changes to the program code, thereby avoiding recompilations and relinkings.

For concentrated solutions, activity coefficient equations are employed. The equation used is a version of the extended Debye-Hückel equation. Several assumptions are made in formulating the chosen model. First, it is assumed that all monovalent ions have the same activity coefficients. Second, it is assumed that the activity coefficient z of a multivalent ion of charge z may be expressed in terms of the monovalent activity coefficient (1) as follows:

The third assumption is that NaCl is the model substance and that all monovalent cations behave like Na+ and all monovalent anions behave like Cl-. The fourth assumption is that the Meissner-Lindsay equation for the activity coefficient of NaCl is accurate from 50 to 335 C and for ionic strengths up to 100 molal. In fact, this formulation is quite accurate from 150 to 300 C and for ionic strength less than 10, and it gives reasonable results over the entire range. The above assumptions were made to simplify the complex question of activity coefficients and to provide a first order approximation that does not depend on individual ionic species. Neutral species also are assumed to have the same activity coefficient, o, and the activity of liquid water is estimated using a correlation.

2.3.2.6. PHREEQE

The geochemical code PHREEQE (52) was originally written to model the interactions of natural waters with mineral phases and simulate the chemical speciation of the aqueous solution. The program calculates the equilibrium water chemistry for a particular chemical inventory and associated minerals, and draws on a database of thermodynamic data. It can model a large number of aqueous chemical reactions including the mixing of solutions, the titration of one solution against another, and the addition of a stoichiometric chemical reaction. In each of these cases, PHREEQE can simultaneously maintain the reacting solution at equilibrium with multiple phases. A series of coupled chemical equations are solved iteratively by the program to yield the following:

• The pH;
• The Eh;
• The total concentration of elements;
• The amounts of minerals (or other phases) transferred into or out of the aqueous phase;
• The distribution of aqueous species;
• The saturation state of the aqueous phase with respect to specified mineral phases.

• The Debye-Hückel correction;
• The Davies correction.

The original version of PHREEQE can handle a database of a maximum of 27 elements, 250 species and 40 minerals. A preliminary thermodynamic database is provided with the code, derived from the WATEQ2 (61) database, and includes 120 aqueous species for 19 elements and 24 mineral species. A code PICKER (62) has been developed which can select a PHREEQE-sized database for a particular model from a much larger master-database. This allows information on a large number of elements and species to be stored as a single database whilst maintaining efficiency by not increasing the array sizes in PHREEQE.

Some users of PHREEQE have modified the code for their own specific applications and some examples are given below.

The program PHRQPITZ (63) is essentially a version of the PHREEQE code which employs Pitzer's equations for the activity coefficients of the aqueous species. A modified database is provided with this version of the code.

The program HARPHRQ (64) is a modified version of PHREEQE developed at Harwell. As PHREEQE has been applied to an increasing number of problems it became a necessity to incorporate additional features into the code, as options alongside the original facilities. These include an additional choice for the method of ionic strength correction, a method of minerals accounting and the facility for fixing the pH of the solution.

A new version of the code, PHREEQC ⁽⁴⁹⁾ developed by USGS, can also be obtained from the NEA.

2.3.2.7. WATEQ4F (US Geological Survey)

WATEQ4F Version 2.1 (65) is a chemical speciation code for natural waters which also includes a self contained database of thermodynamic data. The code uses field measurements of temperature, pH, Eh, dissolved oxygen and alkalinity and the chemical analysis of a water sample as input and calculates the distribution of aqueous species, ion activities and mineral saturation indices that indicate the tendency of a water to dissolve or precipitate a set of minerals. The model assumes homogeneous aqueous phase equilibria except for redox species and equilibrium with respect to the solubility of the minerals is not assumed. The database may also be used for reaction modelling with the codes BALANCE and PHREEQE.

WATEQ4F solves a set of non-linear mass action and mass balance equations using the mathematical method known as the continued fraction method. The Davies equation is used in most cases to calculate individual ion activity coefficients for the solute species. The temperature range over which WATEQ4F can be used is 0 to 100°C but uncertainties increase considerably when there are large departures from 25°C. Calculations are performed at 1 bar pressure, although it should be noted that pressure is not a variable parameter, and for a range of salinity up to that for seawater. The database contains thermodynamic data for 650 species (and reactions) with the log K and DH of reaction stored. Data is obtained by critical assessment of the literature with internal consistency checked. Where possible the database is tested against laboratory measurements or well characterised water chemistry. Further developments of the database could lead to more data for trace elements and improvements are required for applications to higher salinity. The database is freely available from the US Geological Survey.

2.3.2.8. Other computer codes

Other closely comparable programs available include WATEQ (66), WATEQF (67), WATEQ2 (61), and WATEQ3 (68). The equilibrium condition is derived by solving the set of nonlinear equations provided by the mass law and the mole balance equations. Most of these programs have been designed to deal with specific aqueous solutions, in particular geochemical issues, and as such are difficult to extend their application to other problem areas. There are also other codes available, similar to ESP, that form part of a larger suite of programs developed for chemical process modelling, for example ASPEN PLUS.

Also of note is the SUPCRT92 code developed by Johnson et al (69) which is a software package which can be used to calculate the standard molar thermodynamic properties of minerals, gases, aqueous species and reactions from 1 to 5000 bars and 0 to 1000°C The Helgeson model, described by Oelkers et al (70), uses an alternative approach to the Pitzer model and a separate database which can be used with the SUPCRT92 code has been developed.

One of the major problems associated with developing a kinetics database is the dependence of the reaction kinetics on the environmental conditions. It is therefore difficult to apply a standard and then extrapolate from this. Also for complex systems, the number of possible interdependent reactions increases as a power series and therefore obtaining experimental rate data or estimating rate constants for a multicomponent multiphase problem can be very onerous. To overcome this lack of kinetic data, a commonly used approach in assessment studies is to perform thermodynamic calculations with chemical reactions, which are unlikely to form due to kinetic constraints, disabled. For example, in geochemical modelling, most codes consider local equilibria only. Few kinetic data are available for geochemical systems and, in most cases, meta-stable phases are hardly considered, even when they are stable for relatively long periods of time.

As revealed in the survey conducted by NEA, there are very few kinetic databases which are readily available for widespread applications. However, NIST market two chemical kinetics databases (12). The NIST Chemical Kinetics Database provides access to kinetic data for 7800 gas phase reactions, including data for 23500 rate constants, 3800 compounds and 6000 references. The database can be used to provide all of the literature on a particular reaction, all of the reactions for a particular species, subsets of all of the reactions and the data available from a given paper. Selected sets of data may also be fitted to Arrhenius equations using least squares fitting. The NDRL/NIST Solution Kinetics database provides access to rate constants for radical processes involving inorganic radicals and carbon centred organic radicals in aqueous solution and organic peroxyl radicals in various solvents. This database is derived from data evaluations at the Radiation Chemistry Centre at the University of Notre Dame and includes over 13000 entries for 9500 reactions and 7500 chemical species which may be reactants or products. The database can be used to search for reactant pairs, all reactions of a single reactant, reactions of species containing a particular element, reactions generating a particular product and author's names. The software also includes the ability to search by chemical name fragments, as well as to collect various acid-base forms of a particular species in the same search.

The HARKIN database (71) is currently being developed by AEA Technology from critically assessed or calculated rate constants for thousands of chemical reaction steps. Classes of reactions that are covered include molecule-molecule, radical-molecule, radical-radical, ion-molecule, electron collision and ion-ion both in the gaseous and liquid phases. The data are suitable for use with software such as FACSIMILE (72). The data are available commercially, but usually a customised package is prepared for customers to suit their particular problem.

Of the other responses to the survey, only one referred to the development of a kinetic database. This was the PATH.ARC.DAT database (29) developed by the Alberta Research Council where rate data for the dissolution and precipitation of ~30 minerals was stored in the form of rate constants.

The databases referred above are applicable to gas phase and aqueous systems. Software is also available which enables diffusion and the precipitation of phases to be modelled in condensed phase systems using suitable thermodynamic and kinetic databases. DICTRA (73) is a general software package for the simulation of diffusion controlled transformations in multicomponent systems of a simple geometry. Any number of components can be treated provided that the necessary thermodynamic and kinetic data are available. DICTRA is interfaced with THERMOCALC and solves multicomponent diffusion equations in the various regions of a material assuming that thermodynamic equilibrium (determined by THERMOCALC) holds locally at all phase interfaces. Applications include single phase problems such as the homogenisation of alloys; moving boundary problems including the reactions in clad steels and diffusion in dispersed systems such as the carburizing of superalloys.

The most common problems in developing models for reaction kinetics are how to treat surface catalysed reactions, reversible reactions, the identification of rate limiting steps in a multicomponent system and the formation of intermediate and meta-stable compounds. However, it is likely that kinetic data could be used in applications such as those which are relevant to the reprocessing field. Computational methods in this area are still being developed, in particular the combination of thermodynamic and kinetic data. There is also development in the use of thermodynamic packages, such as MTDATA with Computational Fluid Dynamics (CFD) codes such as FLUENT and CFX ⁽⁷⁴⁾. An example of current applications in this area include the modelling of high temperature lamp chemistry. Nuclear applications which use CFD codes, such as fuel performance modelling, may benefit from the further development of links between such codes. It is this combination for clearly defined applications and systems, where all the constraints can be identified at the start of the model development, which may yield the best progress in the future.

It is beyond the scope of this report to critically assess the merits of each database which has been described in the preceding sections. However some general comments are made here with regard to the databases and their use.

As indicated in the preceding sections, there are three main types of thermodynamic database; those for pure substances, solution phases and aqueous systems. The pure substance databases tend to be very generic and can be applied to solid and gas phase systems. In contrast, the solution databases tend to be produced for a specific purpose and are developed around specific models which represent the excess Gibbs energy in the solution phases. Generally solution databases are applied to metallic systems and oxide systems where the complex interactions between miscible phases can have a strong influence on the chemistry of the system. The aqueous databases tend to be in the form of stand-alone databases or integrated within codes and can be applied, for example, to the assessment of leaching behaviour from active waste sites.

The response to the questionnaires show that there is a wide range of substances and data covered by the existing databases and in particular by the pure substance databases. In general the data are often obtained from the same standard sources although differences between the databases arise from the different methods of critical assessment used; different sets of data being preferred over another; or the use of different reference states. Weaknesses in the databases tend to be associated more with the development of adequate models to represent, for example, the modelling of the excess Gibbs energy for particular solution phases. Also the ability to model aqueous systems is still limited at the moment to systems which are very dilute and most of the data available are restricted to temperatures near 25°C. The development of solution models to describe complex systems will be continued in areas where it is important to reduce the uncertainties associated with treating systems as ideal mixtures. However, the development of such models and databases is labour intensive and very specialised because of the effort required to optimise the thermodynamic data in order to reproduce experimental phase diagrams. For many simple applications and in particular gas phase chemistry at high temperatures the use of pure substance databases is probably adequate.

The extent to which a database is complete and therefore contains all the possible compounds that can be formed for a given set of components is referred to as coherency. Not all users of a particular database will be aware, without performing a literature review of their own, whether the database is complete or not. Performing calculations with missing compounds can lead to totally misleading results. There are two reasons why the substances may be missing. The first is that there are no data for those substances and the second is that the formation of that compound is unknown. One method of informing users of the substances that have been identified for a particular system and where assessed data may be found is to establish an international register of thermodynamic databases with a list of the substances contained in them. Ideally this would be accessible on-line through a computer network and could be managed by the NEA.

The use of estimating data is relatively common for substances where no data are available. In compilations (e.g. such as Barin) these estimated data are clearly labelled. However, it is recognised that in the case of databases supplied and used in conjunction with computer codes, the user will not be aware of whether the data are measured or estimated. Some methods are available for the estimation of data but these rely on the expertise and experience of the estimator. The reasons why data are estimated are to reduce the problems of incomplete databases, i.e. the lack of coherency referred above, but it must be recognised that estimated data have a large degree of uncertainty associated with them.

A number of the databases use CODATA values as standard references. Also most of the databases are compiled by critical assessment of data from the open literature. It is likely that many of the databases will rely on the same original sources although the format in which the data is stored and the 'fit' obtained by the critical assessment of the data may be different and therefore could lead to slight differences between the databases. In general the databases enable the Gibbs energy of the components to be either stored as a polynomial function describing its variation with temperature or calculated from DH0, S0 and a function used to describe Cp(T). For aqueous databases log K values are usually preferred.

Many of the databases have been described as being internally consistent. This means for aqueous databases in particular that a suitable reference species is used. Also in the case of solution databases it means that if data for a single component has been changed, then the phase diagrams affected by that particular component have been re-assessed to ensure that they are still also correct. In these particular cases, it can be very important not to mix data from one database with another since they may not be compatible. For example, databases such as HATCHES, which contain some data for condensed phase minerals, have a different reference state from that used by a pure substance databases, such as the SGTE Pure Substance Database, and therefore should not be used together. In general users need to be aware that databases are often developed for specific purposes and the level of review imposed on the data and the inclusion of data for less common substances will be reflected by this.

In the responses to the questionnaires it was evident that Quality Assurance procedures were being followed in both reviewing the data and in managing the databases. In the case of many of the commercial databases the data were critically evaluated by a group of experts rather than a single individual. In some instances, the data processing associated with assessing data from the literature was performed using a set of standard frozen computer codes and therefore was done in accordance with a standard procedure. The use of conventions such as the CALPHAD/SGTE method have also been referred. Changes to the databases were also typically done in a controlled manner by either restricting access to the raw database to a single database manager or using change control software to record all changes to the database.

The method of peer review and the competence of the peer reviewers is another factor which needs to be considered. For all of the databases which are freely available, the data and the review of that data are presented in documentation which is openly available. Conversely, in the case of most of the commercial databases this is not the case and the peer review process is not published. While it is recognised that a commercial supplier of databases will not readily publish their assessed data or the review process to justify their selection of data, it should be possible to include the names of the peer reviewers with the product to provide an element of accountability.

Most of the databases referred to in this report are available commercially with a few which are freely available on request. The free databases tend to be flat file structures which can be accessed by either standard database management codes such as DBASE, spreadsheet packages or word processors i.e. they are not specific to any particular computer codes. The commercial databases tend to be available in formats which can only be directly accessed by a number of computer codes and the data are in a format which supports the use of specific models used by these codes. These have the advantage that they do not need reformatting for use with these software packages, but they cannot be accessed directly as standalone databases.

It has been highlighted that there are very few kinetic databases in use and although there are some which are commercially available, it is unlikely that these are sufficiently focused on substances relevant to nuclear applications to meet practical requirements. The commercial databases are restricted to reactions in gas phase and aqueous systems and some data are available on precipitation rates in solutions. Generally those modelling complex systems using thermodynamic codes can 'disable' specific reactions or products which are unlikely to form due to kinetic constraints. High temperature gas phase chemistry is also likely to achieve near equilibrium conditions. However, there are probably a large number of applications, in particular involving non-ideal systems, which could be modelled more accurately if kinetic data were available and this is perhaps an area which needs to be highlighted and discussed further by expert groups in considering future requirements. Modelling techniques are now suitably advanced to use kinetic and thermodynamic data together as exemplified by the development of software such as DICTRA and the linking of some thermodynamic packages with CFD codes. In these areas the development of such modelling techniques further supports the need to collate kinetic data appropriate to such applications.

Following the preparation of the first draft of this paper, a meeting (75) was held at NEA headquarters to discuss aspects raised by this review. A second draft of the report was issued following the meeting. After further comments, received by correspondence, a 3rd draft was prepared; and following a wider distribution for comment of the 3rd draft to members of the Nuclear Science Committee Task Force on Chemistry and contributors to the database survey this final report has been prepared. The following recommendations were proposed during the meeting on the basis of the first draft of this review.