The NEA-TDB system

One of the principal objectives of the NEA-TDB development effort is to provide an idea of the uncertainties associated with the data selected in the reviews. In general the uncertainties should define the range within which the corresponding data can be reproduced with a probability of 95%. In many cases, a full statistical treatment is limited or impossible due to the availability of only one or a few data points. Appendix C describes in detail the procedures used for the assignment and treatment of uncertainties, as well as the propagation of errors and the standard mles for rounding. 

A database system has been developed at the NEA Data Bank that allows the storage of thermodynamic parameters for individual species as well as for reactions. The stracture of the database system allows consistent derivation of thermodynamic data for individual species from reaction data at standard conditions, as well as internal recalculations of data at standard conditions. If a selected value is changed, all the dependent values will be recalculated consistently. The maintenance of consistency of all the selected data, including their uncertainties (c/ Appendix C), is ensured by the software developed for this purpose at the NEA Data Bank. The literature sources of the data are also stored in the database. 

The following thermodynamic parameters, valid at the reference temperature of 298.15 K and at the standard pressure of 1 bar, are stored in the database  

For aqueous neutral species and ions, the values of AjG°, Af//°, 5 ° and correspond to the standard partial molar quantities, and for individual aqueous ions they are relative quantities, defined with respect to the aqueous hydrogen ion, according to the convention [1989COX/WAG] that Af//°(H, 7) = 0 and that S°(H, T) = 0. Furthermore, for an ionised solute B containing any number of different cations and anions  

As the thermodynamic parameters vary as a function of temperature, provision is made for including the compilation of the coefficients of empirical temperature functions for these data, as well as the temperature ranges over which they are valid. In many cases the thermodynamic data measured or calculated at several temperatures were published for a particular species, rather than the deduced temperature functions. In these cases, a linear regression method is used in this review to obtain the most significant coefficients of the following empirical function for a thermodynamic parameter, X  

AfG° the standard molar Gibbs energy of formation from the elements in 

Most temperature variations can be described with three or four parameters, a, b and e being the ones most frequently used. In the present review, only C° T), i.e., the thermal functions of the heat capacities of individual species, are considered and stored in the data base. They refer to the relation  

Af//° the standard molar enthalpy of formation from the elements in their reference states (kJ-mol ) 

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All the selected data presented in Table lll-l. Table III-2, Table IV-1 and Table IV-2 are internally consistent. This consistency is maintained by the internal consistency verification and recalculation software developed at the NEA Data Bank in conjunction with the NEA-TDB data base system, cf. Section II.6. Therefore, when using... 

This is the seventh volume of a series of expert reviews of the chemical thermodynamics of key chemical elements in nuclear technology and waste management. This volume is devoted to the inorganic species and compounds of selenium. The tables contained in Chapters 111 and IV list the currently selected thermodynamic values within the NEA-TDB Project. The database system developed at the NEA Data Bank, see Section 11.6, assures consistency among all the selected and auxiliary data sets. 

GAM/BUG] and [2005OLI/NOL]), and their use is recommended by this review. Zirconium forms compounds with many of the elements of the Periodic Table. The sources of auxiliary data mentioned do not eontain all data that were needed. Additional auxiliary data were obtained from the literature and, when used, the source has been indicated in Chapter VI. Recalculation of data is presented in Appendix A and, where necessary, the re-evaluation has involved new auxiliary data when needed. Care has been taken that all the selected thermodynamic data at standard conditions are internally consistent. For this purpose, special software has been developed at the NEA Data Bank that is operated in conjunction with the NEA-TDB data base system, cf. Section II.6. To maintain consistency in the application of the values selected by this review, it is essential to use these auxiliary data. 

The need to make available a comprehensive, internationally recognised and quality-assured chemical thermodynamic database that meets the modeling requirements for the safety assessment of radioactive waste disposal systems prompted the Radioactive Waste Management Committee (RWMC) of the OECD Nuclear Energy Agency (NEA) to launch in 1984 the Thermochemical Database Project (NEA-TDB) and to foster its continuation as a semi-autonomous project known as NEA-TDB Phase 11 in 1998. 

In neither case should the selection of a value for the enthalpy of solution be taken to mean that it is certain that the solid is a stable phase in the Ni(N03)2-H20 system. These selections yield, using NEA-TDB auxiliary data ... 

These programs are able to model the geological systems soil/rock-aqueous solution systems that is the concentration and distribution of the thermodynamically stable species can be determined based on the total concentrations of the components and the parameters just mentioned. In addition, the programs can also be used to estimate thermodynamic equilibrium constants and/or surface parameters from the concentrations of the species determined through experiments. Thermodynamic equilibrium constants can be found in tables (Pourbaix 1966) or databases (e.g., Common Thermodynamic Database Project, CHESS, MINTEQ, Visual MINTEQ, NEA Thermodynamical Data Base Project (TDB), JESS, Thermo-Calc Databases). Some programs (e.g., NETPATH, PHREEQC) also consider the flowing parameters. 

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