Tabulated thermochemical data have been available from a number of sources for several decades. For general substances, the most familiar have been the NIST-JANAF Thermochemical Tables54 and Thermochemi­cal Data of Pure Substances55 The data are generally given as 298.15 K values, and columns of values such as Gibbs free energy, heat, entropy, and heat capacity are listed incrementally with temperature. The NIST-JANAF Thermochemical Tables are also avail­able online through the National Institute for Stan­dards and Technology (NIST). One of the key issues in using thermochemical data is the consistency of the standard states. The current commonplace usage is that the standard state is defined as 298.15 K and 1 bar (100 kPa) pressure. Small, but potentially impor­tant, errors can arise if data with different standard states are combined, for example, values at standard state pressure of 1 atm and of 1 bar are used together.

Much of the thermochemical data compilations are currently available as computer databases. In addition to the NIST-JANAF Thermochemical Tables54 is that of the Scientific Group Thermodata Europe (SGTE),56 which is well-established and has an ongo­ing program to assess data and add new species and phases. The same is true for the databases provided by THERMODATA57 in Grenoble, France, which has compound and solution values. Another source is MALT,46 supplied by Kagaku Gijutsu-Sha in Japan, which is more limited than SGTE,56 focusing on data that directly support industry issues. There have also been databases developed specifically for nuclear applications including THERMODATA,57 which has databases for both ex-vessel applications, NUCLEA, and for mixed oxide fuel (MOX). Kurata58 has developed a limited thermochemical database focused on metallic fuels. A database dedicated to zirconium alloys of interest for nuclear applications called ZIRCOBASE59,60 is available with fully devel­oped representations of a number of zirconium- containing binary systems and some ternaries. The binaries and ternaries can be combined in generating higher order systems often with reasonably good accu­racy. An SGTE56 nuclear materials database is also available containing most ofthe gaseous species and simple compounds of interest. An advanced nuclear fuel-specific database initiated by the Commissariat a l’Energie Atomique, FUELBASE,31 and which is expected to be moved under the auspices of the Nuclear Energy Agency with the Organization for Economic Cooperation and Development, is described in more detail in Chapter 2.02, Thermodynamic and Thermophysical Properties of the Actinide Oxides.

Information on the most common compounds and, in recent years, solution phases for many important systems has become available in the liter­ature and is included in databases such as those noted above. However, much important data and models are not available for nuclear systems, which have not received the same attention as, for example, commercial steels. With advances in first princi­ples modeling, some stoichiometric compounds for which there is limited or no experimental informa­tion can have values computationally determined. This is more likely for gaseous species than for condensed phases because of the greater ease in modeling the vapor. Another approach to filling in needed data is to use simple estimation techni­ques. The heat capacity of a complex oxide can be fairly accurately represented by the linear summa­tion of the values of the constituent oxides. A linear relationship with atomic number is often seen in the enthalpy of formation of analogous compounds. These and other methods are discussed extensively in Kubaschewski et al.14

Equilibrium computational software packages typ­ically will automatically acquire the needed data from accompanying selected databases. The published and commercial databases are generally assessed, meaning that they are compatible with broadly accepted values for the systems and when used with other standard values in the database thus yield correct thermochem­ical and phase relations. However, caution is needed when using those data with additional values obtained from other sources such as published experimental or computed values so that fundamental relationships such as phase equilibria are preserved. Another very significant issue is the completeness ofthe information. A simple example is UO2 where calculations can be performed using database values for the phase, whereas in reality the phase varies in stoichiometry as UO2±x and without including a representation for the nonstoi­chiometry any conclusions will be in doubt. Given the great complexity ofthe fuel and fission product phases described in Chapter 2.01, The Actinides Elements: Properties and Characteristics; Chapter 2.02, Thermodynamic and Thermophysical Properties of the Actinide Oxides; and Chapter 2.20, Fission

Product Chemistry in Oxide Fuels, it is apparent that a thermochemical model of fuel undergoing burnup is far from complete. The metallic fuel compo­sition U-Pu-Zr is reasonably well represented,61 largely from the constituent binaries, yet the fuel after significant burnup will also contain bred actinides and fission products. Similarly, the oxide fluorite fuel phase with uranium and plutonium has perhaps been completely represented (see Chapter 2.02, Thermo­dynamic and Thermophysical Properties of the Actinide Oxides), but it too has yet to be modeled containing other TRU elements and fission products. High burnup fuels will also generate other phases, as noted in Chapter 2.20, Fission Product Chemistry in Oxide Fuels, and these too are often complex solid solutions with numerous components. Thus, the criti­cal question in thermochemical modeling is, does the database contain values and representations for all the species and phases of interest? Without inclusion of all important phases, the accuracy of any conclusions from calculations will be in question.

As noted above, most databases are assessed, which implies that the included data have been eval­uated with regard to the sources and methodologies used to obtain the data. It also implies that the data are consistent with information for other phases and species containing one or more of the same compo — nents/elements. Calculations of properties must return the appropriate relationships between phases and species (e. g., activities and phase equilibria). Thus, the use of data from multiple sources raises the specter of inconsistent values being used, leading to inaccurate representations. Assuring that the data are consistent between sources through checks of relationships such as known phase equilibria is important to maintaining confidence in the informa­tion providing accurate results.