There are a variety of software packages that will perform chemical equilibrium calculations for complex systems such as nuclear fuels. These have become quite versatile, able to compute the thermal difference in specific reactions as well as determining global equilibria at uniform temperature or in an adiabatic system. They also provide output through internal postprocessors or by exporting to text or spreadsheet applications. There are also a variety of output forms including activities/partial pressures, compositions within solution phases, and amounts, which can include plotting of phase and predomi­nance diagrams. The commercial products include FactSage62 and ThermoCalc63 which also contain optimization modules that allow use of activity and phase equilibria to obtain thermochemical values and fit to models for solutions. Other products include Thermosuite,64 MTDATA,65 PANDAT,66 HSC,67 and MALT.57

1.17.5 Outlook

Computational thermodynamics as applied to nuclear materials has already substantially contribu­ted to the development of nuclear materials ranging from oxide and metal fuel processing to assessing clad alloy behavior. Yet, in both development of data and models for complex fuel and fuel-fission product systems and in the application of equilibrium calcu­lations to reactor modeling and simulation, there is much to accomplish. Databases containing accu­rate representations of both metallic and oxide fuels with minor actinides are lacking, and even less is known about more advanced fuel concepts such as carbide and nitride fuels. Representations for mul­tielement fission products dissolved in fuel phases or as secondary phases generated after considerable burnup are also unavailable, although some simple binary and ternary systems have been determined. These are critically needed as they will help govern activities in metal and oxide fuels, influencing ther­mal conductivity and providing source terms for transport of important species such as those contain­ing iodine.

The other broad area that needs significant atten­tion is the development of algorithms for computing chemical equilibria. Although there are robust and accurate codes for computing equilibria within the software packages discussed in Section 1.17.6, these suffer from relatively slow execution. That is not a problem for the codes noted above where only a few calculations are required at any time. However, incorporation of equilibrium state calculations in broad fuel modeling and simulation codes with millions ofnodes to determine the spatial distribution of phases, solution compositions (e. g., local O/M in oxide fuel), and local activities poses a different prob­lem. Current algorithms are far too slow for such use, and therefore, new techniques need to be developed to accomplish these calculations within the larger modeling and simulation codes.68

Acknowledgments

The author wishes to thank Steve Zinkle, Stewart Voit, and Roger Stoller for their valuable comments. Research supported by the U.S. Department of Energy, Office of Nuclear Energy, under the Fuel Cycle Research and Development and Nuclear Energy Advanced Modeling and Simulation Pro­grams. This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05- 00OR22725 with the U. S. Department of Energy. The U. S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U. S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manu­script, or allow others to do so, for U. S. Government purposes.