Thermodynamic treatment of uranium dioxide based nuclear fuel

Abstract

Many projects involving nuclear fuel rest on a quantitative understanding of the co-existing phases at various stages of burnup. Since the fission products have considerably different abilities to chemically associate with oxygen, and the metal-to-oxygen molar ratio is necessarily increasing, the chemical potential of oxygen is a function of burnup. Concurrently, well-recognized small fractions of new phases such as inert gas, noble metals, zirconates, etc. also develop. To further complicate matters, the dominant UO2 fuel phase may be non-stoichiometric and most of the minor phases themselves have a variable composition dependent on temperature and possible contact with the coolant in the event of a sheathing breach.A thermodynamic database has been in development to predict the phases in partially burned CANDU (CANada Deuterium Uranium) nuclear fuel containing the major fission products. The building blocks are the standard Gibbs energies of formation of the many possible compounds expressed as a function of temperature. To these data are added mixing terms associated with the appearance of the component species in particular phases. In operational terms, the treatment rests on the ability to minimize the Gibbs energy in a multicomponent system using the algorithms developed by Eriksson. The treatment, considered applicable in the range 300 to 2000 °C, is capable of handling non-stoichiometry in the UO2 fluorite phase, dilute solution behaviour of significant solute oxides, noble metal inclusions, a second metal solid solution U(Pd – Rh – Ru)3, zirconate, molybdate, and uranate solutions as well as other minor solid phases, and volatile gaseous species. The paper highlights the current capability of an ongoing project.

Implementation of A Gibbs Energy Minimizer In A Fission-Product Release Computer Program

“…That model was based on lookup tables generated with the FACT (Facility for the Analysis of Chemical Thermodynamics) computer program [15][16][17] and was tractable on computers of that day. The Royal Military College of Canada (RMC) model of irradiated uranium dioxide fuel, its fission and activation products has been expanded [18], as most recently documented by Corcoran [19] and Piro [20]. The advantage of the new model is that many of the chemical elements produced as activation or fission products during irradiation of UO 2 fuel are treated.…”

“…The phases include pure substances, ideal solutions and non-ideal solutions. The data file used to describe the RMC thermochemical model is based on earlier work by a large number of contributors at the Royal Military College of Canada [18][19][20].…”

“…FACT [15][16][17] has evolved into FactSage [21] and the thermodynamic solver is available as ChemApp [22], which consists of a library of subroutines that can be called from users' computer programs. [18][19][20], the commercial ChemApp user-callable thermodynamics subroutine library [22] and supporting subroutines and modules. Incidental changes were required to support this work.…”

“…The revised nuclide set consists of 18 actinides and 174 fission products (see Table 1 Kr. elements modelled in the RMC thermochemical model [18][19][20] is within 20% for all chemical elements, and is frequently much better. This comparison is documented in [23].…”

Overview of high-temperature fuel behaviour with relevance to CANDU fuel

“…This U-O phase diagram can be used to provide the thermodynamic boundary conditions for models, which account for the fuel oxidation kinetics for fuel possibly containing a molten core [71]. In addition, a multi-component thermodynamic treatment can be further advanced to evaluate how the melting temperature would be affected with the addition of fission products and dopant material [72][73][74].…”