Thermodynamic Modeling of Uranium Nitride with Select Actinides and Fission Products

Uranium mononitride fuel is being investigated as a potential nuclear fuel for a Westinghouse lead-cooled fast reactor due to factors such as its high uranium density, high melting point, and favorable thermal conductivity. To obtain an understanding of the thermochemical behavior of UN fission products and minor actinides, thermodynamic data was obtained from various resources to create a database to represent the behavior of the fuel. This database was used to generate an Ellingham diagram to represent the relative thermodynamic stability of the phases over the reactor temperature range. A simulation was run computing the equilibrium state of the fuel, using the model comprised of all the potential phases based on the fuel’s composition at a burnup of 60MWd/kgU, over the temperature range of 800K-2100K to observe the formation of these phases. This model simulation yielded information on what solutions were present, the amount of these solutions, and their composition at each temperature step. In analyzing the results from the simulation, at a temperature of 1520K the sesquinitride phase ceases to be present, the FCC phase reaches its maximum concentration at 1520K before slowly decreasing over the remainder of the temperature range, and the content of UN in the reactor reaches its peak value at 1540K and maintains it. Additionally, more research is needed into the fission product nitride systems to understand their interaction parameters within the FCC solution so as not to be assumed an ideal solution and to gather data on the systems that were unable to be represented (and those that were used as surrogates) to provide a more accurate model of the fuel.