Simulation of uranium mononitride spent fuel: A thermodynamic approach

Abstract

Uranium mononitride (UN) is a potential versatile fuel for use in both thermal and fast spectrum reactors. Knowledge of the thermodynamic properties of actinides and fission products in spent UN fuel is required to understand their properties such as phase stability and retention during long term storage and disposal. The present study reviews thermodynamic data and calculates the free energies of formation (ΔfGm) of the nitrides and other components formed in the spent fuel to predict the actinide and fission product behaviour.An End of Life (EoL) spent fuel inventory was calculated for a high burnup UN fuel (60 MWd kg−1) using the FISPIN fuel inventory code. The spent fuel consisted predominantly of a solid solution of nitrides (U, An, Ln, Y, Zr, Nb)N forming a single homogeneous and stable phase as expected from the ΔfGm variation with T of its components or of the fuel as calculated for an ideal solid solution. Since δΔfGm/δT > 0 for the reduction: MN ⇔ M + ½ N2, dissociation is consequently more likely at high temperature.Other fission products are expected to be divalent (Ba, Sr), monovalent (Cs, Rb) or non-valent (Tc, Ru, Rh, Pd) as well as noble gases (Xe, Kr), and halides (I, Br) which may form nano-precipitates (e.g. with metal ions). The behaviour of Mo is more complex. At low fuel temperature (<1100 K) it may form nitride precipitates while at higher temperature (>1100 K) MoN and MoN0.5 decompose to Mo metal. The stoichiometry of the spent fuel is related to the burn-up and the temperature of the fuel during operation. It is also shown to be dependent on the molybdenum species generated in pile (metal or nitride precipitate type).Thermodynamic calculations for potential Pellet Clad Interaction (PCI) showed that with Zr alloys interactions are expected while with stainless steel clad no reaction between the steel components (Fe, Ni, and Cr) and UN is expected. Finally, the free energy evaluation for UN hydrolysis shows that UN reaction with water is spontaneous.