  The uranium mononitride (UN) is a material considered as a promising
candidate for Generation-IV nuclear reactor fuels. Unfortunately, oxygen in air
affects UN fuel performance and stability. Therefore, it is necessary to
understand the mechanism of oxygen adsorption and further UN oxidation in the
bulk and at surface. Recently, we performed a detailed study on oxygen
interaction with UN surface using density functional theory (DFT) calculations.
We were able to identify an atomistic mechanism of UN surface oxidation
consisting of several important steps, starting with the oxygen molecule
dissociation and finishing with oxygen atom incorporation into vacancies on the
surface. However, in reality most of processes occur at the interfaces and on
UN grain boundaries. In this study, we present the results of first DFT
calculations on O behaviour inside UN grain boundaries performed using GGA
exchange-correlation functional PW91 as implemented into the VASP computer
code. We consider a simple interface (310)[001](36.8{\deg}) tilt grain
boundary. The N vacancy formation energies and energies of O incorporation into
pre-existing vacancies in the grain boundaries as well as O solution energies
were compared with those obtained for the UN (001) and (110) surfaces.
