FIRST PRINCIPLES ATOMISTIC MODELLING OF NUCLEAR FUELS

E. Kotomin, Yu. Zhukovskii, Yu. Mastrikov, D. Gryaznov, and D. Bocharov,
P. Van Uffelen (EC Joint Research Center, Institute for Transuranium Elements, Karlsruhe, Germany).


Actinide nitrides are promising as advanced nuclear fuels for future fast reactors, since they exhibit higher thermal conductivity and higher metal density over the oxides, most commonly used to fabricate commercial nuclear fuels so far. To predict fuel performance under different operation conditions and understand the evolution as spent fuel over long times in a repository, it is necessary to study the defect-induced processes caused by material self-irradiation and the accumulation of fission products. At the same time, handling and disposal of new nuclear materials, including uranium nitride, require a deeper knowledge of the surface reactivity. Since UN samples contain considerable amounts of O impurities, which affect fuel properties, it is necessary to understand mechanism of oxygen adsorption and further penetration of O inside the uranium nitride. In collaboration with Institute for Transuranium Elements, Karlsruhe, we have performed DFT plane-wave calculations on perfect and defective UN fcc crystal using computer code VASP 4.6 with the Perdew-Wang-91 GGA non-local exchange-correlation functional employed and the scalar relativistic PAW pseudopotentials representing the core electrons of U and N atoms. Neutral vacancies were modelled by removing a U (N) atom from the supercell, the Frenkel and Schottky defect pairs were also modeled. Nitrogen Frenkel defects were described by moving a N atom from a regular site into the interstitial position in the cube center. Results of our calculations reproduce quite well the basic properties (lattice constant a0, bulk modulus and cohesive energy) of pure UN. The calculated effective (Bader) atomic charges are ±1.6 e, which is indicative of the complex chemical bonding, with covalency contributions due to U 5f and N 2p orbital hybridization. We calculated defect formation energies, changes of the macroscopic lattice parameter and local lattice distortions. The formation energies for intrinsic Frenkel and Schottky defect pairs using a 4x4x4 supercells were found 4.6 eV and 3.8 eV, respectively. Consequently, the lattice relaxation energy associated with the Frenkel pair is quite substantial (2.3 eV) while that for the Schottky pair is only 1.7 eV. Charged defects and electrons/holes ''see'' U vacancy as negatively charged whereas N vacancy looks as almost neutral. This makes a considerable difference in the rates of diffusion-controlled reactions with participation of these basic defects. We have also performed DFT calculations of the atomic and electronic structure of perfect and defective UN substrate as well as the early stages of surface oxidation. The focus was placed on a study of (i) (001) substrate relaxation, (ii) basic properties of surface point defects and adsorbed oxygen (including dissociation of O2 molecules at surface), and (iii) modification of substrate properties as a result of defect formation and oxygen adsorption. We have considered supercells with 2x2 and 3x3 extensions (i.e., adsorbate surfaced density was 0.25 and 0.11 ML, respectively). To reduce computational efforts, we have considered symmetric two-side arrangement of both point defects and adatoms. The surface relaxation was studied in detail. The lattice relaxation energy of 1.35 eV calculated for the surface N vacancies, VN at the 0.25 ML concentration is twice as larger as that in the bulk (0.7 eV). The formation energy of the surface vacancy is also smaller than in the bulk (8.9 vs. 9.1 eV, respectively). This indicates that vacancies would like to segregate to the UN grain boundaries. The local lattice deformation around surface N vacancy is also larger than that for VN in the UN bulk. Due to metallic-covalent chemical bonding in uranium nitride, we observe high affinity of atomic oxygen towards the UN (001) substrate: the binding energies are found to be 8.1 and 7.1 eV per adatom atop surface U or N atoms, respectively. Lastly, we found that O adatom atop surface U and N ions transforms into O- ion.

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