Ab initio DFT+U study of He atom incorporation into UO2 crystals

TL;DR

This study uses advanced DFT+U calculations to analyze helium atom incorporation in UO2 crystals, revealing how various computational factors influence incorporation energies and electronic states, with implications for nuclear materials.

Contribution

It provides a detailed ab initio analysis of He incorporation into UO2, highlighting the effects of different functionals, supercell sizes, and electronic states on incorporation energies and crystal properties.

Findings

He incorporation energy varies with computational parameters.

Jahn-Teller effect influences UO2 symmetry and properties.

Restoring semiconducting state affects energy estimates.

Abstract

We present and discuss results of a density functional theory (DFT) study of a perfect UO2 crystals and He atoms in octahedral interstitial positions. We have calculated basic bulk crystal properties and He incorporation energies into the low temperature anti-ferromagnetic UO2 phase using several exchange-correlation functionals within the spin-polarized local density (LDA) and generalized gradient (GGA) approximations. In all these DFT calculations we included the on-site correlation corrections using the Hubbard model (DFT+U approach). We analysed a potential crystalline symmetry reduction and confirmed the presence of the Jahn-Teller effect in a perfect UO2. We discuss also the problem of a conducting electronic state arising when He is placed into a tetragonal antiferromagnetic phase of UO2. Consequently, we found a specific lattice distortion which allows us to restore the semiconducting state and properly estimate He incorporation energies. Unlike the bulk properties, the He incorporation energy strongly depends on several factors, including the supercell size, the use of spin polarization, the exchange-correlation functionals and on-site correlation corrections. We compare our results for the He incorporation with the previous shell model and ab initio DFT calculations.