Thorium, a potential nuclear fuel candidate, has been recently suggested for use in some new advanced reactors designs including molten-salt reactors, high-temperature gas reactors and some accelerator-driven reactors. In most reactor applications, the oxide form of thorium is considered as a possible fuel choice, however, metallic thorium alloys, which received some interest in the early days of reactor fuel development, could provide some distinct advantages over the oxide form, such as higher thermal conductivities and higher fuel densities.
Both experimental and theoretical calculations of metallic thorium and its alloys are rare, resulting in a sizeable gap between theory and applicability. In this work, we seek to lessen that gap by probing the nature and formation of point defects in the two allotropes of metallic thorium (an FCC α-phase and a high temperature BCC β-phase). We use the Density Functional Theory (DFT) ) implemented in the Vienna Ab-Initio Simulation Package (VASP) to calculate the electronic groundstate of both phases, first determining lattice constants, elastic properties, and cohesive energies. We use a revised version of the Perdew-Burke-Ernzerhoff (PBE) generalized gradient approximation (GGA) of the exchange-correlation functional known as the RPBE. Comparing it to the PBE we find the RPBE to produce the better lattice constants and slightly better elastic constants, while the PBE produced the most accurate cohesive energies. The elastic constants of the BCC phase show it to be mechanically unstable at 0K while also having a lower cohesive energy than the FCC phase. From the elastic constants, the elastic moduli of both the FCC and BCC allotropes and the Debye temperature of the FCC allotrope are calculated, for which the RPBE and PBE results are nearly equivalent. Using the RPBE, we then introduce vacancy and interstitial point defects into the lattice and calculate defect formation energies based on the relative supercell total energies. For the FCC phase, the vacancy defect had the lowest formation energy at 2.110 eV while the octahedral position had the lowest formation energy of the interstitials at 4.502 eV. The BCC defect formation energies were lower than the FCC energies across the board with the vacancy being lowest overall at 1.232 eV and the <110> DB being the most favorable interstitial at 3.140 eV. We also investigate formation energies associated with uranium atoms in defect positions in the FCC Th lattice finding the substitutional position to be most favorable with a formation energy of 1.478 eV and the U-interstitial energies to be, in general, slightly lower than their Th self-interstitial counterparts.