Our recent computational work using density functional theory on “Fission gas in thoria” has been accepted and is now available from the Journal of Nuclear Materials.

• Navaratnarajah Kuganathan, Partha S. Ghosh, Conor OT. Galvin, Ashok K. Arya, Bijon K. Dutta, Gautam K. Dey, Robin W. Grimes, “Fission gas in thoria”, J. Nucl. Mater., 485 (2017) 47–55. doi:10.1016/j.jnucmat.2016.12.011

This study uses first-principles density functional theory together with a dispersion correction (DFT+D) to calculate the energetics of neutral and charged point defects and defect cluster geometries in ThO₂. It also investigates their interaction with Xe and Kr. ThO₂ has been identified as a possible alternative nuclear fuel, partly because spent ThO₂ fuels give rise to considerably smaller inventories of minor actinides, especially Pu. Furthermore, ThO₂ is a highly stable oxide, and exhibits higher thermal conductivity, higher melting temperature, higher corrosion resistance and lower thermal expansion compared to UO₂. At high fuel temperatures, fission gas atoms migrate and are accommodated at point defect sites in the fuel matrix. Over time some of these aggregate into bubbles. Formation of bubbles is important as it leads to swelling and degrades mechanical properties of the material. In order to understand fuel performance, it is necessary to understand the interaction of gas atoms with point defects.

Our study finds the most favourable charge state for a point defect (vacancy or interstitial) is that with full ionic charge and in all cases gas atoms occupy the fully charged vacancy sites. The number of fission gas atoms accommodated in ThO₂ is linearly proportional to the number of neutral tri vacancies (NTVs) present in the system. In ThO_2-x the most favourable solution equilibrium site is the NTV1 while in ThO₂ it is the di-vacancy (DV). We compare the current solution energies for Xe in ThO₂ and ThO_2-x with values calculated previously for Xe in UO₂ and UO_2-x. In particular, while the values in UO₂ are ₅₀% are higher, the preference for solution site is identical; DV and NTV1 for UO₂ and NTV1 for UO_2-x.