Stability of fission products in ThO₂: an ab-initio study

Study of fission products in thorium dioxide (ThO₂) is an important field of research in nuclear chemistry. Here we report an ab-initio study on the structural stability, energetics and clustering behavior of platinum group fission products (Ru, Rh, Pd, Ir and Pt) in ThO₂ using plane wave based density functional theory. Energetics calculations were performed considering the possible insertion sites in ThO₂, mainly in interstitial sites, thorium vacancies, oxygen vacancies, oxygen thorium divacancy, and three types of Schottky defects. The thorium vacancy is the most energetically favourable trap site for all of these fission products with incorporation energies ranging from -11.82 to -6.94 eV. All the fission products Ru, Rh, Pd, Ir and Pt have general tendency to form metallic clusters in ThO₂. Moreover, Ru and Ir dimers are the most stable clusters in the thorium and interstitial vacancy positions in ThO₂, suggesting that metal clusters in ThO₂ may be formed of these dimers trapped in those vacancy sites. The clustering behavior of the fission products (dimer and trimer) in different Schottky defects has also been investigated