Helium Trapping And Clustering In ThO2

Helium, generated in nuclear fuel, accomodates into bubbles and degrades mechanical stability. Atomic scale simulations were used to study the interaction of He atoms with point defects and defect clusters. The incorporation of a single He atom was unfavourable at an octahedral interstitial site by 0.92 eV compared to the gas phase reference state, by 0.68 eV at an oxygen vacancy and by 0.32 eV at a Th vacancy. The decreasing values reflect the greater space available for the inert He atom. Defect clusters consisting of multiple oxygen and Th vacancies provide more space. Thus, incorporation at a di-vacancy required 0.31 eV, at a neutral tri-vacancy (NTV) 0.25 eV and at a tetra-vacancy 0.01 eV. Clusters formed of two and three NTVs exhibited no energy penalty for the incorporation of multiple He atoms. Relative to incorporation at an interstitial site, clusters offer space for multiple He and may therefore be effective traps to form proto-bubbles. A relationship was generated that describes the incorporation energy of the xth He atom, Ex (n, m), into a cluster consisting of n thorium vacancies and m oxygen vacancies. Solution energies for He, where equilibrium with the solution site is taken into account, were also determined.

Highlights

• Atomic scale simulations based on density functional theory with dispersion corrections (DFT + D) have been employed to systematically
study the stability of single He atoms at pre-existing neutral and charged point defects, smaller and larger defect clusters.

• Incorporation of He at octahedral interstitial site is energetically unfavourable compared to gas phase He. Oxygen and thorium vacancies
offer more favourable He incorporation sites but still demand 0.68 eV and 0.32 eV respectively compared to the gas phase.

• The energy penalty to incorporate a He into the charged tetra vacancy (CTV) is almost zero. Multiple neutral tri vacancy (NTV) clusters
accommodate multiple He atoms with no energy penalty.

• A relationship was generated that describes the incorporation energy of the xth He atom, Ex (n, m), into a cluster consisting of n
thorium vacancies and m oxygen vacancies.

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