Initial stage of helium clustering in UN and PuN

Helium bubbles degrade fuel performance and integrity. Using spin-polarized DFT simulations, we examine the incorporation of single and multiple He atoms at defect sites in UN and PuN and thus possible initial stages of cluster formation. While all incorporation energies are unfavorable, the order of preference, starting with the least favorable is: interstitial, nitrogen vacancy, uranium or plutonium vacancy, di-vacancy (consisting of a nitrogen vacancy adjacent to a uranium or plutonium vacancy). Incorporation energies at equivalent sites are less unfavorable in PuN than in UN. Vacancy sites act as strong traps for interstitial He, and the di-vacancy will trap multiple He initially in a nitrogen vacancy, as will the uranium vacancy in UN but not the plutonium vacancy in PuN. While this study has identified differences between accommodation energies in UN and PuN, in future, it will be necessary to identify the transition states between trap sites in order that rates can be investigated.