The clustering behavior of helium atoms in thorium dioxide has been investigated by first-principles calculations.The results show that He atoms tend to form a cluster around an octahedral interstitial site (OIS).As the concentration of He atoms in ThO2 increases,the strain induced by the He atoms increases and the octahedral interstitial site is not large enough to accommodate a large cluster,such as a He hexamer.We considered three different Schottky defect (SD) configurations (SD1,SD2,and SD3).When He atoms are located in the SD sites,the strain induced by the He atoms is released and the incorporation and binding energies decrease.The He trimer is the most stable cluster in SD1.Large He clusters,such as a He hexamer,are also stable in the SDs.