BSTRACTNanocrystalline metals are considered highly radiation-resistant materials due to their large grain boundary areas. Here

the existence of a grain size threshold for enhanced irradiation resistance in high-temperature helium-irradiated nanocrystalline and ultrafine tungsten is demonstrated. Average bubble density

projected bubble area and the corresponding change in volume were measured via transmission electron microscopy and plotted as a function of grain size for two ion fluences. Nanocrystalline grains of less than 35 nm size possess ∼10–20 times lower change in volume than ultrafine grains and this is discussed in terms of the grain boundaries defect sink efficiency.