Abstract Cascade damage simulations in a 12 nm nanocrystalline Ni sample are presented. Sinks present in the sample such as grain boundaries and vacancy defects influence the movement of self-interstitial atoms (SIAs). Two temporal mechanisms of SIA activity during their movement to GBs are distinguished: replacement collision sequences and 1D/3D thermal motion. Clustering of SIAs is biased by nearby GBs, forming SIA clusters at the large limit of that seen in displacement cascades of single crystal samples for the same primary knock-on atom energy. SIA clusters containing up to six SIAs are shown to undergo 1D/3D motion due to the presence of the attractive GB sinks in contrast to the traditional picture of purely 1D motion observed for clusters of four or more SIAs in single crystal simulations.