A nuclear island is a complex of buildings and structures that ensure the functioning of a nuclear reactor of an NPP.
"God willing, the first concrete pouring of the nuclear island will be carried out this year at the Karun nuclear power plant ... I hope [it will happen] in the fall," Eslami was quoted as saying by the Iranian government on Telegram.
Pouring concrete into the foundation of the nuclear island means a full-fledged start of construction not only of the reactor building, but also of the entire NPP power unit.