The development leaves Tehran with substantially less of the 20 per cent enriched uranium that it would need to for such a purpose. Uranium at that level is only a technical step away from weapons-grade material.
The UN's International Atomic Energy Agency reported the development today in a confidential report obtained by The Associated Press.
Iran denies any interest in atomic arms. But it has agreed to some nuclear concessions in exchange for a partial lifting of sanctions crippling its economy under a deal in effect since January.