It calls for Iran to trade about 80 percent of its domestically produced low-grade uranium for more highly enriched nuclear fuel from France and Russia.

But Iran's response to Brazil's offer to serve as a neutral repository for Iran's stocks of enriched fuel as part of an attempt to revive a mooted solution to the nuclear stalemate has been vague.

Scientists have long sought easier ways to make the costly material known as enriched uranium - the fuel of nuclear reactors and bombs, now produced only in giant industrial plants.