Updated December 28, 20157:57 PM ET Published December 28, 20155:28 PM ET
A ship loaded with more than 25,000 pounds of low-enriched uranium left Iran for Russia Monday as part of a deal aimed to limit Iran's nuclear program.
"The uranium was not bomb grade, but the U.S. worried Iran could have used it to make a bomb in a matter of months if it had chosen to," NPR's Geoff Brumfiel reports.
In a statement, Secretary of State John Kerry said the move was "one of the most significant steps" in fulfilling an agreement between Iran and six world powers — called the Joint Comprehensive Plan of Action — that was struck in July.
"The shipment today more than triples our previous 2-3 month breakout timeline for Iran to acquire enough weapons grade uranium for one weapon, and is an important piece of the technical equation that ensures an eventual breakout time of at least one year by Implementation Day."
"Implementation day" will happen when the International Atomic Energy Agency verifies that Iran has complied with all of the nuclear commitments included in the nuclear deal. At that time, the "breakout time," or the amount of time Iran would need to obtain enough nuclear material to make a nuclear weapon, will be one year. Also on implementation day, "roughly $100 billion in Iranian assets will be unfrozen, and the country will be free to sell oil on world markets and operate in the world financial system," the New York Times reports.
In exchange for relief from crippling sanctions, Iran must agree to inspection by the IAEA.