When the U.S. and Israel launched the war against Iran on Feb. 28, President Donald Trump said one of the main objectives was to prevent Iran from building a nuclear weapon.
That has put a focus on Iran's stockpile of uranium and its program to convert this naturally occurring element into material that can sustain a nuclear reaction — a process called enrichment. Several key nuclear sites in Iran were attacked by the U.S. and Israel last summer, and again during the latest war.
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FILE - The International Atomic Energy Agency (IAEA) flag flies during a special session of an IAEA Board of Governors meeting in Vienna, Austria, March 2, 2026. (AP Photo/Heinz-Peter Bader, File)
FILE - In this photo released by the Iranian Presidency Office, President Masoud Pezeshkian, second right, listens to head of Atomic Energy Organization of Iran Mohammad Eslami as he visits an exhibition of Iran's nuclear achievements, in Tehran, Iran, April 9, 2025. (Iranian Presidency Office via AP, File)
FILE - A B-2 bomber arrives at Whiteman Air Force Base Mo., on June 22, 2025, after returning from a massive strike on Iranian nuclear sites. (AP Photo/David Smith, File)
FILE - In this photo released by the Atomic Energy Organization of Iran, technicians work at the Arak heavy water reactor's secondary circuit as officials and media visit the site near Arak, Iran, Dec. 23, 2019. (Atomic Energy Organization of Iran via AP, File)
Iran's leaders insist the country's nuclear program is only for peaceful purposes. Before the war began, it had a stockpile of enriched uranium that was a short, technical step from weapons-grade material, according to the International Atomic Energy Agency. Iran had an organized nuclear weapons program up until 2003, the U.N. agency and Western nations say.
Here’s a deeper look at uranium enrichment and Iran's nuclear program.
Uranium, like other elements, is found in slightly different configurations. One, called U-235, is best at powering nuclear reactions. It is less stable than the more common form, which means it’s easier to split to release the energy needed for producing electricity or a weapon. But U-235 makes up less than 1% of the uranium found in nature.
To isolate the more desirable U-235, scientists strip away its more stable cousin, U-238, through a process known as enrichment.
The most common way to enrich uranium is by spinning uranium gas in a centrifuge, where lighter U-235 separates from the heavier U-238. This is done in stages, using multiple centrifuges that enrich the uranium bit by bit.
The early stages are especially labor-intensive because there is so much more U-238 to strip away, said nuclear security specialist Sébastien Philippe at the University of Wisconsin-Madison. Over time, the enrichment process gets easier.
The process can take anywhere from days to months to years, depending on the size of the enrichment plant, the technology being used and how far the uranium is being enriched.
For a nuclear reactor that makes electricity, uranium is typically enriched up to 5%. Nuclear reactors used for science experiments run on uranium enriched up to 20%.
Anything enriched to 20% and above is considered “highly enriched” uranium and above 90% is considered weapons-grade, though some very highly enriched uranium can be used for research and medical purposes, according to the Center for Arms Control and Non-Proliferation.
Iran is thought to have 972 pounds (441 kilograms) of uranium that is enriched up to 60% purity, according to the IAEA.
A crude nuclear weapon could be made with uranium at that enrichment level, Philippe said, and it’s relatively easy to enrich it to the 90% level needed to make a bomb light enough to be carried by a missile.
Iran’s stockpile could allow Iran to build as many as 10 nuclear bombs, should it decide to weaponize its program, IAEA director general Rafael Grossi told the AP last year. He added that it doesn’t mean that Iran has such a weapon.
Once enriched, uranium gas undergoes some final processing and then is converted back into a solid, pressed into pellets and stacked into a sealed metal fuel rod that goes into a nuclear reactor.
It can also be converted to form the core of a nuclear weapon. The process of weaponization is highly complex and involves several engineering challenges, among them miniaturizing the nuclear warhead to make it small and light enough to be carried by a missile.
Over 100 countries including the U.S. and Iran have signed the Treaty on the Non-Proliferation of Nuclear Weapons, which originally went into effect in 1970 and has been periodically updated. As part of the agreement, countries without nuclear weapons agree to use nuclear material for peaceful purposes and countries that have nuclear weapons agree to a process of disarmament. The treaty involves regular onsite inspections by the IAEA to confirm that countries are complying with the treaty’s terms.
Iran was found to be out of compliance in the early 2000s due to a secret nuclear weapons program. It was in noncompliance again in June 2025 due to its failure to provide “technically credible” explanations to the IAEA about the origin of uranium particles found by inspectors at several undeclared locations in Iran.
Tehran has not allowed the IAEA access to its nuclear facilities bombed by Israel and the United States during a 12-day war in June 2025, according to a confidential IAEA report circulated to member states and seen by AP in February.
The report stressed that it “cannot verify whether Iran has suspended all enrichment-related activities,” or the “size of Iran’s uranium stockpile at the affected nuclear facilities.”
Stephanie Liechtenstein in Vienna contributed to this story.
FILE - The International Atomic Energy Agency (IAEA) flag flies during a special session of an IAEA Board of Governors meeting in Vienna, Austria, March 2, 2026. (AP Photo/Heinz-Peter Bader, File)
FILE - In this photo released by the Iranian Presidency Office, President Masoud Pezeshkian, second right, listens to head of Atomic Energy Organization of Iran Mohammad Eslami as he visits an exhibition of Iran's nuclear achievements, in Tehran, Iran, April 9, 2025. (Iranian Presidency Office via AP, File)
FILE - A B-2 bomber arrives at Whiteman Air Force Base Mo., on June 22, 2025, after returning from a massive strike on Iranian nuclear sites. (AP Photo/David Smith, File)
FILE - In this photo released by the Atomic Energy Organization of Iran, technicians work at the Arak heavy water reactor's secondary circuit as officials and media visit the site near Arak, Iran, Dec. 23, 2019. (Atomic Energy Organization of Iran via AP, File)
