Iran’s stockpile of enriched uranium has gained renewed attention after US President Donald Trump said Tehran had agreed to hand it over as part of a deal to end the war.
However, on Monday, Iranian Deputy Foreign Minister Saeed Khatibzadeh denied this claim. He told the AP news agency that such a notion was “impossible.”
As both sides negotiate a path to new peace talks, the future of this fabric is sure to be a key topic of debate.
But what exactly is enriched uranium and why is it so important?
What is enriched uranium?
Uranium is a natural element found in the Earth’s crust.
It is mainly composed of two isotopes: U-238 and U-235.
More than 99% of natural uranium is U-238, which does not easily sustain a nuclear chain reaction. Only about 0.7% is U-235, an isotope that splits easily, releasing energy in a process known as nuclear fission.
For uranium to be useful, the proportion of U-235 must be increased through a process called enrichment.
First, the uranium is converted into gas. This gas is introduced into centrifuges, machines that spin at extremely high speeds.
As they rotate, the heavier U-238 shifts slightly outward, while the lighter U-235 remains closer to the center.
This allows U-235 – the rarest and most useful form of uranium – to be gradually separated from the more common U-238.
This more concentrated uranium is extracted from one end of the centrifuge.
What is the difference between uranium used in nuclear reactors and that used in weapons?
Different levels of enrichment make uranium suitable for various uses.
Low-enriched uranium, which typically contains 3% to 5% U-235, is used as fuel in commercial nuclear power plants.
This level is enough to maintain a controlled chain reaction, but far more horrid than needed for a weapon.
Highly enriched uranium, with levels of 20% or more, can be used in research reactors, and military-grade uranium is typically enriched to about 90%.
At that concentration, the conditions are favorable for a nuclear reaction to go out of control almost instantly. When enough of this type of fabric is gathered, atoms begin to split extremely quickly, releasing enormous amounts of energy in a fraction of a second.
This is what differentiates the civilian and military uses of uranium: in a reactor, the fuel is slightly enriched and the reaction is deliberately slowed and carefully managed, allowing the energy to be released gradually over months or years.
In a bomb, the objective is the opposite: to provoke a rapid and instantaneous reaction.
Under a 2015 agreement with six world powers (China, France, Germany, Russia, the United States and the United Kingdom), Iran was capped at uranium enrichment at a maximum of 3.67%.
The agreement also limited its reserves to 300kg, restricted the number of centrifuges it could operate and prohibited enrichment at its underground Fordo plant.
However, in May 2018, during Donald Trump’s first term, the United States withdrew from this agreement.
Why does the level of enrichment matter?
The higher the level of enrichment, the closer the uranium will be to being usable in a nuclear weapon.
Achieving 20% enrichment represents a significant milestone, as most of the technical effort required to produce nuclear weapons-grade fabric will already be completed by then.
Transforming natural uranium into 20% enriched fabric requires thousands of repeated separation steps and a large amount of time and energy.
In comparison, enriching uranium from 20% to around 90% requires far fewer additional steps.
This means that uranium enriched to higher levels can be further refined to military grade relatively quickly.
How much uranium does Iran have?
The central issue in current negotiations is what should happen to Iran’s existing stockpile of enriched uranium.
At the beginning of the war, Iran possessed approximately 440kg of 60% enriched uranium, according to senior US officials. This fabric can be enriched relatively quickly to the 90% threshold needed to obtain nuclear weapons-grade uranium.
Iran also has approximately 1,000kg of uranium enriched to 20% and 8,500kg enriched to around 3.6%, which is usually used for civilian purposes: for energy production or for medical research.
Most of the highly enriched uranium that could be transformed into fabric for nuclear weapons is believed to be stored in Isfahan. This facility is one of three underground nuclear sites in Iran that were targeted by US and Israeli airstrikes last year.
However, the amount of highly enriched uranium stored elsewhere is unknown.
According to sources, Tehran has rejected the demand for a 20-year moratorium on nuclear enrichment. Instead, he has proposed the five-year pause he had presented before the outbreak of hostilities.
It has also refused to hand over its stockpile of 440kg of highly enriched uranium, maintaining its previous concession to dilute enriched uranium to 60%.
The director of the International Atomic Energy Agency (IAEA), Rafael Grossi, told AP in October last year that this amount, if further enriched, would be enough to make 10 nuclear bombs.
Is Iran developing a nuclear weapon?
Iran insists its nuclear facilities are completely peaceful, and the IAEA says it has found no evidence of an active nuclear weapons program.
Producing weapons-grade uranium is just one step in building a nuclear weapon. A functional bomb also requires complex additional work, including the design and assembly of a warhead and the development of a delivery system.
“Iran had developed some capability in warhead design until 2003, when it apparently stopped the program,” says Patricia Lewis, an independent expert on weapons aid.
But he adds that “following the collapse of the 2015 nuclear deal and the continued failure of negotiations for a new agreement, Iran may have decided to resume developing its warhead manufacturing capacity.”
A May 2025 assessment by the US Defense Intelligence Agency indicated that Iran could produce enough military-grade uranium for a single device in “likely less than a week.”
However, he also stated that Iran was “almost certainly not producing nuclear weapons,” although it had taken steps that could allow it to do so if it chose to do so.
Israel said it has intelligence suggesting that Iran has made “concrete progress” in developing components for a nuclear weapon.
Additional reporting by Nadia Suleman.
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