EXPLAINER: What you need to know about enriched uranium

Peace-talks between the U.S. and Iran have put a spotlight on Iran's stockpile of roughly 440 kilograms (970 pounds) of uranium enriched to 60%, material that could be used for several nuclear weapons if further enriched.

U.S. President Donald Trump wants Iran to give up highly enriched uranium and forgo further enrichment to prevent it building a nuclear weapon. 

Iran insists it has the right to enrich uranium for civilian purposes and denies seeking a bomb.

A senior Trump administration official said Washington’s redlines included ending uranium enrichment, dismantling major enrichment facilities, recovering highly enriched uranium and accepting a broader de‑escalation framework involving regional allies.

But what is enriched uranium, and how would it be transported?

WHAT IS ENRICHED URANIUM

Enriched uranium is uranium with a higher added proportion of a specific isotope - uranium 235. Natural uranium ore possesses around 0.7% of uranium 235, and around 99.3% of a different isotope - uranium 238.

U-235 is what is needed for enriched uranium, as it breaks apart more easily than U-238, said Darya Dolzikova, Senior Research Fellow at the Royal United Services Institute's Proliferation and Nuclear Policy programme. 

"That's what enrichment is, it's increasing the percentage of uranium 235, the fissile isotope that can sustain a chain reaction within the uranium," she told Reuters. 

WHY DOES URANIUM NEED TO BE ENRICHED 

Enriched uranium is not just for nuclear weapons, but also for nuclear energy. 

For energy production, there needs to be between 3-5% of U-235 in the uranium, which would be the levels for civil nuclear reactors.

"For a nuclear weapon, we're talking about closer to 90% uranium 235," Dolzikova explained. 

When uranium contains 20% or more uranium‑235, it is considered highly enriched.

HOW IS URANIUM ENRICHED

Dolzikova explained that, while there are other methods for uranium enrichment, Iran was using centrifuges. 

"It's a very long tube," she said. 

Multiple centrifuges are linked together in what are known as “cascades”. Uranium hexafluoride — the gaseous form of uranium — is then inserted.

The centrifuges spin at extremely high speeds, and during that process, they gradually separate the uranium‑238 from the uranium‑235.

"So by passing that uranium hexafluoride through the cascades of centrifuges, that's how you start accumulating greater percentages of uranium 235 in the output material," Dolzikova said. 

HOW IS ENRICHED URANIUM TRANSPORTED

Enriched uranium for energy producing - between 3-5% - is moved around the world on a regular basis. 

In terms of what is needed to move the material, Dolzikova said specialized containers and "highly protected" specialized vehicles are used. 

The uranium would be moved in "large casks," she said. 

For highly enriched uranium, the process is much more complex. It would need much smaller containers, as well as needing to be packed and sealed in a very precise way to prevent any shaking or water droplets getting inside the casks. 

"Because of the concentration of uranium 235 in it, you run a much higher risk of criticality, essentially an explosion of that material, which would not be the same as nuclear weapon explosion, but is still a concern," Dolzikova said. 

"It's not a simple throw it on the back of a truck and move it situation."

She also said that the enriched uranium in Iran throws up extra complications. "We're not 100% sure where the material is or what state it's in." 

-Production: Hannah Ellison/Reuters