What is uranium enrichment and how quickly could Iran build a nuclear bomb?

United States President Donald Trump has claimed that a new nuclear deal being negotiated with Iran will be “far better” than the 2015 Joint Comprehensive Plan of Action (JCPOA), which the US withdrew from in 2018 during his first term.

On Tuesday, Trump extended the two-week ceasefire with Iran a day before it was set to expire, with hopes for a second round of talks in Islamabad, Pakistan.
Key among the US demands is that Iran stop all enrichment of uranium.

Iran has always insisted its nuclear programme is for civilian use only, such as for power generation, which requires uranium enrichment of between 3 percent and 5 percent. To build nuclear weapons, uranium needs to be enriched to 90 percent.

In this explainer, we visualise what uranium is, how it is enriched and how long it could take Iran to make a nuclear weapon.

What is uranium, and which countries have it?
Uranium is a dense metal used as a fuel in nuclear reactors and weapons. It is naturally radioactive and usually found in low concentrations in rocks, soil and even seawater. About 90 percent of the world’s uranium is produced in just five countries: Kazakhstan, Canada, Namibia, Australia and Uzbekistan. Reserves of uranium have also been found in other countries.

Uranium is extracted either by digging it out of the ground or, more commonly, through a chemical process that dissolves uranium from within the rock.
Before it can be used as nuclear fuel, uranium is processed through several different forms, including:

Yellowcake: Mined ore is crushed and treated with chemicals to form a coarse powder known as yellowcake, which, irrespective of its name, is usually dark green or charcoal in colour, depending on how hot it has been treated.
Uranium tetrafluoride: Yellowcake is then treated with hydrogen fluoride gas, which turns it into emerald-green crystals known as uranium tetrafluoride or green salt.
Uranium hexafluoride: Green salt is further fluorinated to create a solid white crystal known as uranium hexafluoride. When heated slightly, this crystal turns into a gas, making it ready for enrichment.
Uranium dioxide: The gas is spun in a centrifuge machine, which chemically converts it into a fine, black powder.
Fuel pellets: The black powder is pressed to form black ceramic pellets, which can then be used in a nuclear reactor.