Explainer-Key issues the US and Iran must address in nuclear talks

VIENNA, June 15 - Iran and the United States say they have reached a deal to end their war, which is expected to pave the way for talks on Iran's nuclear programme. Below is an outline of the key issues they face in those talks.

WHY THE CONCERN?

Iran's uranium-enrichment programme has long caused the U.S. and its allies to worry that Tehran is at least keeping the option open of one day producing nuclear weapons, since enriched uranium can be used to fuel power plants or make the core of a nuclear bomb.

While modern nuclear power plants generally use fuel enriched to up to 5% purity, Iran was enriching uranium to up to 60%, a short step from the roughly 90% of weapons grade, until Israel and the U.S. bombed its nuclear facilities last June.

Israel argued that Iran was getting too close to being able to produce a nuclear weapon, but the U.N. nuclear watchdog said it had no credible indication of a coordinated nuclear weapons programme in Iran.

As a party to the nuclear Non-Proliferation Treaty (NPT), Iran has the right to develop nuclear technology, including enrichment, for peaceful purposes. Iran says it would never produce nuclear weapons.

It is, however, the only country to enrich to 60% without making an atom bomb. The amount it has enriched to that level is "a matter of serious concern", the International Atomic Energy Agency has said.

WHAT REMAINS OF THE PROGRAMME?

The Israeli-U.S. bombings destroyed or badly damaged Iranian uranium-enrichment plants, but much of the enriched uranium they produced is thought to have survived.

One of U.S. President Donald Trump's top priorities is removing at least the most highly enriched material.

Prior to Israel and the U.S. launching their attacks on Iran at the end of February, the IAEA estimated that Iran had 440.9 kg of uranium enriched to up to 60%. That is enough, if enriched further, for 10 nuclear weapons, according to an IAEA yardstick.

The IAEA believes more than 200 kg has survived in a tunnel complex at Isfahan, and some more is at Natanz, IAEA chief Rafael Grossi has said.

Iran also had more material at lower enrichment levels.

The exact status of Iran's bombed sites, including the tunnel complex, is not known because Iran has not let IAEA inspectors return there, nor has it informed the IAEA of what happened to its enriched uranium stocks.

HOW DO YOU DEAL WITH THAT?

Trump has said Iran must not be able to develop a nuclear weapon. While that is almost impossible to prevent completely, it can be made much more difficult with effective constraints.

That is what the now-defunct 2015 nuclear deal between Iran and major powers did until Trump pulled the U.S. out of it in 2018. Iran retaliated by expanding its programme far beyond the deal's limits.

Any new deal will need to address these points:

* What to do with the most highly enriched uranium?

Trump has said he wants it removed and shipped out or destroyed. Iran wants neither. A compromise may be to "downblend", or dilute, it. They would then have to agree by how much and at what enrichment level it can stay in the country.

A senior Iranian official told Reuters on Sunday Iran had agreed in principle to downblend its highly enriched uranium.

* Some or no enrichment?

The U.S. has at times demanded zero enrichment in Iran. Iran says it will not give up its right to enrich. If it is allowed to do so, up to what level, and how much of the product can Iran keep? The 2015 deal specified those, as well as where Iran could enrich and with what machines.

Officials have said a moratorium on enrichment of 10 or more years is being discussed.

* How to get a full picture?

With machines and material under rubble, how do you know that you've accounted for everything and that none has been taken away? Iran also has an unknown number of uranium-enriching centrifuges stored at unknown locations. A precise inventory is vital.

* How do you police all that?

The 2015 deal granted the IAEA intrusive extra powers, including the right to conduct snap inspections at undeclared locations, which Iran revoked as that deal fell apart. Something similar is likely to be needed. REUTERS