Explainer-How much damage have Israeli strikes caused to Iran's nuclear programme?

By Francois Murphy

VIENNA (Reuters) -Israel has carried out wide-ranging military strikes on Iran, hitting sites including some of its most important nuclear facilities.

Below is a summary of what is known about the damage inflicted on Iran's nuclear programme, incorporating data from the last quarterly report by the U.N. nuclear watchdog on May 31.

OVERVIEW

Iran is enriching uranium to up to 60% purity. This could easily be refined further to the roughly 90% which is weapons grade.

The International Atomic Energy Agency, which inspects Iran's nuclear sites including its enrichment plants, says that is of "serious concern" because no other country has enriched to that level without producing nuclear weapons. Western powers say there is no civil justification for enrichment to that level.

Iran denies seeking nuclear weapons. It points to its right to nuclear technology for peaceful purposes, including enrichment, as a party to the nuclear Non-Proliferation Treaty.

Israel, which is not a party to the NPT, is the only country in the Middle East widely believed to have nuclear weapons. Israel does not deny or confirm that.

HEART OF THE PROGRAMME: URANIUM ENRICHMENT

Iran had three operating uranium enrichment plants when Israel began its attacks:

* The Fuel Enrichment Plant (FEP) at Natanz (power supply hit)

A vast underground facility designed to house 50,0000 centrifuges, the machines that enrich uranium.

There has long been speculation among military experts about whether Israeli airstrikes could destroy the facility given that it is several floors underground.

There are about 17,000 centrifuges installed there, of which around 13,500 were operating at last count, enriching uranium to up to 5%.

Electricity infrastructure at Natanz was destroyed, IAEA chief Rafael Grossi told the U.N. Security Council on Friday, specifically an electrical sub-station, the main electric power supply building, emergency power supply and back-up generators.

While there was no indication of a physical attack on the underground hall containing the FEP, "the loss of power ... may have damaged the centrifuges there", he said.

Grossi has cited unspecified "information available to the IAEA". While Iran has provided the agency with some information, the IAEA generally makes extensive use of satellite imagery.

* The Pilot Fuel Enrichment Plant (PFEP) at Natanz (destroyed)

The smallest and, being above-ground, the softest target of the three enrichment plants. Long a research and development centre, it used fewer centrifuges than the other plants, often connected in smaller clusters of machines known as cascades.

It did, however, have two interconnected, full-size cascades of up to 164 advanced centrifuges each, enriching uranium to up to 60%. Apart from that, there were only up to 201 centrifuges operating at the PFEP enriching to up to 2%.

Most of the research and development work attributed to the PFEP had recently been moved underground to the FEP, where more than 1,000 of its advanced centrifuges were enriching to up to 5%.

The PFEP was destroyed in the Israeli attack, Grossi said.

* The Fordow Fuel Enrichment Plant (no visible damage)

Iran's most deeply buried enrichment site, dug into a mountain, suffered no visible damage, Grossi reiterated on Monday.

While it has only about 2,000 centrifuges in operation, it produces the vast majority of Iran's uranium enriched to up to 60%, using roughly the same number of centrifuges as the PFEP did, because it feeds uranium enriched to up to 20% into those cascades compared to 5% at the PFEP.

Fordow therefore produced 166.6 kg of uranium enriched to up to 60% in the most recent quarter. According to an IAEA yardstick, that is enough in principle, if enriched further, for just under four nuclear weapons, compared to the PFEP's 19.2 kg, less than half a bomb's worth.

OTHER FACILITIES

Israeli strikes damaged four buildings at the nuclear complex at Isfahan, the IAEA has said, including the Uranium Conversion Facility (UCF) and facilities where work on uranium metal was conducted.

While it has other uses, mastering uranium metal technology is an important step in making the core of a nuclear weapon. If Iran were to try to make a nuclear weapon, it would need to take weapons-grade uranium and turn it into uranium metal.

Uranium conversion is the process by which "yellowcake" uranium is turned into uranium hexafluoride, the feedstock for centrifuges, so that it can be enriched. If the UCF is out of use, Iran will eventually run out of uranium to enrich unless it finds an outside source of uranium hexafluoride.

SCIENTISTS

At least 14 Iranian nuclear scientists have been killed in Israeli attacks since Friday, including in car bombs, two sources in the Gulf said on Sunday.

Israel's armed forces named nine of them on Saturday, saying they "played a central part of the progress toward nuclear weapons" and that "their elimination represents a significant blow to the Iranian regime's ability to acquire weapons of mass destruction". That assertion could not immediately be verified.

Western powers have often said Iran's nuclear advances provide it with an "irreversible knowledge gain", suggesting that while losing experts or facilities may slow progress, the advances are permanent.

URANIUM STOCKPILE

Iran has a large stock of uranium enriched to different levels.

As of May 17, Iran was estimated to have enough uranium enriched to up to 60% for it to make nine nuclear weapons, according to an IAEA yardstick.

At lower enrichment levels it has enough for more bombs, though it would take more effort: enough enriched to up to 20% for two more, and enough enriched to up to 5% for 11 more.

Much of Iran's most highly enriched uranium stockpile is stored at Isfahan under IAEA seal, officials have said. The IAEA does not report where it is stored, nor has it said whether it was affected by the strikes.

OPEN QUESTIONS

* How will Iran respond?

Iranian Deputy Foreign Minister Kazem Gharibabadi told state TV on Saturday Iran would take measures to protect nuclear materials and equipment that would not be notified to the IAEA and it would no longer cooperate with the IAEA as before.

Lawmakers are also preparing a bill that could prompt Iran to pull out of the NPT, following in the footsteps of North Korea, which announced its withdrawal in 2003 and went on to test nuclear weapons.

The IAEA does not know how many centrifuges Iran has outside its enrichment plants. Any further reduction in cooperation with the IAEA could increase speculation that it will or has set up a secret enrichment plant using some of that supply.

Existing centrifuge cascades can also be reconfigured to enrich to a different purity level within a week, officials have said.

* What is the status of the uranium stock?

If Iran can no longer convert, its existing stock of uranium hexafluoride and enriched uranium becomes even more important.

* How bad is the damage?

The IAEA has not yet been able to carry out inspections to assess the damage there in detail.

* Will there be more attacks?

Soon after the attacks started on Friday, U.S. President Donald Trump urged Iran to make a deal with the United States to impose fresh restrictions on its nuclear programme "before there is nothing left". Talks scheduled for June 15 were called off.

(Reporting by Francois Murphy, Editing by Timothy Heritage)