What nuclear technology does Iran have?
Iran has an almost complete nuclear fuel cycle – the infrastructure to manage the entire process from digging the uranium out of the ground to generating power with it. That includes uranium mines and processing plants to refine the ore into yellowcake. In Isfahan it has a uranium conversion facility that turns the yellowcake into uranium hexafluoride, a gas that can be enriched with the use of centrifuges. It has about 16,000 centrifuges in a big enrichment plant in Natanz and an underground facility at Fordow, and they produce both low-enriched uranium (LEU) and 20%-enriched uranium. The enriched uranium is taken to a fuel fabrication plant in Isfahan to be turned into oxide pellets, which are put in metal tubes to make fuel rods.
Iran has two reactors: a nuclear power station in Bushehr on the Gulf coast that was built with Russian help and uses Russian-supplied fuel rods; and a research reactor in Tehran that uses 20%-enriched uranium fuel and produces isotopes for medical, agricultural and other scientific uses. Iran has built a plant to make heavy water (deuterium oxide) in Arak and is building a reactor there that would use natural uranium fuel in the heavy water for its fissile reaction.
It does not have a reprocessing plant, the last link in the fuel cycle, which would extract plutonium from the spent fuel from the Arak reactor. That is important as plutonium is an efficient fuel for making a warhead.
How does the Geneva deal restrict it?
The deal would stop Iran making any uranium enriched to above 5% purity. It would dilute its roughly 200kg stockpile of 20%-enriched uranium, the most immediate proliferation concern, or convert it to oxide which is harder to turn back into the hexafluoride gas, which can in turn be enriched further to make weapons-grade fuel. Iran undertakes not to build a plant for re-converting uranium oxide back to uranium hexafluoride.
It would also not be allowed to increase its stockpile of 3.5%-enriched uranium, so any that is produced during the six months of the interim agreement would have to be blended down or converted to oxide.
Iran would also have to freeze its enrichment capacity. It would not be allowed to install any new centrifuges and would be able to use only the roughly 8,000 which are currently being used for enrichment. The other 8,000 that have been installed but are not being fed with uranium hexafluoride would have to be "left inoperable".
Major development work at the Arak heavy water reactor would be suspended for six months. No fuel could be put into the reactor and it could not be turned on. Furthermore, no more fuel rods can be made for the reactor and no major reactor components could be introduced.
What is the significance of enrichment levels?
The percentage enrichment refers to the concentration of the fissile isotope U235 in uranium. Low enriched uranium (LEU) is defined as under 5%. Highly enriched (HEU) or weapons-grade plutonium is considered to be over 90%-enriched. Because of the technical aspects of centrifuge enrichment, the manufacture of 20%-enriched uranium is considered to be nine-tenths the way along the road to HEU in terms of difficulty.
Does the deal recognise Iran's right to enrichment?
This is hotly contested. The US says it does not. But the text does suggest that the comprehensive settlement to be hammered out in the next six months would involve at least acceptance. It says: "This comprehensive solution would involve a mutually defined enrichment programme with practical limits and transparency measures to ensure the peaceful nature of the programme." Washington put emphasis on the "mutually defined" phrase, suggesting Iran would require negotiated consent.
Why did Iran build an enrichment plant underground?
The plant carved into the rock under a mountain at Fordow was discovered by western intelligence in 2009. Iran reported its existence to the International Atomic Energy Agency just days before Barack Obama revealed it at the UN general assembly in September of that year. Western states said Fordow demonstrated Iran's desire to cheat and develop a covert weapons programme. Iran said that it needed a protected facility because of the threats from the west and Israel to bomb the Iranian nuclear programme. The Iranian government said it was only required to declare its existence to the IAEA six months before it intended to put radioactive material in it. However, the IAEA argues that is an old rule. It says Iran should inform the agency of new nuclear facilities as soon as they are designed.
Could Iran still develop a nuclear bomb under these conditions?
It would be hard to develop a bomb in secret, particularly because the deal involves some uniquely intrusive inspections, including daily visits to the enrichment plants at Natanz and Fordow, and for the first time inspections of centrifuge assembly workshops and rotor production facilities. Those inspections are especially important as IAEA experts would be able to check if any extra centrifuges were being made for an undeclared plant. Because the nuclear fuel cycle is checked by the IAEA all the way along, any yellowcake or uranium hexafluoride that was diverted to a parallel programme would be noticed. To escape detection, the Iranians would have to construct an entire covert fuel cycle – which would be very difficult. The chances of being caught and the penalties involved would be great.
The other way to way make a bomb would be to "break out" – to race to build a warhead before inspectors find out and raise the alarm. But by taking away the 20% stockpile and curbing the number of centrifuges, this deal would double the break-out period Iran would need. It would take about three months to make enough HEU for a warhead, but it would take more than that to turn that HEU into uranium metal and shape it into a perfect core for a warhead and then assemble the weapon, let alone test it. Iran would almost certainly be caught before it had made a single weapon.
What is likely to happen now?
Worst case scenario: The deal falls apart almost immediately in the face of hardliners in Washington and Tehran. A congressional vote now for more sanctions, for example, would almost certainly derail it. Iranian conservatives would see such an act as American perfidy and it would make it extremely hard ever to seal another agreement. Iranian conservatives would be likely instead to accelerate Iranian nuclear development, bringing a conflict closer.
Best case scenario: The interim deal holds and the comprehensive settlement is negotiated on time, laying to rest the Iranian nuclear crisis once and for all. The prompt lifting sanctions leads to an economic boom in Iran, strengthening moderates. The end of isolation would strengthen liberals and weaken the hold of the clerical and military elite.
Most likely scenario: The interim deal more or less holds despite squabbles, but negotiations over the comprehensive agreement are dogged by disagreements and delays, requiring an extension to the stopgap measures and an erosion of trust. Success would require another vast investment of effort and political capital from relatively weak presidents, and would be hostage to developments elsewhere, such as Syria.