Will the hoped-for deal with Iran do enough to prevent Tehran developing nuclear weapons under the guise of its civilian activities, as world powers hope?
As negotiators in Switzerland rushed to meet a midnight Tuesday deadline to agree the outlines of a deal, this is a brief lowdown on the current status of Iran's nuclear activities.
Iran denies wanting nuclear weapons and says that its programme is purely for peaceful purposes such as power generation.
- Plutonium -
There are two possible materials to make the explosive core of a nuclear bomb: plutonium or highly-enriched uranium.
Plutonium can be extracted from spent nuclear fuel rods.
Western powers fear that a reactor Iran was building until an interim deal in November 2013 at Arak could have produce a bomb's worth of the material a year.
Now the powers want the design of the reactor to be changed so that the amount of plutonium Iran could obtain is significantly reduced.
Iran is not thought to possess the capability to extract the plutonium, and a secret facility would be easily detected. Arak would also have to run for at least a year before plutonium could be obtained.
- Uranium -
Of greater concern is uranium.
Signup to our newsletter and follow us on Facebook and Twitter!
Enriching uranium raises the percentage of a certain isotope by using sophisticated machines called centrifuges spinning at supersonic speeds. For nuclear power 3.5-5.0 percent is needed, 20 percent is for medicines and 90 percent for a bomb.
Iran is already enriching to low levels and until January 2014 was doing so to 20 percent. It is not thought to have purified to weapons-grade -- although it has the knowhow and the equipment.
- Breakout -
The name of the game for world powers is to extend the "breakout" time to around a year from several months at present.
This is the time period Iran would in theory need to process enough weapons-grade uranium or plutonium for one bomb.
This could happen through reducing the number of centrifuges to a few thousand from the current 19,000, around half of which are operating.
Iran could also reduce its stockpile of low-enriched uranium -- some 8,000 kilos, enough for eight bombs if further enriched, experts say -- by exporting it or converting it to another form.
In addition, the UN atomic watchdog, already responsible for painstakingly accounting for Iran's every ounce of nuclear material, could conduct additional inspections.
- 'Sneakout' -
Experts stress that the "breakout" concept should however be treated with caution.
It fails to include the additional time needed for Iran to work out how to incorporate the fissile material into a warhead and mount it on a missile, steps which would take many months, experts say.
More likely, some say, Iran would go for a "sneakout" -- using facilities that the UN watchdog does not know about.
But this possibility would be covered by greater oversight by the watchdog, the International Atomic Energy Agency.