Debate is swirling about how to stop Iran from getting a nuclear weapon, should it take the decision to develop one.
In the United States, Republicans argue that President Obama is doing too little to confront Tehran, despite his pledge to use military means to deny Iran the bomb if diplomatic pressure fails. Obama, in turn, has criticized the Republicans for irresponsibly "beating the drums of war."
The question is also a sticking point between Israel and the U.S., which disagree about what level of Iranian nuclear development should trigger military action. Meanwhile, Iran says its nuclear program is for peaceful uses only.
There is no certainty about how close Iran might be to being able to build a nuclear weapon. However, here is what is generally accepted about the state of its nuclear program.
How do you make a nuclear weapon?
The hardest part is getting “fissile material,” such as highly enriched uranium or plutonium, which can be split by neutrons to set off a chain reaction that releases massive amounts of energy.
Raw uranium contains less than 1% of a crucial isotope called U-235. It takes uranium that is roughly 3.5% U-235 to fuel a reactor to generate electricity and about 90% U-235 to make a nuclear weapon.
To increase the amount of U-235, uranium must be enriched. There are a few ways to do so, but the main method to enrich enough uranium for a nuclear arsenal requires thousands of sophisticated gas centrifuge machines that spin at extremely high speeds, concentrating the U-235 isotope.
Once the uranium is highly enriched, it still needs to be put into a form that can be detonated. The gas has to be converted into metal. And to set off the explosive reaction, the fissile material has to be compressed so that neutrons bump into other atoms and make them split one after another.
Then there’s testing. Although a crude nuclear weapon does not require an explosive test, countries typically do a test blast to make sure their weapons would work the way they were designed. Finally, a nuclear power needs a missile or some other means of delivering the bomb.
There’s a different process for making nuclear weapons using plutonium, which can be extracted from spent fuel that has been irradiated in a nuclear reactor. But highly enriched uranium is seen as the main path to nuclear weapons for Iran, if its leaders decide to go that route.
Is Iran trying to create a weapon?
U.S. intelligence agencies don’t think that Iran is actively trying to build an atomic bomb. Intelligence reports say it stopped trying to create a nuclear warhead in 2003.
Iranian leaders have said that producing, possessing or using nuclear weapons is “a great sin.” They say their nuclear program is solely aimed at producing electricity and other peaceful purposes.
But Iran hasn’t fully cooperated with the United Nations nuclear watchdog group, the International Atomic Energy Agency, creating doubt in the minds of many analysts about what it is up to. A November report by the IAEA found evidence that Iran might still be doing clandestine research on how to create a weapon, undercutting its claims that it has purely peaceful aims.
Iran is also enriching uranium to higher levels than experts believe necessary for most peaceful uses. Iran says it is enriching uranium to roughly 20% to fuel a research reactor, well above the minimum needed to generate electricity but still far below what would be needed for a weapon.
With those conflicting signals, most outside experts believe Iran is trying to master the technology needed to build a nuclear weapon, if it later decides to actually take that step.
Would that trigger other countries to step in?
The U.S. says it will prevent Iran from actually getting a nuclear weapon. Israel has pushed for a firmer line: Stopping Iran from becoming capable of making a nuclear weapon or moving enough of its uranium enrichment program underground to insulate it from military attack.
What would stand between Iran and a nuclear weapon?
There are signs that if Iran does want to build a nuclear weapon, some technical and logistical hurdles remain. First and foremost, Iran would need to enrich its uranium to higher levels.
Some experts believe that Iran could keep enriching uranium to higher and higher levels by putting it through the same centrifuges without rearranging them.
Others say that to efficiently make highly enriched uranium at its existing Natanz facility, Iran would probably need to reconfigure the plant. That would probably involve rearranging its centrifuges and changing the piping between them, which would take at least a few months, said Paul K. Kerr, a Congressional Research Service analyst.
Whether it would need to take the same steps at one of its other facilities is unclear. Either way, it would be very difficult to avoid being detected by IAEA inspectors, who regularly visit Iranian nuclear facilities. Detection could spur other countries to try to stop them or simply attack.
So if Iran wanted to pursue a bomb without being detected, it would probably have to kick out the inspectors or use secret facilities. Kicking out inspectors would raise alarm bells, and so far Iran hasn’t been able to keep its nuclear work under wraps, said Mark Fitzpatrick, non-proliferation and nuclear disarmament program director at the International Institute for Strategic Studies in London.
There are a few other obstacles. Iran has had problems with operating its existing centrifuges, which use older, less efficient technology. Making more centrifuges would probably be difficult: Iran is believed to depend on foreign supplies to manufacture centrifuges, so strict controls on trade and a careful eye on the international black market could slow it down.
It’s also unclear how prepared Iran is to make highly enriched uranium into a weapon. White House officials have argued that Iran still hasn’t mastered all the necessary technologies. Experts disagree on how difficult it would be; Fitzpatrick estimates it would take six months.
“It’s not something you can do in short order,” said Daryl Kimball, executive director of the Arms Control Association. “It’s very hard to say how far along they are.”
And before Iran could create a workable weapon that could be delivered at a longer range, it would probably need to test it. Other countries have carried out tests underground, on the side of a mountain, in the desert or from a tower. Any blast would probably be detected by remote monitoring.
Even if Iran could muster enough highly enriched uranium to make one weapon, it would probably want to wait and amass enough for a small arsenal, since a single weapon would not be as useful. That drags out the timeline for making and testing a nuclear weapon.
Could Iran get around those obstacles?
Nuclear experts say Iran could probably get the enriched uranium, skills and equipment to build nuclear weapons, if it decided to do it, but it would probably still take years to build anything more than a crude weapon.
Political calculations may be more important than technical ones. The U.S. national intelligence director told Congress that “Iran has the scientific, technical, and industrial capacity to eventually produce nuclear weapons, making the central issue its political will to do so.”
-- Emily Alpert in Los Angeles
Photo: Iranian President Mahmoud Ahmadinejad inspects the Natanz nuclear plant in central Iran in March 2007. Credit: Iran's Presidency Office