A MANAGEABLE PROBLEM:

How to build a bomb: How close is Iran to building a nuclear weapon? And what can the US do to stop it? (Mark Fitzpatrick, June 2006, Prospect)

Uranium is enriched by passing it through a series of centrifuges—1.8cm-high spinning tubes that use centrifugal force to alter the concentration of the different uranium isotopes. Connecting 164 of the centrifuge machines together in a cascade, where the gas is successively enriched in each of several stages, provides a basic module for an enrichment facility. For nuclear fuel for reactors, such as the one Russia is completing at Bushehr, the U-235 content must be enriched to about 3.5 per cent for a controlled nuclear reaction. By contrast, weapons-grade uranium requires enrichment to over 90 per cent. Although that seems to be a far greater leap, once you have reached the 3.5 per cent fuel threshold, half the work is done. To get to weapons-grade, the low-enriched uranium is simply run through the centrifuges more times.

In early April, Iran announced that it had mastered the uranium enrichment process. The International Atomic Energy Agency (IAEA) confirmed that the pilot enrichment plant at Natanz had enriched uranium to the 3.5 per cent level in a connected series of 164 centrifuges. Iran’s claimed achievement came twice as fast as analysts had predicted when the enrichment program resumed in January. In an almost reckless hurry, the Iranians had skipped many of the intermediate testing steps. They presumably wanted to establish new "facts on the ground," so that were they persuaded in the future to again suspend the enrichment programme, they would do so at a higher starting point.

Iran might not be as far along as its leaders would like us to believe. Only a small amount of enriched uranium has been produced, and it is possible that Iran’s haste will eventually make waste. But we should not underestimate Iran’s technical skill. Last year, western intelligence analysts judged that the uranium hexafluoride Iran was producing at Esfahan was overly contaminated with heavy metals. Now experts, including the IAEA, judge it is good enough for Iran’s initial purposes. And Iran has already produced 110 tonnes of the feed material—enough, when enriched, for at least 15 nuclear weapons.

Iran originally planned to build five more 164-centrifuge cascade modules at the pilot plant, then to assemble 54,000 centrifuges in larger cascades in the underground fuel production facility at Natanz. Once the Iranians are confident the smaller cascades work, they can enlarge and replicate them at the underground site.

The Iranians could also replicate the centrifuge cascades in a hidden facility. If they seek to enrich uranium to the level and amount needed for a nuclear weapon, they could do so with 3,000 centrifuges operating for at least nine months. Assembling that number of centrifuges and getting them working smoothly would take some time—three years at least, in the estimate of the International Institute for Strategic Studies (IISS), taking into account the time it takes to build and assemble the centrifuges, and to do all the diagnostic, calibration and sustainability testing that Iran skipped over in its race to demonstrate an enrichment capability this spring. Thus, in the IISS's estimation, the earliest Iran could have a nuclear weapon is at the end of the decade: 2010.

Other reputable analysts believe the earliest timeline for obtaining nuclear weapons could be 2009 or even 2008, while the official CIA estimate remains 2010 to 2015 years. [...]

Seymour Hersh’s claim in the April New Yorker that American military planners had included a tactical nuclear weapon among the alternatives considered to destroy underground nuclear facilities produced a flurry of comment and debate, mostly disparaging the notion. In addition to the disastrous consequences, weapons experts note that there is no need for a tactical nuclear weapon; the American GBU-28 Paveway III laser-guided penetration bombs—the so-called "bunker-buster"—can pierce 30 metres of soil or six metres of concrete. The fuel production facility at Natanz is buried ten metres underground. The depth of tunnels at Esfahan evident in satellite imagery last year are harder to judge, but most of the Esfahan facilities are above ground. The tunnels are presumably for storage of the uranium hexafluoride and associated machinery in the event of a foreseen attack.