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'Iran could have enough material for bomb within 4 months' actually its within 3 months now. I feel, iran will be able to produce nuclear weapons, given the weakness of Western. because West has no real policy to deal with this application.

And they are preparing their selves for the bang

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Maybe you should be getting professional help for your obsessions? Pakistan has bombs, is a muslim nation. –  anna v Jun 30 '12 at 16:02
Multiple flags on this. I"m closing it as being a question on intelligence technique and not physics. –  dmckee Jun 30 '12 at 17:11
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closed as off topic by Qmechanic, dmckee Jun 30 '12 at 17:11

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2 Answers

The estimate starts with the calculation how much enriched uranium is needed for a bomb.

The amount of uranium has to exceed the "critical mass" for which the rate at which the nuclear reaction "feeds on itself" exceeds the loss of the nuclear reaction by escaping neutrons. In principle, this situation may be achieved whenever the percentage of the rare uranium-235 isotope, which is the fissile (i.e. it can sustain chain reaction), exceeds about 6 percent (the remaining uranium, uranium-238, the dominant one on Earth, isn't too useful for the reactions). At 6% enrichment, the critical mass is infinite and drops from infinity as you increase the enrichment level.

For a 20% "low-enriched uranium", the critical mass is about 85 kilograms for one bomb. Iran arguably has more than 75 kilograms of the 20% low-enriched uranium at this point. Given the estimated rate at which Iran is producing the 20% uranium in its centrifuges, they were estimating how quickly they could get the required amount for one bomb.

This estimate is probably overly optimistic. Not only they probably have some additional enriched uranium unknown to the nuclear watchdog. Also, they are probably able to increase the enrichment above 20% which means that the critical mass would be much lower. If Iran isn't there yet, it's probably a matter of weeks for them to get to the critical point. The prediction for the coming weeks or months is a combination of the extrapolation of the known centrifuges that produce uranium at a known enrichment level; and a speculation about the speed with which they are capable of improvements of their centrifuge systems.

At the same moment, the relevant Western intelligence agencies claim that Iran hasn't made the official decision to really build a nuclear weapon in a particular timeframe yet. This interpretation of the Iranian decisions is unfortunately a falsifiable statement and the falsification could hurt.

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It's based on paperclip purchasing. The IAEA looked at the vats number of paperclips required to handle the paperwork in a typical modern defence project and then carefully track Iran's imports of paperclips to estimate the project status –  Martin Beckett Jun 30 '12 at 15:54
@MartinBeckett's comment is all too close to the truth. They really do use the usual tools of strategic intelligence which includes logistical analysis for monitoring uncooperative states. Cooperating states attract on-site inspectors, fuel burn up sampling and other more direct measures. –  dmckee Jun 30 '12 at 16:56
@dmckee, a famous American scientist, don't remember who but was obviously annoyed with the bureaucracy, claimed that if you wanted to stop nuclear weapons development you should ban paperclips and filing cabinets –  Martin Beckett Jun 30 '12 at 18:30
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So the IAEA does put out reports that you can read to get a good idea of the sort of information they use. Of course, some of the sources are sensitive and are not discussed at length.

There is, however, a very quick general method of making that sort of determination. If you assume that they already have all of the supplemental technology, i.e. high precision electrical timers and other necessary components, then the primary hurdle is fuel enrichment. Though the supplemental technologies are not straightforward and require plenty of special knowledge, enriching fuel takes not only special knowledge and equipment but also lots of time. It can generally be assumed that enrichment is the bottleneck in the development of nuclear weapons.

The reason enriching nuclear fuels takes so much time is because there are no large scale chemical methods in use for separating isotopes of the same chemical element. Consequently, you have to rely on physical methods of separation that are inherently based on the very small difference in mass of the isotopes in question. In this case, ${}^{235}$U only weighs one percent less than ${}^{238}$U. Further complicating matters is that ${}^{235}$U only makes up $0.72\%$ of natural uranium. So, to get highly enriched uranium, you have to process a lot of material for a long time.

The amount of time it takes to achieve a specific level of enrichment, however, is not linear. The vast majority of the work of enrichment is performed achieving a moderate enrichment. Either way, these process are well understood and so it is possible, if you know the highest enrichment yet achieved, and the rate at which the country in question can enrich generally (i.e. how many facilities with how many centrifuges), you can make a pretty good estimate about how long it would take to achieve a certain enrichment.

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