Excuse me sir, but is that covert nuclear facility making nuclear weapons or nuclear power?

Allison Loudermilk

U.S. President Barack Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy talk nukes and Iran at the G-20 summit, Sept. 25, 2009, in Pittsburgh. (Scott Olson/Getty Images)

By now, you've probably already heard that Presidents Obama and Sarkozy, along with British Prime Minister Gordon Brown have jointly accused Iran of keeping a secret nuclear facility. The leaders are now asking the International Atomic Energy Agency to immediately inspect the facility.

But how can they tell the Iranian operation is intended to make nuclear weapons rather than nuclear power? Secrecy, of course is a big clue. If the facility is above board, then there's no point in hiding it, say 100 miles southwest of Tehran. Iran might even let the U.N.'s nuclear detectives inspect it occasionally, rather than bar them and stockpile plutonium as North Korea has done. (Speaking of the IAEA, you can check out the agency's special section on Iran here, but it doesn't have the letter everyone's talking about.)

Size also matters. The New York Times quoted President Obama as saying at the Pittsburgh Summit this morning that "the size and type of the facility is inconsistent with that of a peaceful facility." OK, but how? Apparently, it doesn't have enough stuff -- not enough equipment in the form of centrifuges to generate the nuclear fuel necessary to run a reactor, but sufficient to make bomb-making materials, says CNN. You'd think it would be the other way around since nuclear weapons require a highly modified material, which brings me to my last point.

The end product, if the plant is operating (and this new site apparently isn't), is another clue. For example, highly enriched uranium designed for a state-of-the-art nuclear weapon might be enriched to the tune of about 85 to 90 percent. For nuclear power, however, 3 percent enrichment might do the trick, but anywhere in between is game, too, especially, say, if you're a terrorist looking to make a dirty bomb.

If you're fascinated with the element, Tom Zoellner wrote a book about it called "Uranium: War, Energy and the Rock that Shaped the World." Of course, we have a ton of nuclear content at HowStuffWorks.com, too, on stuff like stealing a nuclear bomb. Look 'em over:

How easy is it to steal a nuclear bomb?What's a uranium centrifuge?How Nuclear Power WorksHow Nuclear Bombs Work How Nuclear Detectives Work