Iran's Presidential Office / Xinhua / Corbis Iranian President Hassan Rouhani attends a press conference at the presidential palace in Tehran, on Nov. 22, 2013.

What just happened?

Over the weekend, a meeting in Geneva between Iranian officials and representatives from the permanent members of the U.N. Security Council (the U.S., the U.K., France, Russia, China) and Germany reached what amounts to a temporary deal on Iran’s controversial nuclear program. For the next six months, the Iranians have agreed to halt any activity that could boost their ability to build a nuclear weapon and agreed as well to what seems to be rigorous new international inspections. In return, the U.S. and its allies will afford Iran some relief — though the roughly $7 billion on offer is not much — to the crippling regime of international sanctions that has hobbled the Iranian economy in recent years.

Why is Iran’s nuclear program considered such a threat?

Iran’s nuclear program launched in the late 1950s with considerable American aid — at the time, the country’s ruling monarchy was firmly in Washington’s geopolitical camp. That all ended when the 1979 revolution toppled the Shah and ushered in the Islamic Republic. Thereafter, successive Iranian governments have insisted their nuclear program is intended entirely for peaceful, civilian purposes and that they have no interest in building a bomb. The West, chiefly the U.S. and Israel, looks upon this claim with great skepticism.

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In the past decade, tensions have deepened, with former U.S. President George W. Bush lumping Iran into the “axis of evil” and Iranian counterpart Mahmoud Ahmadinejad earning notoriety for his bellicose rhetoric, particularly when aimed at Israel. Critics of Tehran see it as a destabilizing force with unsavory friends, propping up Syrian President Bashar Assad and the powerful Lebanese militant organization Hizballah. Israeli Prime Minister Benjamin Netanyahu has claimed repeatedly that an Iranian nuclear weapon poses an existential threat to his country. Israel is believed to have waged a covert war against Iran’s nuclear interests, ranging from stealth-missile strikes to computer viruses to the assassination of Iranian scientists. NATO justifies its own long-running plan to build a European missile shield on the existence of an Iranian nuclear threat. Finally, an Iranian nuclear weapon, some analysts fear, would spur an arms race in the Middle East, leading regional rivals like Saudi Arabia to pursue their own nuclear deterrents.

Have other budding nuclear powers faced this scrutiny?

No, not really. It’s an open secret that Israel already has nukes. Like Israel, India and Pakistan are not signatories to the U.N.’s Non-Proliferation Treaty on nuclear weapons (Iran is). When the two South Asian archrivals tested their own nuclear bombs in quick succession in 1998, it triggered a fair amount of international outrage and hand-wringing. But that didn’t last too long — most glaringly, the U.S. inked a potentially lucrative nuclear-energy deal with India in 2005. The spotlight shined on Iran’s nuclear program has little to do with upholding international norms and much more with the realpolitik of the fractious Middle East.

We keep hearing about uranium enrichment. What’s that all about?

You have to enrich uranium for both reactors and bombs because natural uranium is mostly U-238, which means the atoms have three extra neutrons in their nuclei. But U-238 nuclei don’t break apart (or fission, to use the technical term) very easily. That means they can’t generate a self-sustaining nuclear reaction. To make that happen, you need atoms of U-235, which are more fissionable but also much rarer — less than 1% of a chunk of natural uranium, by weight. The most efficient way to boost the percentage of U-235 is with a series of centrifuges that whirl uranium gas around at very high speeds. Some of the heavier U-238 atoms are whipped out to the edges, leaving the gas in the core slightly enriched with U-235. Then take out the enriched gas, put it in another centrifuge, and repeat the process until you get the purity you need.

(MORE: Israel Renews Warnings of Military Action After Iran Nuclear Deal)

So what does the deal do to Iran’s nuclear capabilities?

At least for the next six months, Iran will cease enrichment of uranium beyond 5%, will not install new centrifuges in its facilities and will have to keep some of its existing centrifuges inactive. Iran has also agreed to dilute or convert into oxide its existing stockpile of uranium enriched at 20%. The deal also puts a freeze on work at the heavy water reactor at Arak. And Iran has agreed to a strict, new regime of monitoring by the U.N.’s atomic energy agency that will make it much harder for the international community to be taken by surprise should the Iranians attempt what is known as nuclear breakout.

What’s the difference between 5% or 20% or 90% enriched uranium?

Uranium that’s been enriched to 5% is pure enough to be used in many types of power-generating reactors. Some power reactors, and some research reactors used to make radioactive isotopes for medical treatments, need uranium enriched to 20%. An atomic bomb requires much purer U-235 — up to 90% enriched, although some crude bombs can be made with slightly lower-enriched uranium. (This all applies to a conventional atomic bomb, like the one dropped on Hiroshima in 1945; a hydrogen bomb uses an atomic bomb as a trigger to ignite a much more powerful thermonuclear explosion, a fusion not a fission reaction.) It’s important to note that it’s much more difficult to get from 1% U-235 up to 20% than it is to get from 20% to 90%.

Is it easy to dispose of enriched uranium stockpiles?

It’s laborious, but not complicated. You thoroughly mix enriched uranium with U-238 until the percentage drops back down to natural levels. Once you’ve done that, it’s just as tough to purify the uranium as it was the first time around.

What is plutonium, then, and is the process to use it different?

Plutonium is an entirely different element from uranium, but it’s radioactive as well, and, like U-235, it’s fissionable, so you can use it in bombs or reactors. The bomb that the U.S. dropped on Hiroshima used uranium; the one that destroyed Nagasaki used plutonium. Plutonium bombs are technically more difficult to manufacture. The reason why the facility at Arak concerns those who want to halt Iran’s nuclear program is that it uses plutonium, which is even more dangerous and toxic than uranium, and an air strike on the Arak plant would lead to unconscionable, deadly fallout.

(MORE: Iranians Celebrate Nuclear Deal)

So what does Iran get in return for its concessions and cooperation?

Some $4.2 billion of the less than $7 billion of sanction relief tendered to Iran comes from the country’s own assets frozen in foreign banks. Moreover, the grinding sanctions on Iran’s oil exports and banking sector put into place by the E.U. remain almost totally intact (the concessions involve loosening commercial restrictions on gold, automobiles and pharmaceuticals). As London-based analyst Shashank Joshi puts it: “Iran will still be forfeiting over three times as much in foregone oil revenue as it will gain in relief.” But what Iran does come away with from Geneva is an implicit recognition of its right to enrich uranium — a sticking point for a nation that stressed repeatedly ahead of the Geneva talks the importance of recognizing Iranian dignity and sovereignty. Given the celebrations that greeted the returning Iranian negotiating team, it’s clear many Iranians welcome what is an important step away from the isolationist drift of the Ahmadinejad years.

Who’s unhappy with the deal?

Not long after the deal was brokered late Saturday night, Israel’s hawkish Netanyahu deemed it a “historic mistake” (the stock market in his own country seemed to disagree). Though far less outspoken, the largely Sunni Gulf states, chiefly Saudi Arabia, are wary of American rapprochement with Iran, a Shi‘ite theocracy and main regional rival. Some reports suggest the Saudis are poised to kick-start their own nuclear program, perhaps with Pakistani help. Congressional Republicans have criticized the Obama Administration for being supposedly soft on an untrustworthy regime. Some hard-liners in Iran have also found cause to complain: opposition factions in both Tehran and Washington could yet scuttle the whole process.

What changed in U.S.-Iran relations for the deal to happen?

The surprise election of the moderate cleric Hassan Rouhani as President of Iran earlier this year did much to dispel the bad odor left behind by eight years of Ahmadinejad. Rouhani’s historic overtures to the U.S. that followed were received by a cautious, yet willing Obama Administration. And Rouhani’s appointment of the suave former Iranian ambassador to the U.N., Mohammad Javad Zarif, as the country’s Foreign Minister and lead nuclear negotiator proved crucial. So too, it seems, were secret, back-channel meetings between American and Iranian officials in Oman. For the moment, diplomacy and dialogue have won the day, but the talks will have to continue — and trust between Iran and its Western interlocutors will have to deepen — before a lasting deal can be reached.

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