The cornerstone of Iran's nuclear disputes with the West has been over uranium enrichment — which leaders in Tehran claim as a right and is now a key sticking point in negotiations in Geneva.
Iranian Foreign Minister Mohammad Javad Zarif and Catherine Ashton, the European Union's top diplomat, have met repeatedly since Wednesday trying to resolve that and other differences.
Here are answers to some important questions about uranium enrichment, the central process in turning concentrated uranium into nuclear fuel.
Q: WHAT IS URANIUM ENRICHMENT?
A: It is the process of turning uranium gas feedstock into nuclear fuel. It's done with centrifuges that separate and concentrate the uranium. About 3.5 percent enrichment is needed for an energy-producing reactor such as Iran's Russian-built plant at Bushehr on the Persian Gulf coast. Higher levels of enrichment, about 20 percent, are needed for research reactors that produce isotopes for cancer treatment and other applications, such as agricultural to enhance fertilizers. Iran has one main research reactor.
Q: SO WHY THE WORRY ABOUT NUCLEAR WEAPONS?
A: Because uranium enriched to 20 percent is only several steps away from being boosted to weapons-grade levels at more than 90 percent. Iran says it has no intention of building a bomb. But the West and others worry that Iran could one day start a fast-track weapons program with its stockpile of 20 percent enriched uranium or stop just short of making weapons and become a de facto nuclear-armed state.
Q: WHY WON'T IRAN GIVE UP ENRICHMENT?
A: This is what Iran has frequently called its "red line" in the nuclear talks. Iran's leaders say they will never relinquish control over the entire nuclear cycle as a matter of national pride. Iran portrays itself as an emerging technological giant of the Islamic world. The nuclear energy program is a pillar of Iran's self-image as a center of scientific advances independent of the West. Iran has made some other important strides, including claims of sophisticated drone development, a homegrown auto industry and an aerospace program that officials say has sent rockets to the edge of space with animals aboard.
Q: WHERE IS THEIR ROOM FOR COMPROMISE?
A: Iran says it could discuss capping the level of enrichment at 5 percent or lower. Such a promise would also require additional monitoring by the U.N.'s nuclear watchdogs, the International Atomic Energy Agency, which already visits many Iranian nuclear sites. Keeping the enrichment labs at lower capacities would add more time to watch for any breakout attempts at higher levels. It also would freeze the stockpile of 20 percent, currently about 200 kilograms (440 pounds). Iran also could agree to accelerate the transformation of the 20 percent enriched uranium into a reactor-ready state, which effectively takes it out of the loop for further enrichment.
Q: IS IT POSSIBLE TO MAKE A BOMB WITH ENRICHMENT AT 5 PERCENT OR LOWER?
A: No. But Israel and others worry that giving Iran the capacity to enrich could open the door to a secret program for higher levels someday. Iran denies this.
Q: IS IRAN CORRECT IN CALLING ENRICHMENT A "RIGHT?"
A: Iran is a signer of the U.N.'s Nuclear Nonproliferation Treaty, which governs the spread of atomic technology. The document does not specifically spell out any "rights" for enrichment. Iran, however, sees its support of the treaty as granting it the "right" of enrichment. The U.S. and allies have balked at Iran's previous demands to acknowledge the "right" of enrichment. Instead, the West appears to support the position that Iran can continue some level of enrichment, but only under strict U.N. monitoring.
Q: WHEN DID IRAN START ENRICHMENT?
A: It was announced in 2006, but enrichment was part of the nuclear disputes between Iran and the West for more than a decade. In late 2003, Iran agreed to suspend its work on installing centrifuges and related facilities as part of nuclear talks with European envoys. The negotiations faltered and Iran moved ahead with its enrichment plans.
Q: WHERE ARE IRAN'S ENRICHMENT SITES?
A: Iran has two main uranium enrichment facilities. The oldest and largest — in Natanz, about 260 kilometers (160 miles) southeast of Tehran — is largely built underground and is surrounded by anti-aircraft batteries. Uranium enrichment began in 2006. Another site is known as Fordo, which is built into a mountainside south of Tehran. Its construction was kept secret by Iran until it was disclosed in September 2009 in a pre-emptive move before its existence was revealed by Western intelligence agencies. The area is heavily protected by the Revolutionary Guard. U.N. nuclear inspectors have visited both sites and have installed round-the-clock monitoring systems.
Q: HOW MANY OTHER COUNTRIES ENRICH URANIUM?
A: More than a dozen countries have enrichment programs, but several of those do not have nuclear weapons.