Efforts to build diplomatic momentum in nuclear talks between Iran and the six world powers must confront hard realities about the scope of Iran's uranium enrichment labs and the stockpile of material it has produced. Any potential deal must work out a formula on both these issues, which have been the main stumbling blocks in past negotiations.
These machines spin uranium gas feedstock at supersonic speeds in a cascade process to purify it at various levels. Uranium that is enriched to lower levels — about 3.5 percent — is used for power-generating reactors such as Iran's Russian-built Bushehr plant. Higher enriched levels of about 20 percent are needed for more specialized purposes such as reactors that produce isotopes for cancer treatment. Iran has a medical research reactor in Tehran.
Uranium enriched to more than 95 percent is needed for an atomic weapon.
Iran currently has about 18,000 centrifuges, with about 10,000 currently in operation. That's an increase from about 12,000 total early this year with about 9,000 in operation.
The latest report by the U.N.'s International Atomic Energy Agency says Iran has about 185 kilograms (407 pounds) of 20 percent-enriched uranium. This is the highest level acknowledged by Tehran. Another nearly 187 kilograms (411 pounds) of 20 percent uranium have been repurposed into powder that makes it only useable as nuclear fuel and unable to be further boosted.
Iran has more than six tons of uranium enriched below 5 percent.
Experts say 250 kilograms (550 pounds) of 20 percent-enriched uranium are needed to create enough higher-enriched material for a single warhead. Iran has consistently denied that it seeks nuclear weapons, pointing to a religious edict by Supreme Leader Ayatollah Ali Khamenei that described nuclear arms as against Islamic teaching.
Iran has said it won't consider shipping uranium stockpiles abroad as part of a possible nuclear accord with the West. Israel has pushed for a full stop to Iran's enrichment program. The U.S. and its negotiating partners want to limit enrichment levels well below the grade needed for nuclear weapons and they want Tehran to ship out the bulk of its enriched stockpile to other countries over the long run.
Iran's main enrichment facility is in Natanz, about 160 miles (260 kilometers) southeast of Tehran. It is largely built underground and is surrounded by anti-aircraft batteries to protect it. A more fortified facility, built into the side of a mountain, was disclosed in 2009 near Qom, about 100 kilometers (60 miles) south of Tehran. The area is heavily protected by the Revolutionary Guard. U.N. nuclear inspectors have toured both sites.
Iran has nearly completed a heavy water reactor in Arak, about 255 kilometers (152 miles) southwest of Tehran. Heavy water is a compound used to cool nuclear reactors, which do not need enriched uranium to operate. Heavy water reactors also produce plutonium as a byproduct, which could be used to make warhead material.
Another area mentioned in the nuclear standoff is Parchin, a military base and weapons development facility outside of Tehran. The site has also been suspected of housing a secret underground facility used for Iran's nuclear program, a claim denied by Iran. U.N. nuclear inspectors twice visited the site, but seek a third tour. Iran says it will approve another visit to Parchin provided an agreement is reached on the extent of the inspection and disclosure guidelines about it.
Writer Ali Akbar Dareini contributed from Tehran.