The other method is the production of plutonium, a material that can be used in medical research or -- again -- for nuclear weapons.
Earlier this month, U.S. Secretary of State Colin Powell reiterated Washington's concerns over how Tehran intends to use this technology.
"We have to be nervous when a nation such as Iran continues to take action that, at least suggests to us, that it continues to be interested in a nuclear weapons program," Powell said.
Iranian officials said Tehran will not give up its right under international treaties to produce its own reactor fuel, but said they have no interest in nuclear weapons.
Iranian President Mohammad Khatami put Tehran's position this way in late October: "We are ready for complete cooperation and [to reach an] understanding with the world and also with the [International Atomic Energy Agency, or IAEA] to make sure that Iran's [nuclear] activities do not move toward nuclear weapons."
Shannon Kile, an expert in nonproliferation issues at the Stockholm International Peace Research Institute (SIPRI) in Sweden, noted that although Iran maintains that its programs are entirely aimed at civilian nuclear energy and research, there are aspects of each that are highly troubling to experts because they appear to go well beyond normal civilian activities.
"Well, Iran basically has two uranium-enrichment facilities that we know about," Kile said. "They are both located at Natanz, which is south of Tehran. One is a very small-scale facility, holding about 1,000 centrifuge cascades. The other one is a much larger facility, holding up to 50,000 centrifuges. And what is striking about it is that it is built deep underground with heavily reinforced walls and roofs, which would indicate that, a) the Iranians are interested in hiding it, and b) they are concerned about the possibility of military strikes against it."
Tehran did not declare the existence of these facilities to UN arms inspectors -- as required under the NPT -- until the sites were exposed by an exile Iranian opposition group in 2002. Follow-up UN inspections of the facilities raised serious questions about whether they were being used to enrich uranium to levels above that needed for nuclear fuel.
"There are some specific activities that are troubling," Kile said. "The International Atomic Energy Agency has detected the presence of high-enriched uranium on some of the centrifuge components that they have examined. Now, they do accept that it is possible that some of that contamination has come, in part, from a third-country supplier, which would most likely be Pakistan. But it is difficult to accept that all of it has come from a third-country supplier. And that means that Iran might have enriched uranium. And it is difficult to know why it would enrich [uranium] to that level if it were going to simply use it for a nuclear fuel program."
The UN nuclear agency's inspectors found traces of uranium enriched to 20 percent -- far higher than the usual 2 to 3 percent enrichment level required for nuclear fuel.
Kile said many nuclear experts believe that unless Iran commits to abandoning its uranium-enrichment activities, it could acquire enough weapons-grade material for a bomb by 2007 or 2008.
However, he said it remains uncertain whether Iran is seeking to produce a bomb immediately or is merely trying to perfect a technical capacity for future production. That would permit Tehran to "break out" as a nuclear power anytime in the future, should it feel the need.
As for the second route to making a nuclear weapon, Iran has a program to produce plutonium that centers around a heavy-water nuclear reactor to be built near the central city of Arak. The project -- which was again not declared to arms inspectors until it was exposed in 2002 -- is described by Tehran as an effort to produce isotopes for medical use.
But Iran's plans worry many nuclear experts because it is building what is commonly known as a "breeder reactor." Such reactors are efficient at quickly producing significant amounts of plutonium, particularly for military use.
Kile said the "breeder" design exceeds normal specifications for reactors generating plutonium for civilian uses.
"The 40-watt heavy-water reactor at Arak is ideally suited for producing weapons-grade plutonium," Kile said. "And, in fact, this is the type of reactor that was used by all of the [original] nuclear weapons states [United States, Russia, Britain, France, and China] in the early years of their nuclear programs."
Construction of the reactor is just now getting under way, and it will be eight to 10 years before it becomes operational.
Kile said there is ample precedent for countries successfully using both uranium enrichment and plutonium production as clandestine routes to nuclear weapons. He noted that Pakistan is believed to have derived a bomb using uranium enrichment, while India and Israel are thought to have taken the plutonium route.
The five "nuclear-weapons nations" recognized under the NPT -- the United States, Russia, Britain, France, and China -- have used both technologies to produce their nuclear arsenals.