In the last few months, Iran has advanced to the brink of having a nuclear weapon.

It has accumulated at least two tons of enriched uranium - enough to make two nuclear bombs, according to a U.N. report released Monday. Though the uranium is meant to be used for power generation and a medical reactor, it's a short step from there to bomb-grade fuel.

The same technology can easily be ramped up to make nuclear weapons, said physicist David Albright, president of the Institute for Science and International Security in Washington. "If they wanted to make highly enriched uranium, they could do it," he said, referring to bomb-grade material. He estimates the Iranians are 18 months away.

Albright and other scientists who have followed the situation say Iran is moving toward a uranium bomb - the same type used on Hiroshima. Such weapons are relatively easy to engineer, they say. The challenge, as it was for Manhattan Project scientists, is getting enough of the explosive form of uranium, known as uranium 235.

But new technology makes that easier all the time.

Late last month, the five permanent member states of the U.N. Security Council proposed a tougher set of sanctions than any yet imposed on Iran. And a full Security Council vote could come sometime this month. Individual countries also are expected to impose their own sanctions. But experts doubt it will stop Iran's program.

Nor are experts optimistic about a plan for Iran to send some of its nuclear materials to Turkey. Many U.S. experts see the plan as a diversion.

There is evidence that Iranian scientists are working on ways to make and deliver nuclear weapons, said Albright, who worked with U.N. weapons-inspection teams in the 1990s to sniff out and dismantle Iraq's nuclear program. His group, ISIS, aims to stop the proliferation of nuclear weapons.

Iran's efforts prompted Defense Secretary Robert M. Gates to issue a secret memo last year warning that the United States was not prepared for a nuclear Iran. (Parts of that memo were leaked to the media this spring.)

"The U.S. position publicly is that we're going to stop them from getting nuclear weapons," Albright said. But short of waging war, he said, there is no clear plan for how to achieve that.

"Most people who are rational say we have to think about the day after they get nuclear weapons," he said.

Iran is allowed to develop nuclear power and even refine uranium for fuel under the rules of the international Nuclear Non-Proliferation Treaty, adopted in 1970.

The constraint is the Iranians must let inspectors from the International Atomic Energy Agency come in to make sure the country is not pursuing weapons.

The Iranians have weakened those inspections since 2002, said Albright, by refusing to answer questions and building enrichment sites in secret.

Iran is still officially working within the international community, said Ivan Oelrich, vice president of the Strategic Security Program for the Federation of American Scientists. By contrast, North Korea rejected the nonproliferation treaty outright in 2003, after which it threw out the IAEA inspectors.

Iran's strategy may be to get as close to a nuclear weapon as possible without explicitly violating the treaty, Oelrich said. "They might like to occupy that spot where they have this ongoing latent capability without explicitly crossing the nuclear threshold."

What kept Iran from doing that years ago is the same challenge that faced scientists from the beginning of the nuclear era - concentrating enough U235. Unlike conventional explosives, where a small amount makes a small blast, nuclear weapons need a "critical mass" of material or they won't go off at all.

For better or worse, natural uranium contains only about 0.7 percent of U235. Most of the rest is U238. The two forms are isotopes - the same element, but with different atomic weights.

Both can be induced to release energy through fission; when blasted with particles, the nuclei of these jumbo atoms split into smaller fragments. That process converts a small amount of mass into a large amount of energy.

Back in the 1930s, physicists found that a trace component of natural uranium - U235 - could induce its own fission in a chain reaction. When the U235 atoms split into fragments, they release particles called neutrons and these hit other nuclei, shattering them and shooting off more neutrons.

They realized that could cause an explosion of unimaginable power, but some prominent physicists thought it would be impossible to amass enough U235 to get such a chain reaction going.

During the Manhattan Project, scientists found a way, eventually raising the concentration of U235 from less than 1 percent to close to 90 percent - the standard used for uranium weapons.

The Manhattan Project scientists started by turning the uranium into a gas - uranium hexafluoride - then running it through membranes to separate out the molecules with U238, which is heavier than U235. The machines were enormous and the process slow and expensive.

Now the preferred technique involves spinning the uranium hexafluoride gas in centrifuges. Centrifugal force pulls the heavier uranium 238 outward, leaving more U235 near the center.

It's a simple principle but a complex technology, since the centrifuges have to spin at more at 60,000 r.p.m. The idea is to spin it as fast as you can without having your machinery fly to pieces, said Arjun Makhijani, a nuclear engineer who heads the nonprofit Institute for Energy and Environmental Research.

Spinning that fast could start a centrifuge wobbling like an unbalanced spin cycle on a washing machine, he said. The bearings would also burn up unless the centrifuge used special materials.

Iran reportedly got the blueprints for its centrifuges from the infamous Pakistan-born engineer A.Q. Kahn, said Makhijani. Kahn learned about centrifuge technology in the Netherlands, where he had worked at a nuclear-enrichment facility while married to a Dutch woman. He took the technology back to Pakistan in the 1970s and is thought to have shared it with Syria, as well as Iran, in the 1980s.

Iran now has thousands of centrifuges spinning uranium, experts say.

Fuel for a nuclear power plant needs only 20 percent U235, said Makhijani, but once the concentration becomes that high, it takes only a few weeks to get it to bomb grade.

"It's a very short step from 20 percent to 90 percent," he said.

There was a time when the world encouraged Iran to develop nuclear power, said Makhijani. In the 1960s, the shah was in charge and "everyone wanted his money."

That all changed in 1979, when the shah was overthrown, but since then Iran again has begun working with other countries to build reactors.

The Iranians also have built a research reactor in Tehran designed to create radioactive materials used in cancer treatment, said Oelrich of FAS. "That puts us in a pickle," he said. If the international community tells the Iranians to stop, "they can claim we want their cancer patients to die."

One solution was for another country to give Iran finished fuel rods - uranium processed to 20 percent and shaped specifically for use in this reactor. In exchange, Iran would send out some of the uranium it has been enriching.

An original plan to send the uranium to Russia fell through.

Last month, Turkey and Brazil renegotiated a similar pact. The problem is that since the original deal was offered, Iran has accumulated much more enriched uranium, experts say.

One policy that has worked to some extent, said Albright, is sabotaging Iranian efforts by feeding bad materials and equipment into the black market that supplies Iran. The wrong metals or oils can break or ruin centrifuges.

"It makes sense to do this," he said, since it's illegal for Iran to be buying these materials in the first place. "If the Mafia is buying something, it's hard to say we have a responsibility to deliver them a high-quality product," Albright said.

A couple of cases of sabotage already have been made public - including one in which the bad equipment was manufactured at a U.S. national lab. That's all Albright knows, since it remains a classified project.

Even if Iran gets the bomb, said Albright, that doesn't mean the situation cannot be reversed, as occurred in South Africa after it acquired nuclear weapons in 1979.

The South African program wasn't discovered until 1984, said Albright. In 1989, South Africa gave up its weapons to become a respected member of the international community, he said. That occurred after extensive negotiations.

"That strategy can actually work."

Contact staff writer Faye Flam
at 215-854-4977 or fflam@phillynews.com.