Iran has made steady progress toward producing nuclear fuel and could make significant quantities of enriched uranium in less than a year, according to new estimates by diplomats, scientists and intelligence officials.
Mastering enrichment will move Tehran a big step closer to being able to build an atomic bomb. Iran's progress already has intensified its confrontation with the United States and other countries that fear it is trying to develop nuclear weapons.
Despite persistent suspicions, however, a report due next month by the U.N. nuclear watchdog agency is not expected to provide proof that Tehran has a weapons program, diplomats said.
Nearly two years of inspections have uncovered a pattern of concealment and deception by Iran over two decades. But when it comes to whether Iran is secretly pursuing an atomic bomb, the case remains circumstantial.
Iran insists that its goal is to generate electricity. Its leaders have so far rejected demands by the U.N.'s International Atomic Energy Agency, the U.S. and European countries that they freeze enrichment activities.
A showdown appears to be approaching. The U.S. and its allies, arguing that the threat is imminent, want the U.N. Security Council to impose sanctions on Iran for violating the Nuclear Nonproliferation Treaty, which limits the spread of nuclear technology to peaceful purposes.
But since the United States failed to prove its claims about the existence of weapons of mass destruction in Iraq, other countries want more time for a fuller evaluation of Iran's enrichment capabilities and intentions.
At the center of the dispute is the enrichment process itself, which converts uranium ore into fissionable material, the most elusive component of a nuclear weapon. The same basic process produces low-level enriched uranium for civilian reactors or, with technical adjustments, highly enriched uranium for bombs.
This month, Iran said it was gearing up to produce large amounts of gaseous uranium, which is used in enrichment. The gas, known as uranium hexafluoride, can be fed into slender centrifuges, which spin at high speed to transform the gas into enriched uranium.
Iran has moved much faster than expected in manufacturing and assembling these centrifuges, diplomats said. The rapid progress means a pilot centrifuge plant near Natanz, in central Iran, could soon be equipped with enough machines to begin large-scale enrichment.
Two senior European diplomats said the pilot plant could be expanded from the existing 164 centrifuges to 1,000 within weeks and produce enough material in less than a year to fashion a crude nuclear device.
"They need to install more centrifuges and do preparatory work, and they could be in production in shorter than a year," said one diplomat, who, like most of the people interviewed for this article, spoke on the condition that his name and position be withheld.
For now, the International Atomic Energy Agency is monitoring the gas-production plant at Esfahan, also in central Iran, and preparations at the pilot plant. The pilot operation is part of a complex where an underground enrichment facility for as many as 50,000 centrifuges is under construction.
Western intelligence officials said the big fear is that once the two plants are operating, Iran will shift enrichment operations to hidden installations or follow North Korea's example and kick out the IAEA, allowing Tehran to begin enriching uranium to weapons grade at Natanz.
Uranium enrichment is relatively portable. Experts say 1,000 centrifuges could operate in a small building with little chance of detection by even the most sophisticated sensors or satellites.
There is no evidence that a hidden plant exists, and only hints about weapons research. But even officials who give Iran the benefit of the doubt say Tehran has been caught in so many lies that verifying the absence of a weapons program would take months, if not years, and might be impossible.
"When people have looked you literally in the eye across the table and told you this is black and it turns out to be white, your confidence in them is damaged," said a third senior European diplomat.
Mohamed ElBaradei, the IAEA director-general, has praised Iran's cooperation often enough to evoke U.S. anger, but he also has acknowledged that Iran's actions have created a "deficit of confidence." As before the Iraq war, ElBaradei wants more time to complete inspections before sending the matter to the Security Council.
But the U.S. and allies such as Canada and Australia say time has run out. They argue that the threshold for action is not the discovery of a secret plant or a weapons design. Instead, they say, Iran must be stopped before it begins to enrich uranium.
President Bush says a nuclear-armed Iran would be unacceptable, and Israel's defense minister warned last month that his country would consider "all options" to stop Tehran.
Military strikes against nuclear installations in Iran would be difficult; they could provoke retaliation and would certainly result in international condemnation. But Israeli officials argue that the backlash would be less painful than allowing Iran to obtain nuclear weapons.
Six months after an exile group's August 2002 disclosure that Iran was building an enrichment plant at Natanz and a second nuclear installation, the IAEA began trying to decipher the full scope of Iran's atomic activities.
Inspectors have examined research centers and workshops across the country, interviewed hundreds of scientists and pored over thousands of pages of documents dating to the mid-1980s, when Iran began secretly buying nuclear technology.
At every step, Iran concealed crucial aspects of its program. Iranian authorities twice denied inspectors access to suspect locations while incriminating material and equipment was hauled away. Each time, the inspectors said they still found evidence of nuclear experiments.
"The world at large knew nothing about Iran's nuclear plans two years ago and everything since then has been pried out of them," a U.S. diplomat said.
Under pressure from Washington, the IAEA board last month told the agency's staff to conduct another round of inspections and prepare a comprehensive summary of findings.
The board also ordered Iran to stop its enrichment program. Tehran voluntarily curtailed enrichment in a deal made a year ago with Britain, France and Germany, but it resumed the work this year.
The IAEA summary report will be circulated two weeks before representatives of the 35 member nations on the board meet Nov. 25 in Vienna. The expected conclusion that there is no proof of a weapons program and no new evidence of concealment is unlikely to stop the United States from demanding a vote to refer the issue to the Security Council.
Iran escaped previous U.S. pushes for tough action. That's unlikely this time, diplomats say, unless Iran again halts enrichment -- and even that is no guarantee it can avoid referral.
Diplomats familiar with the U.S. strategy said U.N. sanctions would be the first step in an effort to force Tehran to abandon enrichment efforts. Harsher steps eventually could include military action.
But other diplomats said Russia, China and other governments were reluctant to endorse sanctions, worrying that they might be the first step leading to an attack on Iran.
Tehran's best chance of avoiding being hauled before the Security Council appears to be accepting a new European offer of a package of incentives that would include guaranteeing Iran access to civilian nuclear technology and fuel for a nearly complete reactor in exchange for the country mothballing its enrichment efforts.
Tehran has not responded formally to the proposal, which is to be presented to its representatives today in Vienna. Western diplomats expect some concession before the Nov. 25 meeting.
Foreign Minister Kamal Kharrazi said in an interview last month that Tehran was willing to consider "any kind of verification mechanism ... to make sure there is no secret program." He said the goal of any agreement with the Europeans would be to prevent the issue from going to the Security Council.
Iran's conservatives, who have solidified control of the government since the first agreement with the Europeans, appear divided over whether to strike another bargain.
Defiant hard-liners want to withdraw from the Nuclear Nonproliferation Treaty and go ahead with enrichment. Proposed legislation would require Iran to pull out of the accord and halt IAEA inspections.
"Countries are seriously concerned about Iran withdrawing from the treaty," a Western diplomat said. "That's the second-worst scenario -- getting the bomb being the worst."
IAEA chief ElBaradei warned that Iran's withdrawal could prompt other countries to follow, severely damaging the primary means of controlling the spread of nuclear weapons.
More moderate voices in Iran argue that the country should remain in the treaty and try to avoid sanctions by accepting the European deal.
An Iranian official in Vienna said Tehran was unlikely to sign off on any agreement until after the U.S. presidential election, to avoid boosting Bush's campaign.
Sen. John F. Kerry, the Democratic challenger, has suggested that the U.S. should back a deal to provide Iran with nuclear fuel, something the Bush administration has so far refused to support.
Diplomats speculated that if Kerry won, the IAEA board might delay action against Iran until his administration took office.
Despite the absence of clear evidence, U.S. accusations against Iran have gained wide acceptance in recent months.
The main reason is that the suspicions do not rest as heavily on U.S. intelligence as they did in the case of Iraq's alleged nuclear program. International concerns about Iran are rooted in information uncovered by IAEA inspectors and described in six detailed reports.
"Evidence gathered by the IAEA makes a circumstantial case that is much stronger than the case that Iraq was restarting its nuclear program," said George Perkovich, a proliferation expert at the Carnegie Endowment for International Peace in Washington. "The only thing missing in Iran is a weapons design."
Barring a last-minute surprise, insiders said, inspectors have no concrete evidence of a weapons program or new instances of concealment. Still, the report is expected to conclude that too many big mysteries remain for inspectors to give Iran a clean bill of health.
"There are many reasons for worrying about Iran's intentions, but you have to be careful jumping to saying there is a weapons program or not," one of the European diplomats said.
The most pressing concern is identifying the origins of small amounts of weapons-grade uranium and low-enriched uranium found at four locations during the last 18 months.
Iran says the material came from contaminated centrifuge components bought on the black market. Abdul Qadeer Khan, a Pakistani scientist who helped develop his country's nuclear weapons, has confessed to selling components to Iran, Libya and North Korea.
In its September report, the IAEA said it was plausible that some of the enriched uranium came from Pakistani parts. But some concentrations were larger than simple contamination could explain, and not all of it was necessarily from Pakistan, the agency said.
Despite the high priority, inspectors have made no progress in answering the question because Pakistan refuses to cooperate fully, several diplomats familiar with the inquiry said.
Pakistan provided some data and said pointedly that not all of the enriched uranium came from its program, but diplomats said the Pakistanis had not allowed inspectors inside their nuclear plants to take samples.
The IAEA wants its own samples so independent laboratories can determine conclusively whether the material found in Iran matches enriched uranium produced by Pakistan.
ElBaradei said in late September that Pakistan had refused to let the agency question Khan. U.S. authorities also have been unable to interview the scientist. A Western diplomat complained that the Bush administration was not pressuring Pakistan to allow the IAEA to take samples or interview Khan.
The failure to trace the contamination leaves open the possibility that Iran produced weapons-grade uranium at a secret plant or bought it from an unknown supplier, diplomats said.
"Most of the traces are from Pakistan, but if some of it is not, then it is a serious issue that raises the possibility that Iran produced it," one of the European diplomats said.
Another issue is how much work Iran did on advanced centrifuge machines, known as P-2s.
In October 2003, Iran submitted a multivolume document to the IAEA that it said represented the complete history of its nuclear activities. But it began to unravel three months later.
When Libya decided to give up its clandestine nuclear weapons effort, it turned over information and technology to the IAEA and the U.S. It became clear that, like Iran, Libya had bought nuclear technology from Khan's network.
Comparing Libya's shopping list with what Iran had reported, IAEA inspectors were puzzled that the Libyans had managed to buy designs for Pakistan's P-2 centrifuges. The P-2 was far more efficient than the older P-1, which was what Iran had bought on the black market.
Inspectors had independent suspicions that Iran had been experimenting with another type of centrifuge. When confronted, Iran acknowledged that it had bought a complete set of P-2 designs in 1995.
Iranian officials explained that the P-2s had been left out of the October report because nobody asked about them and because the scientists were concentrating on the P-1.
The P-2 drawings were set aside for seven years and the only work involving them was a contract in 2002 for a small private business to conduct limited experiments, Iran said.
But when inspectors visited the business in Tehran, they found that the contractor had made a modification to the P-2 rotors that indicated extensive research had been done. They also found that he had ties to the military through other contracts.
"The modification didn't come out of thin air," a Western scientist said. "There was concern that work might have been conducted at some unknown place."
So far, the IAEA has neither a good answer from Iran about how much work it did on the P-2 nor any evidence of a P-2 plant.
Centrifuges are half the enrichment equation. The machines need uranium hexafluoride gas to manufacture enriched uranium.
Iran said it planned to transform 37 metric tons of yellowcake, a form of processed uranium ore, into uranium hexafluoride at the Esfahan plant this month.
David Albright, a physicist and former IAEA inspector who runs a think tank in Washington, said 37 tons was enough to make several nuclear weapons.
Iran turned out a small amount of uranium hexafluoride at the plant last spring, but a diplomat familiar with the current operations at Esfahan said it had not yet produced a larger batch.
Some Western officials have speculated that Iran is having technical trouble. But the diplomat said that the plant appeared ready to roll, and that he thought Iran had decided it was politically unwise to produce more uranium hexafluoride now.
Estimates vary on when the pilot plant at Natanz could start turning the gas into enriched uranium. The newest information indicates that Iran has moved much faster than anticipated.
One of the senior European diplomats said the pilot plant could begin operating on a small scale within weeks. In less than a year, he said, it could produce substantial quantities of low-enriched uranium.
"It would take a month to start to spit out enriched uranium, and the serious business would come about a year from now," he said.
Once centrifuges are operating smoothly, increasing the enrichment level to bomb-grade material is straightforward. However, it would require ending IAEA monitoring or carrying out enrichment secretly at another location.
What remains unclear is whether Iran has conducted research and tests to build a nuclear weapon.
It began work on nuclear power in the mid-1970s under the shah. The program was abandoned after the Islamic Revolution in 1979, when Western governments imposed sanctions on the new regime.
Iran acknowledged last year that it had restarted the program in 1985. Officials said technology was bought secretly through front groups because of the sanctions.
Iran was in the middle of an eight-year war with Saddam Hussein, seemingly an odd time to devote scarce resources to an expensive program for generating electricity in a country with vast reserves of oil and natural gas.
Since the nuclear program was discovered, Iran's secret purchases, particularly of technology with both civilian and military applications, have received more scrutiny. The items it recently tried to buy included high-speed switches that could trigger a nuclear weapon and specialized cameras that could test a nuclear explosion.
Some older purchases also attracted new attention. Among them were attempts in the early 1990s to buy weapons-related nuclear technology for a physics research center, a diplomat involved in the review said.
The IAEA was monitoring the research center at Lavizan Shiyan on the outskirts of Tehran in November when U.S. spy satellites picked up heavy equipment beginning to demolish the complex. As buildings were knocked down, all the large chunks of rubble and tons of earth were hauled away.
Asked about the demolition, Iran told the IAEA the center had been built by the military in 1989 to evaluate and treat casualties in the event of a nuclear attack on Tehran. Authorities said it was being leveled to make way for a park.
Iranian authorities allowed inspectors to visit the now-barren site at the end of June but refused to give them access to the material taken away or a list of equipment used at the center, citing security concerns.
The report next month will say that environmental samples turned up no evidence of radioactive material, but diplomats said enough concerns remained that Lavizan Shiyan would not be scratched off the list of suspect sites.
IAEA inspectors are monitoring several other sites where weapons work might have occurred, diplomats said.
One is a military complex containing hundreds of buildings and bunkers at Parchin, about 20 miles southeast of Tehran. The IAEA had been trying to get permission to inspect it for several weeks when ABC News broadcast satellite images of Parchin in September along with U.S. accusations that it was a test site for nuclear weapons.
Tehran denied that nuclear weapons research was underway there and invited the IAEA to visit the site.
Iran wants only an informal visit, arguing that the location does not fall within IAEA jurisdiction because there are no nuclear activities there. The IAEA won't go unless Iran allows a full inspection, which would give inspectors the right to examine any part of the complex and take environmental samples to test for nuclear activity.
Despite the absence of hard evidence, U.S. officials say they remain convinced that Iran is pursuing nuclear weapons.
Estimates differ on when Tehran might be able to produce a bomb. U.S. intelligence reports say it will take at least three more years. A leaked Israeli analysis predicts Iran could have an atomic weapon by 2007.
"The truth is that nobody knows for sure," said a Western intelligence official who monitors Iran. "But this is getting close to the end game."