A recurring fear haunts the West’s increasingly tense confrontation with Iran: Is its work on civilian nuclear power actually a ruse for making a deadly atomic arsenal, as has been the case with other countries?

Next month, the United Nations plans to take up that question at a board meeting of the International Atomic Energy Agency, in Vienna . The diplomatic backdrop includes possible sanctions and even the threat of war.

“If Iran goes nuclear, you worry that Hezbollah goes nuclear,” said Paul Leventhal, president of the Nuclear Control Institute, a private group in Washington , referring to the Iran-backed terrorist group.

The Iranian crisis, and related ones simmering in North Korea and also around Dr. Abdul Qadeer Khan, the Pakistani expert who recently confessed to running nuclear black markets, are giving new urgency to limiting proliferation, a central danger of the atomic era. Recently, international inspectors discovered that North Korea may have clandestinely supplied uranium to Libya , demonstrating how an aspiring state can secretly reach for nuclear arms.

The development of such arsenals is often hard to hide, because it takes place in large industrial complexes where nuclear power and nuclear weapons are joined at the hip – using technologies that are often identical, or nearly so. Today, with what seems like relative ease, scientists can divert an ostensibly peaceful program to make not only electricity but also highly pure uranium or plutonium, both excellent bomb fuels.

Experts now talk frankly about a subject that was once taboo: “virtual” weapon states – Japan, Germany, Belgium, Canada, Brazil, Kazakhstan, Taiwan and a dozen other countries that have mastered the basics of nuclear power and could, if they wanted, quickly cross the line to make nuclear arms, probably in a matter or months. Experts call it breakout.

The question now, driven largely by the perception that the world is entering a dangerous new phase of nuclear proliferation, is whether the two endeavors can be separated. And as difficult as that may seem, new initiatives are rising to meet the challenge.

Last year, North Korea stunned the world by withdrawing from the Nonproliferation Treaty. It was the first time a nation had dropped out of the 1968 pact, setting a grim precedent and prompting warnings of the accord’s demise.

If another virtual power crosses the line, experts fear, it could start a chain reaction in which others feel they have no alternative but to do likewise.

Yet a country like Iran can retain its virtual-weapons status – and the threat of breakout – even if the International Atomic Energy Agency gives it a clean bill of health. That kind of quandary is driving the wider debate on ways to safeguard nuclear power, especially given that the world may rely on it increasingly as worries grow about global warming and oil shortages.

“We can’t give absolute guarantees,” said Graham Andrew, a senior scientist at the agency. “But there will be technological developments to make the fuel cycle more proliferation-resistant.”

Other experts agree. “The future looks better than the past in terms of this whole problem,” said Rose Gottemoeller, a senior associate at the Carnegie Endowment for International Peace. “At the moment, it’s a very, very fast-moving arena that a lot of people are into and thinking about.”

The central compact of the nuclear age – what critics call a deal with the devil – is that countries can get help from other nations in developing nuclear power if they pledge to renounce nuclear arms. That principle was codified in the 1968 treaty and has produced a vast apparatus of the International Atomic Energy Agency that not only helps nations go peacefully nuclear but also monitors them for cheating.

But surveillance has proved far from perfect, and states have proved far from trustworthy.

“If you look at every nation that’s recently gone nuclear,” said Mr. Leventhal of the Nuclear Control Institute, “they’ve done it through the civilian nuclear fuel cycle: Iraq , North Korea , India , Pakistan , South Africa . And now we’re worried about Iran .”

The moral, he added, is that atoms for peace can be “a shortcut to atoms for war.”

Moreover, the raw material is growing. The world now has 440 commercial nuclear reactors and 31 more under construction.

Experts say Iran provides a good example of the breakout danger. With the right tweaks, its sprawling complex now under construction could make arms of devastating force. Recently, mistrust over that prospect soared when inspectors found that Iran had hidden some of its most sensitive nuclear work as long as 18 years.

In the central desert near Yazd , the country now mines uranium in shafts up to a fifth of a mile deep.

At Isfahan , an ancient city that boasts a top research center, it is building a factory for converting the ore into uranium hexafluoride. When heated, the crystals turn into a gas ideal for processing to recover uranium’s rare U-235 isotope, which, in bombs and reactors, easily splits in two to produce bursts of atomic energy.

Nearby at Natanz, Iran aims to feed the gas into 50,000 centrifuges – tall, thin machines that spin extraordinarily fast to separate the relatively light U-235 isotope from its heavier cousin, U-238. It recently came to light that Iran had gained much help in making its centrifuges from Dr. Khan and his secretive network.

Iran says it wants to enrich the uranium to about 5 percent U-235, the level needed for nuclear reactors.

But enrichment is one place that good power programs can easily go bad, nonproliferation experts say. By simply lengthening the spin cycle, a nation can enrich the uranium up to 90 percent U-235, the high purity usually preferred for bombs.

Moreover, a dirty little secret of the atomic world is that the hardest step is enriching uranium for reactors, not bombs. David Albright, president of the Institute for Science and International Security, an arms control group in Washington , said the step from reactor to weapon fuel took roughly 25 percent more effort.

The whirling centrifuges at Natanz could make fuel for up to 20 nuclear weapons every year, according to the Carnegie Endowment. Others put the figure at 25 bombs a year.

The Iranians are building a large power reactor at Bushehr on the Persian Gulf meant to be fueled with low-enriched uranium from Natanz. Here too, experts say, a good program can go bad.

Normally, uranium fuel stays in a reactor for three or four years and, as an inadvertent byproduct of atomic fission, becomes slowly riddled with plutonium 239, the other good material for making atom bombs. But the spent fuel also accumulates plutonium 240, which is so radioactive that it can be very difficult to turn into weapons.

But if the reactor’s fuel is changed frequently – every few months – that cuts the P-240 to preferable levels for building an arsenal. (And since less plutonium than uranium is needed for a blast of equal size, it is the preferred material for making compact warheads that are relatively easy to fit on missiles.)

John R. Bolton, the State Department’s under secretary for arms control, recently told Congress that after several years of operation, Bushehr could make enough plutonium for more than 80 nuclear weapons.

Iran strongly denies such ambitions.

“That we are on the verge of a nuclear breakthrough is true,” Hashemi Rafsanjani , Iran ‘s former president, said recently, according to the Islamic Republic News Agency. “But we are not seeking nuclear weapons.”

If Iran wanted to recover plutonium from Bushehr, or a different reactor under construction at Arak , it would have to extract the metal from spent fuel, a hard job because of the waste’s high radioactivity. Such reprocessing plants have legitimate commercial uses for turning nuclear detritus into new fuel, as France , Britain , Japan and Russia do.

Iran , too, has announced that it wants to master the complete nuclear fuel cycle, apparently including reprocessing. Last year, President Mohammad Khatami said the country wanted to recycle power-plant fuel. “We are determined,” he said in a televised speech, “to use nuclear technology for civilian purposes.”

Around the globe, experts are struggling to find ways to guarantee such good intentions: not just in Iran , but everywhere.

Mohamed ElBaradei, director of the International Atomic Energy Agency, is calling for “multinational controls” on the production of any material that can be used for nuclear arms. If accepted, that would mean no single country could enrich uranium or reprocess plutonium on its own, but only in groups where members would verify each other’s honesty.

Early this month, Iran signaled that it might be interested in teaming with Russia and Europe to enrich uranium, giving arms controllers some hope of a peaceful resolution to the current crisis.

Matthew Bunn, a nuclear expert at the Kennedy School of Government at Harvard, has called for sweetening the deal by guaranteeing members of a consortium lifetime fuel supplies and spent-fuel removal if they forgo enrichment and reprocessing plants.

“What you need is an incentive,” he said. One challenge, he added, would be convincing states that consortiums “won’t change their minds,” given that nuclear policy makers have often done so in the past.

President Bush has taken a harder line, proposing in a February speech to limit drastically the number of nations allowed to produce nuclear fuel. Only states that already have enrichment and reprocessing plants, he said, should do such work, and they in turn would service countries that aspire to nuclear power.

While many experts praise Mr. Bush’s attention to the nonproliferation issue, some have faulted his specifics. “It’s all sticks and no carrots,” said Mr. Bunn, adding that the Bush plan would only feed global resentment toward the nuclear club. “I think you can couch this to be more carrotlike.”

Down the road, a different approach involves developing new classes of reactors that would better resist nuclear proliferation, especially by making the recovery of plutonium 239 much harder. Many studies, including one last year at M.I.T., have championed better fuel cycles and security.

“There is potentially a pathway – diplomatic, technical – to see a significant global deployment” of safer technologies and strategies, said Ernest J. Moniz, a former Energy Department official who helped lead the M.I.T. study. “But it can’t happen without U.S. leadership and the U.S. partnering with other countries, and that will require a re-examination of our policies.”

Mr. Leventhal of the Nuclear Control Institute said too many of the proposals were too timid. Most fundamentally, he said, nations have to turn away from the commercial use of plutonium, which grows more abundant every day.

“Only denial and greed” can explain the world’s continuing to want plutonium for peaceful uses, he said, and added, “It may take the unthinkable happening before the political process can screw up the courage to put an end to this ridiculously dangerous industry.”