Many breathless pundits have argued in recent days that the world faces a now-or-never window of opportunity to bomb Iran's new Bushehr nuclear reactor to neutralize any potential nuclear threat from Tehran.

Former UN Ambassador John Bolton says the deadline for destroying it must be before the first fuel is loaded on Saturday – and before it becomes more of a radioactive and political mess to destroy.

The reactor, once operating, could give Iran enough plutonium-laced spent fuel to make up to 60 nuclear weapons within 12 to 18 months of operation, according to nuclear experts.

Still, many leading US nuclear scientists caution that even after Russia begins fueling the power plant that it built in southern Iran, the international community will have at least two weeks, and possibly longer, while fuel is being loaded to evaluate the situation since Bushehr will not become instantly radioactive.

"A good number of people have erroneously assumed that once the Russians load the fuel in Bushehr, the plant 'goes radioactive,' " Henry Sokolski, director of the Nonproliferation Policy Education Center, a Washington think tank, writes in an e-mail.

Though he opposes bombing, "the amount of reactivity in the reactor initially is relatively low, so low that the amount of radioactive material that might be dispersed if the plant was bombed would be negligible" for weeks or months.

But other experts say the time is just a few weeks at most. Intense radioactive buildup won't begin at Bushehr until the reactor is turned on after fuel loading is completed on Sept. 5, Iranian officials say.

After that, the intensity of the radiation buildup depends on the power output, says Frank Von Hipple, a nuclear nonproliferation expert and physicist at Princeton University.

"I think that the idea of bombing Bushehr – whether in a day or a month – is crazy," he says.

The potential environmental and health impacts of bombing Bushehr after it is operating depends on wind conditions and how much radiation has built up inside the reactor, he says. Long- and short-lived radiation levels would be relatively small until power output reaches perhaps 10 percent or more, Dr. Von Hipple says.

But dangerous levels of radioactivity could be reached in less than a week after Bushehr begins operating, says Edwin Lyman, a nuclear physicist with the Union of Concerned Scientists, who also does not favor bombing.

Radiation will build up significantly in the two-week span after Sept. 5, if Iranians ramp up power to 50 percent as they say they will, he says.

"There will be an enormous quantity of radioactivity even at a fraction of the rated power of the reactor soon after startup," Lyman writes in an e-mail. "Even after a few days at a significant power level, like 20 percent, these [radioactive isotopes] would be numerous enough to be of great concern if released into the environment."

While Von Hipple, Mr. Sokolski, or Dr. Lyman differ on how significant the threat from Bushehr is, all say that the reactor's first year to 18 months – when the first load of nuclear fuel is being irradiated – will be telling. That's where agreement ends.

During that period of time, Bushehr's first load of fuel could produce about 300 kilograms (kg) of weapons-usable plutonium – enough to entice the Iranians to cheat on international inspectors, Sokolski says. The fuel would be lower in radioactivity at this time – so easier to handle – while also higher in plutonium concentration and better for making bombs.

Von Hipple says it would be about one-third that amount. Still, it takes as little as 6 kg of plutonium to build a bomb, he says.

"The reactor uses tons of low enriched uranium that can be diverted and enriched to make bomb grade uranium fuel – and it produces tons of spent fuel that contains weapons useable plutonium," Sokolski says. The plutonium could be "stripped out chemically relatively quickly in small plants that could be built covertly."

Lyman and Von Hipple agree this scenario is conceivable ­– but just barely, they say. It is far less likely, for instance, than the possibility that Iran might simply build a clandestine fuel-enrichment plant to develop bomb fuel with no connection at all to Bushehr.

"Iran would have to steal the fuel out from the under the noses of international inspectors," Lyman says. "The only rationale for using fuel from Bushehr is if they wanted a much higher rate of weapons production which would require a hard-to-conceal large-scale reprocessing plant. I don't think it would be a great benefit to them to build a small quick and dirty plant."

In Lyman's view, Iran is unlikely to tap into Bushehr's closely watched trove and risk the wrath of the world. The reactor does not pose a large proliferation threat as long as close international monitoring continues – and it can exclude the existence of a clandestine plant to reprocess Bushehr fuel.

The spent fuel rods are supposed to go back to Russia and Iran has said all along that it has no intention of building nuclear weapons.

"It's not a slam dunk that the Iranians could get away with it," says Von Hipple of the diversion scenario. "The question is: Is this the most plausible way for Iran, if wants a bomb? What is the lowest risk way to go about it? This doesn't seem like the way to do it."

But whether the International Atomic Energy Agency (IAEA) is up to the task of ensuring that none of the fuel in Bushehr goes missing worries Sokolski. If IAEA inspectors check their cameras and appear on-site only once every 90 days, which is standard practice, then fuel could be stolen, processed, and installed in waiting bombs before anyone could do anything, he says.

Bushehr's fuel could be clandestinely extracted and made into bomb material using a "quick and dirty" reprocessing approach made public by Oak Ridge National Laboratory researchers in the 1970s, Sokolski notes.

"You couldn't filch this stuff without it being noticed eventually," he says in an interview. "But the point is: What is anyone going to do about it?"

While the Russian light-water reactor design is widely regarded as fairly resistant to misuse – the technical capabilities of the reactor to produce bomb material are quite clear, experts say.

"It's technically true that the spent fuel coming out of this light-water reactor will be more radioactive and difficult to use for nuclear weapons as time between refueling grows," says Leonard Spector, a nonproliferation expert who directs the Washington office of the Monterey Institute of International Studies. "It's also absolutely true that the spent fuel from it can be reprocessed [to obtain plutonium] for use in nuclear weapons."

But whether Iran is willing to divert fuel from the reactor in defiance of the global community, reprocess it into bomb fuel, and thereby set off a firestorm of condemnation and likely military attacks is unknown.

"Definitely there's a game going on with Iran building secret underground facilities," Lyman, the nuclear physicist, says. "My point is whether there is a real need for immediate military action when we have already tolerated the Iranians building thousands of centrifuges? I don't think so."