Iran’s nuclear program is facing such severe technical difficulties that it could take four years to produce enough weapons-grade uranium for one bomb and eight years to deploy an operational nuclear weapon, experts say.

President Ahmadinejad’s announcement on April 9 that uranium enrichment on an “industrial scale” had begun was “misleading,” and the time-scale for success is likely to be longer than early estimates suggested.

“It’s very difficult to enrich uranium,” said Norman Dombey, emeritus professor of theoretical physics at Sussex University. “It calls for several different scientific and engineering disciplines. Iran hasn’t yet shown that it has mastered the problem.”

Iran’s underground facility in Natanz has space for 3,000 centrifuges, the devices used to enrich uranium. Iran has told the International Atomic Energy Agency that only 1,312 have been installed so far.

Once all 3,000 are in place, the machines must be fitted together to form 18 cascades. Then technicians must introduce uranium in the form of gas into the centrifuges. The machines must then spin at 1,500 revolutions a second to separate out the uranium-235 of 90% purity, the fissile material capable of producing the chain reaction unleashed by a nuclear bomb.

But the smallest particle of dust — even a fingerprint — can disrupt enrichment. Iran will have to spin all the centrifuges inside a vacuum without any interruption for a period of about one year.

If any machine breaks down — or if dust enters the system or if the power supply is lost — the process must halt and start again.

Mr. Dombey estimates that Iran will need about two years simply to master the process of running centrifuges. Then, making allowances for interruptions caused by breakdowns, it could take another two years to produce enough weapons-grade uranium for one bomb.

Mr. Dombey said: “At the moment, their program doesn’t constitute a threat.”