Its huge centrifuge plant at Natanz is still nearly empty, and the more threatened Iran feels, the more reason it has to hide its program.

First, after the 1991 gulf war United Nations inspectors learned that Iraq had planned to build a centrifuge plant of 1,000 machines.

In 1991, Iraq planned to build a centrifuge plant of 1,000 machines designed to produce 10 kilograms (22 pounds) of highly enriched uranium a year.

About 2,000 such centrifuge plants would be able to produce about 40 to 50 pounds of highly enriched uranium a year or enough explosive for one bomb.

The fuel would then be taken either to the centrifuge facility at Natanz or, perhaps, to some covert centrifuge plant.

Another is that Iran might somehow divert material from Natanz and take it to a secret centrifuge plant for enrichment to weapons-grade material.

And any centrifuge plant big enough for weapons production should be spotted by intelligence satellites, leaving time for diplomatic pressure and military action if necessary.

In addition to an industrial-sized centrifuge plant, they are also building an experimental light water reactor, supposedly for energy production.

Siegfried Hecker, a former chief of the Los Alamos nuclear lab who visited the centrifuge plant, called it "astonishingly modern."

Now it, too, wants to build a centrifuge plant, near a former gaseous diffusion plant in Ohio.