Iran said in June it would shift production of uranium enriched to 20 percent purity to Fordow from its main Natanz plant this year and triple output capacity, in a defiant response to charges that it is trying to make atomic bombs.

Iran only disclosed the existence of Fordow two years ago after Western intelligence detected it and said it was evidence of covert nuclear activities. The facility has yet to start
operating.

“They are preparing (for the centrifuges to be installed) in Fordow,” one diplomatic source said.

Refined uranium can be used to fuel nuclear power reactors and also, if enriched to much higher levels, provide material for atomic arms.