"cheaper gas centrifuge" . "Zippe-type centrifuge" . "gas-centrifuge program" . "centrifuges" . "gas-centrifuge" . "Zippe-type" . "Zippe-method" . . "Zippe uranium centrifuges" . . "ultra centrifuge" . "Zippe method" . . . . "second generation gas centrifuge" . . . "centrifuge" . . "Zippe-type centrifuge" . . "gas centrifuges" . . .