All commercial enrichment methods involve first gasifying the uranium with fluorine, creating the gas uranium hexafluoride.

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The Russians perfected a far less energy-hogging method that uses centrifuges to spin the uranium hexafluoride at 50, 000rpm so that heavier u-238 separates from lighter u-235.

FORBES: Energy & Genius

This opinion was bolstered by a statement in the report (in a footnote) that feeding the centrifuge cascades with its input uranium hexafluoride was stopped on November 16.

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