Six years after the Trump administration withdrew from the Iran nuclear accord, Tehran is rapidly accumulating enriched uranium, some of it very close to weapons grade. Experts fear that a bomb could be a short dash away.

For the past 15 years, the most important clues about Iran’s nuclear program have lain deep underground, in a factory built inside a mountain on the edge of Iran’s Great Salt Desert. The facility, known as Fordow, is the heavily protected inner sanctum of Iran’s nuclear complex and a frequent destination for international inspectors whose visits are meant to ensure against any secret effort by Iran to make nuclear bombs.

The inspectors’ latest trek, in February, yielded the usual matrices of readings and measurements, couched in the clinical language of a U.N. nuclear watchdog report. But within the document’s dry prose were indications of alarming change.

In factory chambers that had ceased making enriched uranium under a 2015 nuclear accord, the inspectors now witnessed frenzied activity: newly installed equipment, producing enriched uranium at ever faster speeds, and an expansion underway that could soon double the plant’s output. More worryingly, Fordow was scaling up production of a more dangerous form of nuclear fuel — a kind of highly enriched uranium, just shy of weapons grade. Iranian officials in charge of the plant, meanwhile, had begun talking openly about achieving “deterrence,” suggesting that Tehran now had everything it needed to build a bomb if it chose.

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