The Iranian government this week published its first detailed study of how long it estimates it would take its scientists and engineers to assemble a nuclear weapon, saying that with its current infrastructure, "the required time span is in years."

The estimate was disputed by U.S. officials, and it contradicts both classified assessments by the U.S. government and many, though not all, estimates by outside experts.

But the very fact that Iran's nuclear energy establishment wrote the eight-page report, titled "How Long Would an Iranian 'Breakout' Really Take?," was itself notable. Until now, Iran's public position has been that its program is entirely peaceful and that it has never studied what it would take to amass the fuel for a weapon, which is known as "breakout."

The report was issued as Iranian officials were announcing that they had agreed to change the design of a heavy water reactor near the Iranian city of Arak in ways that would limit its ability to forge a second path to a bomb, using plutonium. U.S. officials said they were studying the new Iranian proposal but saw it as a hopeful sign.

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