Iran made a veiled threat this week to enrich uranium stocks closer to weapon-grade levels amid rising tensions in the region.

That would mean going beyond the level of potency allowed by the 2015 nuclear agreement with world powers, which the United States withdrew from last year. Tehran is threatening to resume higher enrichment on July 7 if no new agreement is reached to provide relief from U.S. sanctions.

Enriching a supply of uranium means boosting its concentration to the type of uranium that can power a nuclear reactor. That type, or isotope, is called U-235. Enrichment basically means stripping away atoms of another isotope, called U-238.

When uranium is mined, it typically has about 140 atoms of this unwanted isotope for every atom of U-235, noted Christopher Chyba of Princeton University. Refining it to a purity of 3.67 percent, the level now allowed by the nuclear deal, means removing 114 unwanted atoms of U-238 for every atom of U-235.

Boosting its purity to 20 percent means removing 22 more unwanted isotopes per atom of U-235, while going from there to 90 percent purity means removing just four more per atom of U-235, he noted. Ninety percent is considered weapons-grade.

So once one achieves 20 percent purity on the way to 90 percent, “You’re most of the way there already,” Chyba said.

Iran’s Supreme Leader Ayatollah Ali Khamenei reportedly said Tuesday that achieving 20percent “is the most difficult part. … The next steps are easier than this step.” Iran is not known to have enriched beyond 20 percent previously.

Matthew Bunn at Harvard’s John F. Kennedy School of Government said the same centrifuge technology used to reach 3.67 percent can be used to drive enrichment to 90 percent.

Once Tehran had enough uranium enriched to 20 percent, it would only be weeks away from a bomb if it decided to build one, he said. A bomb’s worth of highly enriched uranium “could be the size of a very large grapefruit,” Bunn said.

Iran said it has never sought nuclear weapons. But Western officials and experts said that prior to the nuclear deal, Iran had a breakout capability of just a few months if it were to decide to build a bomb. The Obama administration, which negotiated the deal, said it extended that breakout period to one year or more, giving Western powers more time to detect any such decision and respond to it.

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