Iran enriched uranium to 84 percent — but can it make a nuclear bomb?

Iran appears to have made a new and worrying advance in its nuclear program. Bloomberg reported on Sunday that it has reached the level of 84 percent enriched uranium, a significant advance on the 60 percent previously announced. The magic number needed for making an atomic bomb is 90 percent.

The new figure was discovered by monitoring equipment operated by the world’s nuclear watchdog, the International Atomic Energy Agency (IAEA). But the Bloomberg report did not reveal how much uranium enriched to this extent Iran has produced. The amount needed for a nuclear bomb is about a grapefruit-sized worth, which would weigh around 33 pounds. (Uranium metal is even more dense than lead.)

This latest revelation comes just a few days after news emerged that Iran had altered the piping joining two groups of centrifuges in its Fordow plant, a change that would allow faster enrichment to higher levels.

That change was discovered by chance. The IAEA can carry out three types of inspections: “announced” (i.e., planned in advance with Iran’s cooperation), “unannounced” (inspectors suddenly turning up and demanding access), and “random” (a variation of unannounced but much rarer). The piping alteration at Fordow caught the eye of an experienced inspector on such a random inspection, a detail that his well qualified but less experienced colleagues may have missed.

The Bloomberg report suggests the higher enrichment level may not be definitive. Was it reached on purpose or accumulated by accident? An Iranian nuclear scientist has claimed, correctly, that it is in the nature of the enrichment process that the spinning centrifuges produce a range of enrichment values above and below the target level.

Even at a notional 6 percent less than the level needed for a nuclear bomb, the 84 percent figure is worrying. Uranium enrichment 101 is that the process is all about the separation of two isotopes of uranium, the slightly lighter U235 isotope from the heavier but more numerous U238 isotope. The ratio of such isotopes in natural uranium is 993 U238 atoms to just 7 U235 atoms. The enrichment process alters the ratio. The 90 percent level is when the ratio is just 1:7 — i.e., 992 atoms of U238 have been stripped out. Eighty-four percent is roughly this ratio, so a workable bomb may need just a pound or two more of U235 to function. And Twitter feeds, milking the Bloomberg story, are reminding us that the first U.S. bomb, dropped on the Japanese city of Hiroshima in 1945, used material of roughly the 84 percent level.

So, the new level that Iran reportedly has reached is well beyond most people’s “red line” of concern. It doesn’t necessarily mean that Iran is close to actually making a nuclear bomb. Officials say that Iran still seems to be challenged when it comes to making the gaseous uranium hexafluoride used in centrifuges into solid metal and casting it into hemispheres that, when placed together as a sphere, could be the explosive core of a nuclear bomb.

But officials also acknowledge that their level of confidence in knowing what Iran is doing on weapon design is significantly less than its enrichment activities. And if Iran were to settle for a bomb delivered by an aircraft, rather than on a long-range missile, the sophistication of design needed could be less. Of course, if Iran were to test a device in a remote desert area, it could be much cruder than a deliverable bomb.

A new additional concern for officials is that the Russian military may slip Iran a critical mass or two of 90 percent enriched uranium, as China did to jump-start Pakistan’s program in the early 1980s. Moscow’s historical record against such proliferation has been exemplary. But the Ukraine war and Iran’s supply of drones to Russia may prompt some elements to give Tehran a special “thank you.”

More details may emerge in the next few days. The IAEA, headquartered in Vienna, said Sunday that it is “discussing with Iran the results of recent Agency verification activities.” The issue probably will figure high on the agenda of the next meeting of its board of governors, due on March 6.

Simon Henderson is the Baker Fellow and director of the Bernstein Program on Gulf and Energy Policy at the Washington Institute for Near East Policy. Follow him on Twitter @shendersongulf.