Iran can still build nuclear weapons without further enrichment. Only diplomacy will stop it

Since the successive Israeli and US air strikes on Iran’s nuclear sites in June, much of the fevered and highly politicized public debate has focused on whether the attacks have “obliterated” Tehran’s capability to build nuclear weapons or only set it back a few months or years. But one critical point continues to be largely and inexplicably overlooked: Iran’s stockpile of over 400 kilograms of highly enriched uranium (HEU)—enriched to 60 percent uranium 235—is weapon usable.

This means that Iran’s HEU—which the International Atomic Energy Agency (IAEA) reported in June as being unaccounted for following Israel’s initial air strikes and which may have been moved to secure locations before the attacks—could be used directly to make bombs without requiring further enrichment.

If Iran still has access to some of its HEU stockpile, then direct use of that material may suddenly appear to its leaders as the most attractive and fastest pathway to a bomb, especially if its ability to enrich uranium has indeed been significantly degraded. There may be other chokepoints along the road to weaponization, but access to bomb material would not be one of them.

Right now, whether Iran’s HEU stockpile survived the attacks is a major conundrum for Israel and the Trump administration. There is no plausible military option for destroying or seizing it without being able to pinpoint its location—which by now could be anywhere in Iran, and possibly spread over several sites. The most effective way for the international community to gain full confidence that the HEU has not been diverted for weapons use is therefore through a diplomatic agreement in which Israel and the United States would forswear further attacks, and Iran would provide the IAEA with all the information and access it needs to fully account for the fate of the stockpile and quickly reestablish an enduring verification regime.

An open secret. That HEU at 60 percent enrichment can be used in a nuclear weapon is hardly a state secret. But it has taken on greater significance now that Iran’s ability to further enrich this material or produce additional HEU from lower-enriched stocks has been set back by at least several months—or even years by some estimates—after the attacks on centrifuge enrichment and other supporting facilities at Natanz, Fordow, and Isfahan.

Although Iran may well have every reason—both practical and strategic—not to pursue direct weaponization of its remaining 60-percent enriched HEU, this is a different issue from whether it has the technical capability to do so. The prevailing assertion that Iran needs “weapons-grade” uranium enriched to at least 90 percent uranium 235 to build a nuclear explosive device—widely repeated by government officials, the media, and commentators alike—is simply wrong.

One does not have to dig deep to document the basis for the statement that all HEU is weapon-usable. The IAEA considers HEU, which is defined as uranium enriched to 20 percent or greater, as a “direct use” material, meaning it “can be used for the manufacture of nuclear explosive devices without transmutation or further enrichment.” This is the basis for the international safeguards measures that the IAEA applies to stocks of declared HEU and other direct use materials such as separated plutonium, as well as for the international standards that govern the protection of direct use materials by states from domestic theft.

This is not to say that all grades of HEU are equivalent regarding their usability in weapons, and it is important to understand the differences.

The IAEA’s “significant quantity” of HEU, defined as “the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded,” is 25 kilograms of contained uranium 235. For 90 percent enriched material, this corresponds to 27.8 kilograms of total uranium.

It is not difficult to see that 20 to 25 kilograms of 90-percent enriched HEU could be used to make a first-generation implosion weapon similar in design to the “Fat Man” plutonium weapon that destroyed Nagasaki in 1945—albeit one that is larger in diameter and considerably heavier.

At 60 percent enrichment, the IAEA significant quantity corresponds to 41.7 kilograms of total uranium (or 1.5 times the significant quantity at 90 percent). This means that, at least for a specific weapon design, a fixed stockpile of HEU would support a smaller number of weapons at 60 percent than at 90 percent. An analogous Fat Man-type design at 60 percent could require about twice as much total uranium as at 90 percent. This suggests that Iran’s stockpile of 408 kilograms, as reported by the IAEA, could be used to make roughly 6 to 7 weapons of that type, compared to the 9 to 10 that have been estimated at 90 percent. (More advanced designs—which would require further development and are likely beyond Iran’s capability today—would need less HEU per weapon in either case.)

Design considerations. In February, the New York Times reported that Iran was considering a crash program to develop a relatively unsophisticated nuclear weapon within a matter of months that could not be miniaturized to fit on a ballistic missile but could be delivered by other means. Although not completely clear from the article, it suggests that the target enrichment for such a design would still be 90 percent. But with Iran’s capability to further enrich the material quickly in doubt, it is reasonable to consider whether such an unsophisticated device could accommodate a 60-percent uranium core without significantly compromising its effectiveness.

Using more uranium at a lower enrichment in a first-generation implosion device generally has certain drawbacks associated with the larger and heavier core (about 25 percent greater in this case) and neutron reflector. Such a configuration would require significantly more high explosive to compress the core and other structures to create a highly supercritical state in which the rate of nuclear fission reactions increases exponentially, leading to a nuclear explosion. Another factor is that the fission chain reaction in lower enriched uranium evolves more slowly. This generally reduces the explosive yield because fewer fission reactions have time to occur in a supercritical core before it blows apart and the chain reaction stops. However, other mitigating factors could partially offset these disadvantages, so the weight and size penalty may not be as prohibitive as it first appears.

There is also the possibility that Iran could build gun-type weapons with 60 percent enriched HEU like the “Little Boy” bomb that destroyed Hiroshima, which used about 60 kilograms of HEU with an enrichment of about 80 percent. These would require more uranium per weapon than implosion devices—perhaps 120 kilograms or so—but would be much simpler to build. The risk of a predetonation due to spontaneous neutron generation—an effect that reduces the yield and is more of an issue for gun-type devices—would be higher for 60 percent enriched HEU, but the difference would not likely be decisive.

Avoiding Iran’s nuclear breakout. If it still has access to a significant fraction of its current 60-percent HEU stockpile, Iran would have options for weaponization even if it is unable to further enrich the material. At this stage, the limiting factor to Iran’s possible weaponization could be the lack of a capability to de-convert uranium hexafluoride to uranium metal, following the destruction of its facility in Isfahan by the US bombing raid on June 20. But even that may not be a major obstacle if Iran already has a small-scale clandestine facility or maintains the know-how and equipment to reproduce the capability. Although a large-scale facility may take years to replace and would be difficult to build in secret, a facility sized to produce a few hundred kilograms in a short time does not appear nearly as daunting a task, given that Iran already knows how to do it.

Iran’s technical capability to produce at least a few crude weapons relatively quickly from its stockpile of 60 percent HEU cannot be discounted. The question then hinges on intent: Is weaponization something that makes strategic sense for Iran to pursue at a moment of weakness and threat to the regime? Assuming that the Iranian leadership is rational and understands the potentially catastrophic ramifications of attempting to build and use a nuclear weapon, the answer should be a resounding “no.” But there will always be doubt about Iran’s intentions unless Iran provides the IAEA with full access and authority to verify whether the stockpile still exists and has not been diverted. The only viable solution to prevent an Iranian nuclear breakout is a diplomatic one. Both Jerusalem and Washington should see it that way.