Nuclear terrorists wear suits: How Iran could build a nuclear weapon without state approval

Matt Caplan, Vesal Razavimaleki 

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A nuclear terrorist does not match the profile of a suicide bomber or spree gunman. It’s not someone building a nuclear weapon in a cave from a box of scraps. A nuclear terrorist, rather, could be hiding in plain sight in a mid-tier government post. Such a person could be active in Iran, right now, motivated to build an improvised nuclear weapon after an opportunity to steal weapon-usable uranium—enriched at 60 per cent uranium 235 has just presented itself. This person could be emboldened by the absence of IAEA inspectors overseeing Iran’s known stockpile of fissile material.

Despite popular confusion about the nature of critical masses and what level of enrichment can be used for a weapon, a technical companion shows that as little as 40 kilograms of 60-percent-enriched uranium, representing only 10 percent of Iran’s stockpile, could be used to build a crude gun-type weapon like the “Little Boy” bomb that destroyed Hiroshima with an explosive yield of several kilotons. Such a weapon requires no further enrichment, greatly simplifying and fast-tracking construction.

The question, therefore, is not whether Iran can achieve its nuclear ambitions, but whether and how these can be realised by nuclear terrorists without state approval. If not by air, then by cargo. In their letter to US President Franklin D. Roosevelt, Leo Szilard and Albert Einstein suggested that a nuclear weapon may be too heavy to be delivered by air, but could be brought into a port in a cargo container and detonated. Such a scenario is feasible for aspiring nuclear terrorists in Iran. 

While the technical hurdles of building a nuclear weapon have long been discussed, they may not be insurmountable to a well-resourced group of clandestine sub-state actors. As of July 2025, publicly available intelligence was inconclusive about what remains of Iran’s enrichment capabilities and other nuclear assets following the June attacks by Israel and the United States on its nuclear facilities. Some experts have suggested, reasonably so from a tactical perspective, that Iran may have removed part of its enriched uranium stockpile from the Fordow underground enrichment facility and scattered it into multiple secret caches.