(Bloomberg) -- When US prosecutors charged a Japanese mobster this year with conspiring to traffic nuclear-weapons material to an Iranian general, they exposed one piece of a shadowy international network that continues to preoccupy security officials. 

Investigators convene Monday at the International Atomic Energy Agency in Vienna to assess how organized crime networks are still able to buy and sell fissile material that’s slipped outside of regulatory controls. 

Because of the slim margin for error — less than 25 kilograms (55 pounds) of highly-enriched uranium or 8 kilograms of plutonium are needed to make a bomb — law enforcement tends to be on higher alert ahead of prominent events like this year’s European soccer championships across Germany and Olympic Games in Paris and other venues in France.  

“The recurrence of incidents confirms the need for vigilance,” said Elena Buglova, the IAEA’s top security official. 

While the number of incidents of nuclear material trafficked with malign intent has fallen in recent years, cases like the US Southern District of New York’s against a Yakuza crime boss shows the market for illicit nuclear material hasn’t completely dried up. 

Posing as the middleman acting on behalf of an Iranian general, a US Drug Enforcement Administration official was able to buy a sample of weapons-grade uranium and plutonium during a 2020 sting set up in Thailand. A forensic laboratory later confirmed the authenticity of the material, according to the 20-page indictment. 

The Japanese crime syndicate leader, Takeshi Ebisawa, wanted to trade the nuclear material for military-grade weapons sourced from Myanmar, the US alleges. The undercover DEA agent was offered access to hundreds of pounds of additional uranium.  

“It’s impossible to overstate the seriousness of the conduct,” US Attorney Damian Williams said in February, after a grand jury filed international nuclear-trafficking charges.

To be sure, most of the 152 new incidents that wound up in the IAEA’s trafficking database in 2023 were incidental — radioactive material that just happened to be inside a stolen vehicle, for example, or shipped with a cargo of scrap metal. Only about 2% of all cases were categorized as having had malign intent. 

But though the probabilities are low, the potential costs are enormous. A nuclear-armed terrorist attack on the US port of Long Beach, California, would kill 60,000 people and cost as much as $1 trillion in damage and cleanup, according to a study by the RAND Corporation. Researchers figure a low-level radiological or dirty-bomb attack on Washington, while causing a limited number of deaths, could lead to damages of $100 billion. 

“Nuclear and other radioactive material remains vulnerable,” Buglova said.

The IAEA, the UN’s nuclear watchdog, hosts officials from more than 140 countries for discussions running through May 24.

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