ahem

So the first thing is that if you have cesium 137 or weapons-grade uranium and you didn't buy it, the only people who you can sell to are extremists. It's not like Habitat for Humanity is in the nuke business. North Korea? Iran? Not only do they have centrifuges but if they get caught it totally fucks up their sanction-space.

The next thing is that the Russian Mafia has been selling anything that isn't bolted down (and lots of things that are) since the fall of the Soviet Union. They can't sell submarines to the Cartels anymore because the cartels have gone indigenous. People have been freaked out about the former Soviet Union's nuclear stockpiles since before Yeltsin was hunkered down in the Kremlin.

So it comes down to this: It would be amazing if the Mafiya hadn't sold some nasty shit to some nasty folx at some point in the past 20-plus years. that said nasty shit hasn't been deployed by said nasty people is probably a combination of timing and the general forbearance of the human race. Contrary to popular opinion, there has yet to be a mullah that condones nuclear weapons.

ISIS? ISIS is filthy rich, at least as far as "extremists" go. If you've got expensive contraband, they're the guys. You really gonna go sell uranium to Boko Haram? The Houthis? No, you go for the dudes with oil wells. Not that this makes them discerning buyers bent on world destruction, it just makes them more likely marks.

'member Doc Brown and the Libyans and their pinball machine parts? That's how much I'm worried about ISIS and nuclear material. Dirty bombs are effective as a scare tactic once and then everyone realizes that 50g of cesium 137 dispersed in a bus station is like five chest x-rays. Weapons-grade uranium? 'k, great. Better have a pretty good idea how to use it and a rag-tag bunch of al qaeda in Iraq holdovers don't. Full stop.