Basically its super hard to get a nuclear bomb to detonate all of its material, because the force of the explosion tends to just blow the nuclear material away before it can become part of the runaway chain reaction. From what I have read you need extreme precision and carefully placed essentially "counter explosions" around the core to keep the material close enough together in the chain reaction long enough to get high rate of conversion of the material into as much energy as possible
The "crude terrorist nuke" assumes that terrorists aren't going to be able to get that precision and technique and will only get a very small yield because the bomb will blow itself apart before the chain reaction can consume all the material. After all nation states took many, many tries to get it right
Generally the smaller a nuclear explosion is the greater the danger of radiation relative to the blast. The bombs at Hiroshim and Nagasaki killed many people via radiation sickness. For modern strategic weapons if you're close enough to worry about that you'll have already died from the blast.
Of course fallout, especially from explosions on the ground to destroy hardened bunkers, silos, and sub pens can generate terrifying levels of fallout. It's just the prompt radiation from the explosion itself that usually isn't a concern.
No (nuclear) fizzle necessary. Just blow apart some abused Cobalt-60 source by conventional means, scrapped from medical or materials testing equipment. Imagine the scare. Effective area denial.
Even with all the work they put into making the implosion hold the fissionable material together it still apparently only consumed about 20% of the material.
But yeah its unlikely a terrorist org could get enough material for a "gun type" and the implosion type would also be very, very hard to get right
AFAIU the consensus among non-proliferation experts is that given access to HEU, even a relatively unsophisticated terrorist group could make a gun type weapon. That's why access to HEU is so tightly controlled with a lot of effort expended to minimize use of it. See e.g. the RERTR program to convert research reactors to LEU. And why the international community is aghast when some country like Iran tries to build up enrichment capability.
OTOH a plutonium implosion type device is a lot more difficult to get right, and is practically out of reach for non-state actors. On top of the device engineering difficulty itself, producing the raw materials is difficult too. The actual production of weapons grade plutonium isn't terribly difficult (e.g. a natural uranium fueled graphite moderated reactor), separating the Pu from the spent fuel with the PUREX process is fairly expensive and complicated.
it would be more feasible if it were stolen as theres a fair bit of monitoring going on
we made a bit of a big deal when it was revealed uranium yellow cake was being produced in quantity
>In the Little Boy design, the U-235 "bullet" had a mass of around 86 pounds (39 kg), and it was 7 inches (17.8 cm) long, with a diameter of 6.25 inches (15.9 cm). The hollow cylindrical shape made it subcritical. It was powered by a cordite charge. The uranium target spike was about 57.3 pounds (26 kg).<
The "crude terrorist nuke" assumes that terrorists aren't going to be able to get that precision and technique and will only get a very small yield because the bomb will blow itself apart before the chain reaction can consume all the material. After all nation states took many, many tries to get it right