The bottom line is that a nuclear fission weapon within the technological capabilities of all but the US and Russia is limited in weight to a minimum of between 1,000 and 1,500 lbs, and is the size of a large plastic trash dumpster.
However, this is not a practical weapon for ground delivery, as it would give off a significant radiation signature, which would be very obvious at our ports of entry.
The other alternative is a “radiation bomb”, that is radiological material blown into a cloud with high explosives. It does not create a nuclear reaction. But it has an inherent problem.
Light weight particles that would be carried great distances have short half-lives. Heavier particles fall to Earth quickly, limiting the area of contamination.
A dangerous particle that was created at Chernobyl was radioactive Iodine. Its half-life is two weeks. But from the moment of its creation, the clock was ticking. So if some terrorists got their hands on a quantity, they would have to use it almost immediately or it would be decayed and no longer radioactive.
But if they got a quantity of uranium or plutonium, it is so heavy that the vast majority would just be within a few feet of were the bomb had gone off.
Oh. You mean about the same size as a small load of pot or cocaine brought in from the Bahamas, Mexico, or Cuba?