Radiation detection could help with RDDs and the radionuclides that go into them, but the only thing that’s really going to help with a nuke is an x-ray, to spot the lead, tungsten, or DU box around the bomb, or perhaps the unshielded HEU

That is not quite correct. Detectors are being designed to spot smuggled fissile materials. A neutron beam is sprayed through the ship/truck/train container or trailer or luggage. The Plutonium or uranium will then emit gamma radiation with an extremely precise spectrum.

Ordinary passive radiation detectors such as would be used for people, trucks and cars at tunnels or bridges would not have that capability.

Passive detector:

I think the ideal way to smuggle HEU is embedded within depleted uranium weights, such as used in some aircraft.

Attribution is more than determining the source organization for the attack, but also the source of the nuclear material as well as the design and construction of the device.

Obviously, such after-the-fact research does little to prevent an attack, but it’s still important. Attribution wasn’t a concern during the cold war because we knew where the nukes would come from. Even if we know what groups conducted a nuclear attack, it’s still necessary to determine how they got the nuclear material. Since the early 1990’s a lot of basic research has been done on attribution and it remains an important area of research.

As a layperson, I’m not sure what this means. Does it refer to improving forensics technique, which would determine the source after an attack?

Of course, terrorists aren’t likely to be concerned with an attack being traced back to them (or whoever supplied them with the bomb). In fact, they’d likely take credit for it.

I take it then, the poster was exhibiting more of that black humor endemic to you wonks.