It's safe to then say, that if someone knew what they were doing and skilled in chemistry and nuclear chemistry, in addition to some HVAC type stuff -- they could built a detection-proof container which contains a large nuke. And have a >95% chance of it making it through to turn NY harbor into instant steam and Manhattan into glass, and the near counties of `NY and NJ into dust and ash.
nuclear chemistry??.
If the guys have that level of expertise then they can probably get it in another way. If they have a nuke on ship they could do more damage by blowing in the harbor it before the ship docks and the container can be inspected.