Atomic weapons can be easily detonated by one person. A crude atomic weapon could be built by one person as well.
The only reason they can't is...wait for it...governmental restrictions.
I disagree. Atomic weapons require a great deal if technical expertise and specialized equipment.
A crude bomb is much easier to make than a crude firearm, but you still don't see a whole lot of bombing going on between private citizens in the US.
If you haven't noticed, the average American citizen isn't exactly the sharpest tool in the shed. Even without government restrictions, I wouldn't worry about some nutjob building a crude atomic weapon.