There's a nuclear test site in Georgia where the us government did preliminary tests for a nuclear powered airplane. It was bat shit insane. It seems the idea was not to shield the reactor, but to only shield the crew, and rely on distance and speed to not irradiate basically everything else.

To that end, they built a nuclear reactor that could be hoisted in and out of a hole in the ground so that it could be run unshielded above ground. They tested the effects on all sorts of materials, and a huge swath of surrounding woods, including all the creatures there, which promptly died.

It's now a recreational area, and considered generally safe, except for a few small, fenced off areas.

So my point is, watch what you wish for.

They've tried it. It's too dangerous to have that much nuclear material running around. Not only does it make sourcing for a dirty bomb very easy, but it's also a question of disposal, and how to contain it in a collision.