Is There a Chain Reaction With Uranium?

In general, naturally ccuring uranium can't set off a chain reaction. Those neutrals just go into whatever other material. But if you get uranium that's dense enough and you have it pure enough, then it can reach critical mass,. and so it sustains itself. And as you said, if you have the right kind of fuel, it can create a runaway explosion or a very rapid release of energy That's a bomb. You got to get the right blend, and yougot to get the neutrons going at just the right speed.