First - Metal square, in this whatever what increase pressure (maybe explosive), and next plutionium what make neutrons which makes reaction with uranium in middle.

Go to my creations. I made a plutonium nuke with a uranium 'buffer' or 'pusher'. In this game, uranium is non-reactive to neutrons, so you can't use only uranium.

Just make a metal box, fill it with C4 (the all important thing people leave out), hollow out a bit, press pause, make a square of plutonium (good size), and fill the remaining space between the plutonium and C4 with uranium. Unpause. Use air on the plutonium, until neutrons appear. Now, you should have a reaction.

EDIT: alternately, do what the guy above me said. I didn't see that when I posted this :P

Uranium only generates heat. A lot of heat that is, I used it my desalination plant. But it has problems because it's too hot.

thunder core

uranium outside, covered by metal, thunder makes so much pressure that the uranium heats up and destroys the metal, Nuke

post results

BOOM!

Well, that is my guess. Haven't tried it yet.