Critical mass is over 50kg so let's say I have 2 halves of a sphere of the isotope U-235, each weighing 30 kg. I drop one onto the other so that they form a supercritical sphere. No doubt a chain reaction would begin, but I assume it would produce energy on the level of a nuclear reactor rather than a fission bomb? Is it a matter of force when they collide? Just how soft is uranium? My question stems from learning about the "gun type" fission bombs developed during WWII. My guess is that uranium is a soft metal, and not much force is required to meld the two pieces together although I may be wrong.