Have you calculated out how much energy you can get from a hunk of uranium in a nuclear reactor and figured out of 20 kilotons is a reasonable yield for a nuclear bomb?

My suspicion is that there's way more than 20 kilotons of energy in 30 hunks of uranium. I will count U-235 and U-238 the same as I think the energy release from splitting them is about the same, you just need a fast neutron to split an atom of U-238.

Nope. The current limit is 20 Tons (note: not kilotons) because the engine doesn't support projectiles at a range over 1km. In the next version I'm going to try and circumvent that, but for now larger explosions simply get truncated :l

I plan to look into balancing recipes after I've gotten the core effects (shock wave, flash, and prompt radiation) working.