How much U-235 will make a big bomb?
The type of U-235-based atom bomb that was dropped on Hiroshima is known as the gun bomb design. In essence, super-critical mass is achieve within the device by uniting one sub-critical chunk of U-235 with another, by shooting it down the barrel of an artillery tube, where the propellant used is similar to conventional artillery-shell propellant. The Hiroshima weapon’s yield was about 12 kilotons. Some sources put the total amount of U-235 refined into bomb grade (say, 90% or better purity) that’s needed in this design as something like forty pounds. Less than that, and the gun bomb won’t accomplish super-criticality: no big blast and no mushroom cloud. So, it would seem that forty pounds is a barrier, a limiting factor, in this approach to mass destruction. Less than forty pounds, the bad guys don’t got an A-bomb. However, less widely known is that uranium can be used in the different type of bomb design that was tested in New Mexico and dropped on Nagasaki. This other approach, the i
