How is the super-critical mass achieved in nuclear weapons without it exploding prematurely? — JG, Bateman, Australia
Apart from obtaining fissionable material, this is the biggest technical problem with building a nuclear weapon. Although a fission bomb’s nuclear fuel begins to heat up and explode almost from the instant it reaches critical mass, just reaching critical mass isn’t good enough. To use its fuel efficiently—to shatter most of its nuclei before the fuel rips itself apart—the bomb must achieve a significantly super-critical mass. It needs the explosive chain reactions that occur when each fission induces an average of far more than one subsequent fission.
There are two classic techniques for reaching super-critical mass. The technique used in the uranium bomb dropped over Hiroshima in WWII involved a collision between two objects. A small cannon fired a piece of uranium 235 into a nearly complete sphere of uranium 235. The uranium projectile entered the incomplete sphere at enormous speed and made the overall structure a super-critical mass. But despite the rapid mechanical assembly, the bomb still wasn’t able to use its nuclei very efficiently. It wasn’t sufficiently super-critical for an efficient explosion.
The technique used in the two plutonium bombs, the Gadget tested in New Mexico and the Fat Man dropped over Nagasaki, involved implosions. In each bomb, high explosives crushed a solid sphere of plutonium 239 so that its density roughly doubled. With its nuclei packed more tightly together, this fuel surged through critical mass and went well into the super-critical regime. It consumed a much larger fraction of its nuclei than the uranium bomb and was thus a more efficient device. However, its design was so complicated and technically demanding that its builders weren’t sure it would work. That’s why they tested it once on the sands of New Mexico. The builders of the uranium bomb were confident enough of its design and too worried about wasting precious uranium to test it.