How does implosion increase the density of fissionable material?

In an implosion-type fission device, the increase in density is achieved by surrounding a subcritical spherical core of fissile material—such as plutonium or uranium-235—with a layer of chemical high explosive.1

When these explosives are detonated, they create a powerful inward-directed shock wave that compresses the fissile core. This rapid compression forces the material into a much smaller volume, significantly increasing its density. By increasing the density, the probability that emitted neutrons will strike a fissionable nucleus before escaping the core is greatly enhanced, allowing a subcritical mass to reach a supercritical state and initiate a chain reaction. Advanced designs may further enhance this effect through techniques like core levitation, which introduces an air gap to allow for higher compression upon detonation.12