The high-explosives blocks were actually very sophisticated laminates of two materials, each laced with dust from light and heavy metals. The outer layer in each case was a relatively slow explosive with a detonation speed of just over seven thousand meters per second. The explosive wave in each expanded radially from the detonator, quickly reaching the edge of the block. Since the blocks were detonated from the outside-in, the blast front traveled inward through the blocks. The border between the slow and fast explosives contained bubbles - called voids - which began to change the shock-wave from spherical-shaped to a planar, or flat wave, which was refocused again to match exactly its metallic target, called 'drivers.'

The 'driver' in each case was a piece of carefully-shaped tungsten-rhenium. These were hit by a force wave traveling at more than nine thousand eight hundred meters (six miles) per second. Inside the tungsten-rhenium was a one-centimeter layer of beryllium. Beyond that was a one-millimeter thickness of uranium 235, which though thin weighed almost as much as the far thicker beryllium. The entire metallic mass was driving across a vacuum, and since the implosion was focused on a central point, the actual closing speed of opposite segments of the bomb was 18,600 meters (or 11.5 miles) per second.

The central aiming point of the explosives and the metallic projectiles was a ten kilogram (22 pound) mass of radioactive plutonium 239. It was shaped like a glass tumbler whose top had been bent outwards and down towards the bottom, creating two parallel walls of metal.

Ordinarily denser than lead, the plutonium was compressed further by the million-atmospheres pressure of the implosion. This had to be done very quickly. The plutonium 239 mass also included a small but troublesome quantity of plutonium 240, which was even less stable and prone to pre-ignition. The outer and inner surfaces were slammed together and driven in turn towards the geometric center of the weapon.

The final external act came from a device called a 'zipper.' Operating off the third signal from the still-intact electronic timer, the zipper was a miniature particle accelerator, a very compact mini-cyclotron that looked remarkably like a hand-held hair-dryer. This fired deuterium atoms at a beryllium target. Neutrons traveling ten percent of the speed of light were generated in vast numbers and traveled down a metal tube into the center of the Primary, called the Pit. The neutrons were timed to arrive just as the plutonium reached half of its peak density.

Ordinarily, a material weighing roughly twice an equivalent mass of lead, the plutonium was already ten times denser than that and still accelerating inward. The bombardment of neutrons entered a mass of still-compressing plutonium.

Fission.

The plutonium atom has an atomic weight of 239, that being the combined number of neutrons and protons in the atomic nucleus. What began happened at literally millions of places at once, but each event was precisely the same. An invading 'slow' neutron passed close enough to a plutonium nucleus to fall under the Strong Nuclear Force that holds atomic nuclei together. The neutron was pulled into the atom's center, changing the energy state of the host nucleus and kicking it into an unstable state. The once symmetrical atomic nucleus began gyrating wildly and was torn apart by force fluctuations. In most cases a neutron or proton disappeared entirely, converted to energy in homage to Einstein's law E = MC^2. The energy that resulted from the disappearance of the particles was released in the form of gamma and X-radiation, or any of thirty or so other but less important routes. Finally, the atomic nucleus released two or three additional neutrons. This was the important part. The process that had required only one neutron to start released two or three more, each traveling at over 10 percent of the speed of light - 20,000 miles per second - through space occupied by a plutonium mass two hundred times the density of water. The majority of the newly-liberated atomic particles found targets to hit.