Command and Control

Conventional explosives, like TNT, detonate through a chemical reaction. They are unstable substances that can be quickly converted into gases of a much larger volume. The process by which they detonate is similar to the burning of a log in a fireplace — except that unlike the burning of a log, which is slow and steady, the combustion of an explosive is almost instantaneous. At the point of detonation, temperatures reach as high as 9,000 degrees Fahrenheit. As hot gases expand into the surrounding atmosphere, they create a “shock wave” of compressed air, also known as a “blast wave,” that can carry tremendous destructive force. The air pressure at sea level is 14.7 pounds per square inch. A conventional explosion can produce a blast wave with an air pressure of 1.4 million pounds per square inch. Although the thermal effects of that explosion may cause burns and set fires, it’s the blast wave, radiating from the point of detonation like a solid wall of compressed air, that can knock down a building.

The appeal of a nuclear explosion, for the Manhattan Project scientists, was the possibility of an even greater destructive force. A plutonium core the size of a tennis ball had the potential to raise the temperature, at the point of detonation, to tens of millions degrees Fahrenheit — and increase the air pressure to many millions of pounds per square inch.

Creating that sort of explosion, however, was no simple task. The difference between a chemical reaction and a nuclear reaction is that in the latter, atoms aren’t simply being rearranged; they’re being split apart. The nucleus of an atom contains protons and neutrons tightly bound together. The “binding energy” inside the nucleus is much stronger than the energy that links one atom to another. When a nucleus splits, it releases some of that binding energy. This splitting is called “fission,” and some elements are more fissionable than others, depending on their weight. The lightest element, hydrogen, has one proton; the heaviest element found in nature, uranium, has ninety-two.

In 1933, Leó Szilárd realized that bombarding certain heavy elements with neutrons could not only cause them to fission but could also start a chain reaction. Neutrons released from one atom would strike the nucleus of a nearby atom, freeing even more neutrons. The process could become self-sustaining. If the energy was released gradually, it could be used as a source of power to run electrical generators. And if the energy was released all at once, it could cause an explosion with temperatures many times hotter than the surface of the sun.

Two materials were soon determined to be fissile — that is, capable of sustaining a rapid chain reaction: uranium-235 and plutonium-239. Both were difficult to obtain. Plutonium is a manmade element, created by bombarding uranium with neutrons. Uranium-235 exists in nature, but in small amounts. A typical sample of uranium is about 0.07 percent uranium-235, and to get that fissile material the Manhattan Project built a processing facility in Oak Ridge, Tennessee. Completed within two years, it was the largest building in the world. The plutonium for the Manhattan Project came from three reactors in Hanford, Washington.

A series of experiments was conducted to discover the ideal sizes, shapes, and densities for a chain reaction. When the mass was too small, the neutrons produced by fission would escape. When the mass was large enough, it would become critical, a chain reaction would start, and the number of neutrons being produced would exceed the number escaping. And when an even larger mass became supercritical, it would explode. That was the assumption guiding the Manhattan Project scientists. In order to control a nuclear weapon, they had to figure out how to make fissile material become supercritical — without being anywhere near it.

The first weapon design was a gun-type assembly. Two pieces of fissile material would be placed at opposite ends of a large gun barrel, and then one would be fired at the other. When the pieces collided, they’d form a supercritical mass. Some of the most difficult computations involved the time frame of these nuclear interactions. A nanosecond is one billionth of a second, and the fission of a plutonium atom occurs in ten nanoseconds. One problem with the gun-type design was its inefficiency: the two pieces would collide and start a chain reaction, but they’d detonate before most of the material had a chance to fission. Another problem was that plutonium turned out to be unsuitable for use in such a design. Plutonium emits stray neutrons and, as a result, could start a chain reaction in the gun barrel prematurely, destroying the weapon without creating a large explosion.