Eric Schlosser - Command and Control

The atomic bombs in The World Set Free detonated slowly, spewing radioactivity for years. During the 1930s, the Hungarian physicist Leó Szilárd — who’d met with H. G. Wells in 1929 and tried, without success, to obtain the central European literary rights to his novels — conceived of a nuclear weapon that would explode instantly. A Jewish refugee from Nazi Germany, Szilárd feared that Hitler might launch an atomic bomb program and get the weapon first. Szilárd discussed his concerns with Albert Einstein in the summer of 1939 and helped draft a letter to President Franklin D. Roosevelt. The letter warned that “it may become possible to set up a nuclear chain reaction in a large mass of uranium,” leading to the creation of “extremely powerful bombs of a new type.” Einstein signed the letter, which was hand delivered to the president by a mutual friend. After British researchers concluded that such weapons could indeed be made and intelligence reports suggested that German physicists were trying to make them, the Manhattan Project was formed in 1942. Led by Leslie R. Groves, a brigadier general in the U.S. Army, it secretly gathered eminent scientists from Canada, Great Britain, and the United States, with the aim of creating atomic bombs.

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.

A second design promised to overcome these problems by increasing the speed at which a piece of plutonium might be made supercritical. The new weapon design was nicknamed, at first, “the Introvert.” A sphere of plutonium would be surrounded by conventional explosives. The shock wave from the detonation of these explosives would compress the sphere — and the denser the sphere became, the more efficiently it would trap neutrons. “The more neutrons — the more fission,” a secret government manual on nuclear weapons later explained. “We care about neutrons!” Imploding a ball of plutonium to produce an explosion was a brilliant idea. But it was easier said than done. If the conventional explosives failed to produce a shock wave that was perfectly symmetrical, the plutonium wouldn’t implode. It would blow to pieces.

Many of the physicists who worked on the Manhattan Project — Oppenheimer, Fermi, Teller, Bethe — later became well known. And yet one of the crucial design characteristics of almost every nuclear weapon built since then was perfected by George B. Kistiakowsky, a tall, elegant chemist. Born in the Ukraine and raised in an academic family, Kistiakowsky had fought against the Bolsheviks during the Russian civil war. He later earned a degree at the University of Berlin, emigrated to the United States, and become a professor of chemistry first at Princeton, then at Harvard. By the mid-1940s, he was America’s leading expert on explosives. Creating a perfectly symmetrical shock wave required not just the right combination of explosives but also the right sizes and shapes. Kistiakowsky and his team at Los Alamos molded explosive charges into three-dimensional lenses, hoping to focus the shock wave, like the lens of a camera focuses light. Tons of explosives were routinely detonated in the hillsides of Los Alamos, as different lens configurations were tested. Kistiakowsky considered these lenses to be “precision devices,” not crude explosives. Each weighed between seventy and one hundred pounds. As the date of the Trinity test approached, he spent long hours at the lab with a dentist’s drill, eliminating the air bubbles in lenses and filling the holes with molten explosives. The slightest imperfection could distort the path of a shock wave. The final design was a sphere composed of thirty-two shaped charges — twelve pentagons and twenty hexagons. It looked like a gigantic soccer ball and weighed about five thousand pounds.