Command and Control

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.

The shape and composition of the explosive lenses were irrelevant, however, if the lenses failed to detonate at exactly the same time. The shock wave would travel through the device at a speed of one millimeter per millionth of a second. If a single lens detonated a few ten millionths of a second before the others, it could shatter the plutonium without starting a chain reaction. Blasting caps and Primacord were the detonators usually employed with conventional explosives. But both proved incapable of setting off thirty-two charges simultaneously. The physicist Luis Alvarez and his assistant, Lawrence Johnston, invented a new type of detonator for the job — the exploding-bridgewire detonator. It sent a high-voltage current through a thin silver wire inserted into an explosive. The current vaporized the wire, created a small shock wave, and detonated the explosive. Donald F. Hornig, who was one of the youngest scientists at Los Alamos, devised a contraption, the X-unit, that could store 5,600 volts in a bank of capacitors and then send that electricity instantaneously to all the detonators.

In theory, the X-unit and the exploding bridgewires would set off thirty-two explosive lenses at once, creating the perfect shock wave and imploding the plutonium core. In reality, these new inventions were unpredictable. Cracked insulation frequently caused the detonators to short-circuit. When that happened, they didn’t work. And a week before the Trinity test, an X-unit fired prematurely during a lightning storm. It had been triggered by static electricity in the air. The misfire suggested that a nuclear weapon could be set off by a lightning bolt.