Command and Control

The thermonuclear device that had vaporized Elugelab was too large to be delivered by plane. And that type of device presented a number of logistical challenges. Mike’s thermonuclear fuel, liquefied deuterium, had to be constantly maintained at a temperature of –423 degrees Fahrenheit. Although the feasibility of liquid-fueled hydrogen bombs was being explored, weapons that used a solid fuel, such as lithium deuteride, would be much easier to handle. On March 1, 1954, a solid-fueled device named “Shrimp” was tested at a coral reef in the Bikini atoll. The code name of the test was Bravo, and the device worked. But miscalculations at Los Alamos produced a yield much larger than expected. The first sign that something had gone wrong was detected at the firing bunker on the island of Enyu, twenty miles from the explosion. While awaiting the blast wave, the lead scientist in the bunker, Bernard O’Keefe, grew concerned. He was hardly the nervous type. The night before the Nagasaki raid, he’d violated safety rules and secretly changed the plugs on Fat Man’s master firing cable. In 1953, after an implosion device mysteriously failed to detonate at the Nevada Test Site, he’d climbed two hundred feet to the top of the shot tower and pulled out the firing cables by hand. Now he felt uneasy. About ten seconds after Shrimp exploded, the underground bunker seemed to be moving. But that didn’t make any sense. The concrete bunker was anchored to the island, and the walls were three feet thick.

“Is this building moving or am I getting dizzy?” another scientist asked.

“My God, it is,” O’Keefe said. “It’s moving!”

O’Keefe began to feel nauseated, as though he were seasick, and held on to a workbench as objects slid around the room. The bunker was rolling and shaking, he later recalled, “like it was resting on a bowl of jelly.” The shock wave from the explosion, traveling through the ground, had reached them faster than the blast wave passing through the air.

Shrimp’s yield was 15 megatons — almost three times larger than what its designers had predicted. The fireball was about four miles wide, and about two hundred billion pounds of coral reef and the seafloor were displaced, much of it rising into a mushroom cloud that soon stretched for more than sixty miles across the sky. Fifteen minutes after the blast, O’Keefe and the eight other men in his firing crew tentatively stepped out of the bunker. The island was surrounded by a dull, gray haze. Trees were down, palm branches were scattered everywhere, all the birds were gone — twenty miles from ground zero. O’Keefe noticed that the radioactivity level on his dosimeter was climbing rapidly. A light rain of white ash that looked like snowflakes began to fall. Then pebbles and rocks started dropping from the sky. The men ran back into the bunker, slammed the door shut, detected high levels of radioactivity within the bunker, and after a few moments of confusion, turned off the air-conditioning unit. Inside, the radiation levels quickly fell, but outside they continued to rise. The men were trapped.

The dangers of radioactive fallout had been recognized since the days of the Manhattan Project but never fully appreciated. A nuclear explosion produces an initial burst of gamma rays — the source of radiation poisoning at Hiroshima and Nagasaki. The blast also creates residual radiation, as fission products and high-energy neutrons interact with everything engulfed by the fireball. The radioactive material formed by the explosion may emit beta particles, gamma rays, or both. The beta particles are relatively weak, unable to penetrate clothing. The gamma rays can be deadly. They can pass through the walls of a house and kill the people inside it.

Some elements become lethal after a nuclear explosion, while others remain harmless. For example, when oxygen is bombarded by high-energy neutrons, it turns into a nitrogen isotope with a half-life of just seven seconds — meaning that within seven seconds, half of its radioactivity has been released. That’s why a nuclear weapon exploded high above the ground — an airburst, like the detonations over Hiroshima and Nagasaki — doesn’t produce much radioactive fallout. But when manganese is bombarded by high-energy neutrons, it becomes manganese-56, an isotope that emits gamma rays and has a half-life of two and a half hours. Manganese is commonly found in soil, and that’s one of the reasons that the groundburst of a nuclear weapon can create a large amount of deadly fallout. Rocks, dirt, even seawater are transformed into radioactive elements within the fireball, pulled upward, carried by the wind, and eventually fall out of the sky.