Bomb: The Race to Build—and Steal—the World's Most Dangerous Weapon

A gun assembly is an atomic bomb shaped like a short artillery gun with a clump of uranium at one end and a smaller amount at the other end that’s fired into the big clump, causing it to explode in a chain reaction. “Little Boy,” the weapon that the US detonated over Hiroshima, was this kind of weapon. A more powerful weapon, made of plutonium, required a different system to detonate but became the weapon of choice thereafter. Some years later, this too was superseded by more powerful fusion bombs. Thus, the gun assembly bomb, the first atomic weapon ever used, was retired immediately following its successful use.

Most hydrogen atoms are extremely simple: They contain one proton around which orbits a single electron. A few hydrogen atoms, though, also contain a neutron that’s paired with the proton in the nucleus. When combined with oxygen to create water, these extra-heavy hydrogen atoms form “heavy water.” This material slows the release of neutrons from fissioning uranium atoms so that they can enter other uranium atoms and cause them, in turn, to fission.

Graphite, used by Fermi in his Chicago atomic pile experiments, also slows neutrons, but it’s less efficient. German scientists, who misconstrued graphite as a poor medium for sustained chain reactions, relied on heavy water for their uranium experiments during WWII. When Norwegian commandos destroyed their supply, it effectively knocked Germany out of The Race to Build a Bomb.

Heavy water reactors later were built as sources of electric power, but they also create plutonium, making them security risks. Heavy water is therefore heavily guarded, much of it supervised by the International Atomic Energy Agency.

An implosion bomb contains a central core of plutonium surrounded by high explosives that, when ignited all at once, crush the core and cause it to undergo a high-speed chain reaction. This releases the energy of tens of thousands of tons of TNT. This was the kind of bomb the US detonated over Nagasaki, Japan, on August 9, 1945. It was the second atomic weapon ever used in warfare, and its more effective design made it the preferred shape of atomic bombs to come.

Hidden away in remote canyon country in northern New Mexico, Los Alamos was a research town set up during World War II to design an atomic bomb. Its original purpose was a closely kept secret; scientists, recruited from all over, lived and worked there for up to two years while they developed the theory and engineering of the “gadget.” The town continued its work after the war and today remains an important center for research, often on sensitive projects relating to US national defense.

“Manhattan Project” was the code name for the US Army’s effort to build an atomic weapon. Begun in 1939, the Manhattan Project developed the first A-bombs ever detonated, including two used in combat over Japan. General Leslie Groves managed the Project between 1942 and 1946; the scientific portion was led by Robert Oppenheimer and his team of scientists at the Los Alamos lab in New Mexico. Bomb materials were produced at Oak Ridge, Tennessee, and Hanford, Washington. In all, some 300,000 people worked on the project.

The program culminated in the “Trinity” test in the New Mexico desert, where scientists successfully detonated an implosion-type plutonium bomb. A second implosion bomb, “Fat Man,” was dropped on Nagasaki, Japan, on August 9, 1945—days after the detonation of a gun-assembly uranium bomb, “Little Boy,” over Hiroshima. The Project continued testing atomic bombs until 1947, when its work was folded into the then-new US Atomic Energy Commission.

Some atoms, especially very large ones like uranium, can be unstable; in the presence of neutrons flung from other unstable atoms, such atoms can break apart, or fission, releasing energy and neutrons. When the atoms are closely packed, this can cause a “chain reaction” that causes uranium atoms to split and release neutrons to other uranium atoms, which also split, and so on. If a mass of uranium is exposed slowly to such a process, its energy release can heat water that spins a dynamo to produce electricity. If the uranium is exposed suddenly and violently to radiation, it reaches a “critical mass” and fissions all at once in an explosion.

Nuclear fission thus is the basic principle behind both atomic power and atomic bombs. The simplest atom is hydrogen, with one proton at its center and one electron orbiting it. When a mass of hydrogen is pressed together with enough pressure and heat, it can fuse into helium and release energy. This is the principle behind the much more powerful thermonuclear bomb; it’s also at the center of the ongoing search for a way to harness fusion power to create electric power.

Uranium 238—the number 238 is the total number of nucleons, both protons and neutrons, at the center of the uranium atom—is fairly stable, but one of its variants (or isotopes) is uranium 235. This form splits apart easily when bombarded by neutrons. The process also sometimes creates plutonium atoms, which work better than U-235 in atomic weapons. Plutonium has atomic number 94 and is the heaviest element that occurs naturally. (All heavier elements are created in the lab.) It decays slowly into smaller atoms; this process has a half-life, or a time when half the material has decayed, of 88 years.

Plutonium is one of the most dangerous radioactive elements: During nuclear detonations, a great deal of it gets hurled about, and people who inhale or ingest a dust mote of it nearly always develop cancer from its effects.