The Ultimate Weapon? Nuclear Weapons

^{National Archives from Office of War Information Atomic bomb explodes over Nagasaki}

^{August 8, 1945.}

At 8:15 a.m. on August 6, 1945, an American B 29 bomber flew over the city of Hiroshima, Japan and released something on a parachute. Hiroshima was a medium‑size city, largely untouched by the war because it contained no military objectives worth touching. The object floating earthward under the parachute was the first nuclear weapon to be used in war. When the bomber was far away, but the parachute still above the ground, the bomb exploded.

Between 70,000 and 80,000 people in the city below died instantly or almost instantly. As many as 125,000 more died later as a result of injuries incurred by the blast. Three days later, a similar bomb exploded over Nagasaki, killing from 40,000 to 70,000 more people at once and 50,000 to 100,000 later from radiation sickness, cancer, or other illnesses caused by the explosion. Six days later, Japan surrendered.

The possibility of nuclear weapons had been known in the scientific community for years. All matter is composed of atoms, which have a nucleus composed of protons and neutrons around which electrons orbit. The number of atomic particles in the nucleus of an element’s atom determines its atomic weight, which is expressed in numbers that have bedeviled generations of high school chemistry students. When neutrons, protons, deuterons, and other particles strike a nucleus of high atomic weight, they are absorbed and the nucleus splits into two, forming two lighter atoms. The process releases a million volts of energy per atom. This process goes on continually in radioactive material but causes no trouble, because the released energy simply bypasses the other atoms in a block of material and passes into space.

However, by forming certain radioactive materials in a large enough and dense enough block, you have so many atoms in such limited space that a released neutron simply has to strike another nucleus, and particles released by that splitting of that atom will strike another nucleus. Then you have a chain reaction, with the energy in those trillions and trillions of atoms released all at once. Of the kinetic energy released in the chain reaction, about 50 percent forms a shock wave that flattens buildings, trees, and so on, the way a conventional explosion would. The main difference from conventional explosives is in the strength of the shock wave. The power of atomic bombs is measured in kilotons, each the equivalent of 1,000 tons of TNT, or megatons, the equivalent of a million tons of TNT. Thirty‑five percent of the kinetic energy appears as heat, light, and ultraviolet radiation. The heat is radiated heat – infrared radiation – and travels at the speed of light. At the center of the explosion, the heat reaches 10,000,000 degrees centigrade. Conventional explosives may produce 5,000 degrees. The remaining 15 percent of the kinetic energy forms various nuclear radiations such as neutron rays and gamma rays, which are extremely destructive to living tissue. Some of this radiation kills or injures people in the initial spurt. More of it – about two thirds – is in radioactive dust that falls to earth. Some of this “fallout” may appear a few hours after the explosion, but fallout from a single explosion may continue falling for months or years, depending on how high it was blown into the atmosphere. It may be carried by the wind for thousands of miles.

Weapons using this “fission” reaction are commonly called “atomic bombs.”

There’s another process – fusion – that can produce even more powerful bombs.

This consists of combining the nuclei of two light elements. That forms an element that is lighter than the sum of the two elements that were combined.

The difference in mass is released as energy. The fusion of two light elements may release less energy than the fission of a heavy element such as uranium 235, but a chain reaction is different. Because light atoms are much smaller, there are far more of them in a given volume of material. A fusion bomb may release four times as much energy and six times more neutron rays than a fission bomb of the same size.

Fission bombs were the first kind developed. The most common fissionable materials are U‑235 and U‑233, unstable isotopes of uranium, and plutonium – a man‑made element created by bombarding neptunium by deuterons or by performing other atomic hocus‑pocus on uranium 238.

To reach a critical mass of plutonium 239, you need a lump of about 15 kilograms; for uranium 235, the critical mass is about 50 kilograms. There are two ways to make a critical mass in a bomb. One uses two pieces of the fissionable material, machined to extremely close tolerances to fit tightly together.

These are driven together in the bomb by explosive charges. When they meet, they form a critical mass and a nuclear explosion occurs. The second method uses a spongy ball of the fissionable material – full of holes so a fair proportion of the atoms are not in contact with other atoms. In this kind of bomb, explosives outside the fissionable material squeeze it together to form a critical mass.

Fusion bombs use light elements that fuse only when subjected to enormous heat. Hence they are called thermonuclear bombs. In these bombs, the heat is supplied by a fission explosion.

Much research on nuclear explosions has been directed at miniaturization.

The United States developed an enormous 280 mm howitzer, nicknamed the

“atomic cannon,” to shoot nuclear shells. It was just barely road‑transportable.

But it was hardly out of its testing before the U.S. had a shell that could be fired from an ordinary 8‑inch gun or howitzer. Then there was a still smaller atomic shell that fit the 155 mm cannons. Innumerable rockets, bombs, and shells have been designed for nuclear explosions. There are even nuclear depth charges.

One that seemed to arouse particular horror was a weapon the news media called the “neutron bomb” and the U.S. military called an “enhanced radiation device.” The neutron bomb will explode, but the explosion is, for a nuclear weapon, nothing much. What it does is project massive amounts of neutron rays that would kill everyone and everything in an area while leaving buildings, vehicles, and all man‑made property unscathed and uncontaminated with radiation. It was probably this single effect – killing without destroying property – that led the public to view the neutron bomb with such horror.

None of these weapons have ever been used, and everyone in the world devoutly hopes that they never will be. One reason is that even use of the small “tactical” nuclear weapons might induce an enemy to respond with something bigger, like an ICBM. The other is the largely unknown danger of the fallout from a number of tactical nukes.

Although they have been used only twice in history, nuclear weapons have decisively influenced both warfare and all international relations.