Nuclear Weapons 101

So far this week, we've seen how rockets went ballistic and surveyed "Ballistic Missiles 101." Today, let's turn to nuclear weapons--the deadly devices that make ballistic missiles, which are really delivery vehicles, more terrifying than they otherwise would be.

The nuclear age began in 1942, in a small laboratory underneath the football field at the University of Chicago. There, physicist Enrico Fermi conducted the world's first controlled nuclear reaction. Just three years later, World War II ended with the detonation of nuclear bombs over Hiroshima and Nagasaki in Japan. The scientists of the top-secret Manhattan Project did their job frighteningly well.

Critical Mass

Even before the Manhattan Project, scientists knew that certain elements were unstable, slowly emitting energy as their atomic state changed over time. Some of these radioactive elements, uranium and plutonium in particular, could undergo nuclear fission. That is, the nucleus of their atoms could be split into two fragments, releasing large amounts of energy and a few stray neutrons, too. If these stray neutrons could strike and split other atoms, physicists figured, you could create a sustained nuclear reaction.

The problem is that it's very difficult to create just the right conditions for a sustained reaction. The solution, scientists found, is to concentrate a sufficient mass of radioactive material together, so that when one atom splits, stray neutrons stand a good chance of striking and splitting some neighbors, which then release more neutrons to continue the chaos. The mass necessary to achieve this chain reaction is known as "critical mass."

Yet it's not enough to have a critical mass of uranium or plutonium. You need the right kind of uranium or plutonium, as each exists in several different isotopes. Isotopes of an element may have the same chemical properties, but because they have a different number of neutrons in their nucleus, they have different nuclear properties.

Only uranium-235 and plutonium-239 have the right nuclear stuff for sustainable fission. You can mine uranium ore out of the earth, but almost all of the ore is uranium-238, and you'll need to mine huge amounts to extract even a little U-235. The right plutonium is even harder to get, because it doesn't exist in the earth at all. It has to be produced in sophisticated nuclear reactors. Only after these difficult and expensive processes will you have the purified "weapons-grade" material needed for a nuclear weapon.

The thermonuclear weapons in modern arsenals are even more complex--not to mention a thousand times more powerful than the bomb exploded over Hiroshima. First detonated in 1952, thermonuclear, or hydrogen, bombs use the intense heat of a fission reaction to start a second, fusion reaction, in which hydrogen isotopes combine to form helium. To fit together, the hydrogen atoms must lose mass. The mass becomes energy, and kilotons of destructive force become megatons.

Nightmare Device

A 10-megaton nuclear weapon creates an explosion equal to 10 million tons of TNT--from less than 200 pounds (90 kg) of nuclear fuel. Pressure waves emanating from the blast would generate winds in excess of 700 miles per hour (1,125 km/h). Such winds could knock down steel-and-concrete buildings with ease. Even 20 miles away (32 km), the blast would shatter windows and make projectiles out of the shards.

The thermal effects would reach even farther. Everything within a 2-mile (3.2-km) radius of ground zero would be vaporized in a fireball as hot as the surface of the sun. At 15 miles (24 km), the temperature would reach 1,400 degrees Fahrenheit (760 degrees C), igniting combustible materials and producing innumerable fires. Even 30 miles away (48 km), exposed bodies would suffer serious burns.

And when the dust cleared and the fires were out, the bomb's most insidious effect would remain: radiation. Radiation is all around you. It bombards you every moment of your life, everywhere you go. Don't panic, though. Radiation is simply traveling energy, and most of it is harmless. But some forms, like ultraviolet light and X-rays, are harmful if you're exposed too long. Radiation like this, called ionizing radiation, contains enough energy to break down chemical bonds in substances that absorb it.

Radioactive elements like uranium and plutonium emit, among other things, ionizing gamma rays, packing 10,000 times more energy than visible light. Gamma rays can pass right through humans, penetrating tissues and ionizing atoms in your body. This leads to massive cellular damage, resulting in system-wide "radiation sickness" and, with enough exposure, death.

Although damage from the blast, heat, and even radiation burns may heal over time, the ionizing damage done to the DNA in human cells will remain. Sooner or later, the body's own replication of damaged DNA leads to the final danger of a nuclear blast: cancer, mutations, and a host of genetic abnormalities. These last casualties can take years, or even decades, to occur.