How Nuclear Bombs Work

Hiroshima Peace Memorial stands as a visible reminder of the day the Japanese city was bombed on Aug. 6, 1945. After that fateful day, the structure was the only thing still standing in the vicinity of the explosion.

The first nuclear bomb meant to kill humans exploded over Hiroshima, Japan, on Aug. 6, 1945. Three days later, a second bomb detonated over Nagasaki. The death and destruction wrought by these weapons was unprecedented and might have, in another world with another race of beings, ended the nuclear threat right then and there.
But the events in Japan, although they brought a close to World War II, marked the beginning of the Cold War between the United States and the Soviet Union. Between 1945 and the late 1980s, both sides invested huge amounts of money in nuclear weapons and increased their stockpiles significantly, mostly as a means to deter conflict. The threat of catastrophic destruction from The Bomb loomed over everyone and everything.
Schools conducted nuclear air raid drills. Governments built fallout shelters. Homeowners dug bunkers in their backyards.
During the 1970s and '80s, tensions began to ease somewhat. Then the Berlin Wall fell in 1989, followed by the collapse of the Soviet government itself two years later. The Cold War officially ended. As relations between the two countries improved, a commitment to limit nuclear arsenals emerged. A series of treaties followed, with the latest going into effect in February 2011. Like its predecessors, the new Strategic Arms Reduction Treaty (START) aims to further reduce and limit strategic arms. Among other measures, it calls for an aggregate limit of 1,550 warheads [source: the White House].
Unfortunately, even as Russia and the U.S. step tentatively away from the brink, the threat of nuclear warfare remains. Nine countries can now deliver nuclear warheads on ballistic missiles [source: Fischetti]. At least three of those countries -- the U.S., Russia and China -- could strike any target anywhere in the world. Today's weapons could easily rival the destructive power of the bombs dropped on Japan. In 2009, North Korea successfully tested a nuclear weapon as powerful as the atomic bomb that destroyed Hiroshima. The underground explosion was so significant that it created an earthquake with a magnitude of 4.5 [source: McCurry].
While the political landscape of nuclear warfare has changed considerably over the years, the science of the weapon itself -- the atomic processes that unleash all of that fury -- have been known since Einstein. This article will review how nuclear bombs work, including how they're built and deployed. Up first is a quick review of atomic structure and radioactivity.

An atom, in the simplest model, consists of a nucleus and orbiting electrons.

 
Atomic Structure and Radioactivity
 

Before we can get to the bombs, we have to start small, atomically small. An atom, you'll remember, is made up of three subatomic particles -- protons, neutrons and electrons. The center of an atom, called the nucleus, is composed of protons and neutrons. Protons are positively charged, neutrons have no charge at all and electrons are negatively charged. The proton-to-electron ratio is always one to one, so the atom as a whole has a neutral charge. For example, a carbon atom has six protons and six electrons.
It's not that simple though. An atom's properties can change considerably based on how many of each particle it has. If you change the number of protons, you wind up with a different element altogether. If you alter the number of neutrons in an atom, you wind up with an isotope. For example, carbon has three isotopes: 1) carbon-12 (six protons + six neutrons), a stable and commonly occurring form of the element, 2) carbon-13 (six protons + seven neutrons), which is stable but rare and 3) carbon-14 (six protons + eight neutrons), which is rare and unstable (or radioactive) to boot.
As we see with carbon, most atomic nuclei are stable, but a few aren't stable at all. These nuclei spontaneously emit particles that scientists refer to as radiation. A nucleus that emits radiation is, of course, radioactive, and the act of emitting particles is known as radioactive decay. If you're particularly curious about radioactive decay, you'll want to peruse How Nuclear Radiation Works. For now, we'll go over the three types of radioactive decay:

1. Alpha decay: A nucleus ejects two protons and two neutrons bound together, known as an alpha particle.
2. Beta decay: A neutron becomes a proton, an electron and an antineutrino. The ejected electron is a beta particle.
3. Spontaneous fission: A nucleus splits into two pieces. In the process, it can eject neutrons, which can become neutron rays. The nucleus can also emit a burst of electromagnetic energy known as a gamma ray. Gamma rays are the only type of nuclear radiation that comes from energy instead of fast-moving particles.

Remember that fission part especially. It's going to keep coming up as we discuss the inner workings of nuclear bombs.

Nuclear Fission

Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei. There are two basic ways that nuclear energy can be released from an atom. In nuclear fission (pictured), scientists split the nucleus of an atom into two smaller fragments with a neutron. Nuclear fusion -- the process by which the sun produces energy -- involves bringing together two smaller atoms to form a larger one. In either process, fission or fusion, large amounts of heat energy and radiation are given off.
We can attribute the discovery of nuclear fission to the work of Italian physicist Enrico Fermi. In the 1930s, Fermi demonstrated that elements subjected to neutron bombardment could be transformed into new elements. This work resulted in the discovery of slow neutrons, as well as new elements not represented on the periodic table. Soon after Fermi's discovery, German scientists Otto Hahn and Fritz Strassman bombarded uranium with neutrons, which produced a radioactive barium isotope. They concluded that the low-speed neutrons caused the uranium nucleus to fission, or break apart, into two smaller pieces.
Their work sparked intense activity in research labs all over the world. At Princeton University, Niels Bohr worked with John Wheeler to develop a hypothetical model of the fission process. They speculated that it was the uranium isotope uranium-235, not uranium-238, undergoing fission. At about the same time, other scientists discovered that the fission process resulted in even more neutrons being produced. This led Bohr and Wheeler to ask a momentous question: Could the free neutrons created in fission start a chain reaction that would release an enormous amount of energy? If so, it might be possible to build a weapon of unimagined power.
And it was.

Officials from the Manhattan Project, the code name for the U.S. plan to develop atomic weapons, inspect the detonation site of the Trinity atomic bomb test. That's Dr. Robert J. Oppenheimer in the white hat.

Nuclear Fuel

In March 1940, a team of scientists working at Columbia University in New York City confirmed the hypothesis put forth by Bohr and Wheeler -- the isotope uranium-235, or U-235, was responsible for nuclear fission. The Columbia team tried to initiate a chain reaction using U-235 in the fall of 1941, but failed. All work then moved to the University of Chicago, where, on a squash court situated beneath the university's Stagg Field, Enrico Fermi finally achieved the world's first controlled nuclear chain reaction. Development of a nuclear bomb, using U-235 as the fuel, proceeded quickly.
Because of its importance in the design of a nuclear bomb, let's look at U-235 more closely. U-235 is one of the few materials that can undergo induced fission. Instead of waiting more than 700 million years for uranium to naturally decay, the element can be broken down much faster if a neutron runs into its nucleus. The nucleus will absorb the neutron without hesitation, become unstable and split immediately.
As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two lighter atoms then emit gamma radiation as they settle into their new states. There are a few things about this induced fission process that make it interesting:

• The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a bomb that is working properly, more than one neutron ejected from each fission causes another fission to occur. It helps to think of a big circle of marbles as the protons and neutrons of an atom. If you shoot one marble -- a single neutron -- into the middle of the big circle, it will hit one marble, which will hit a few more marbles, and so on until a chain reaction continues.
• The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (0.000000000001 seconds).
• In order for these properties of U-235 to work, a sample of uranium must be enriched; that is the amount of U-235 in a sample must be increased beyond naturally occurring levels. Weapons-grade uranium is composed of at least 90 percent U-235.

In 1941, scientists at the University of California at Berkeley discovered another element -- element 94 -- that might offer potential as a nuclear fuel. They named the element plutonium, and during the following year, they made enough for experiments. Eventually, they established plutonium's fission characteristics and identified a second possible fuel for nuclear weapons.