Nuclear Bomb

There are two main types of bombs which release energy from the nuclei of atoms.

The simplest kind is an atomic bomb. Like a nuclear power plant, it releases great quantities of energy through a process called nuclear fission, or 'splitting', of a large unstable (radioactive) element like uranium or plutonium.
A more complicated type is the hydrogen bomb, or thermonuclear bomb, which releases an even greater quantity of energy through nuclear fusion, a process which has not yet been put to peaceful uses.

The purpose of this page is to explain how each type of device works, from a physics perspective.
The devices described here are the original designs, perhaps 50 years old, but the physical processes that cause the energy release remain the same in today's bombs and power plants.


The Atomic Bomb
The energy source is a mass of radioactive material such as uranium or plutonium. This material is very unstable; every atom's nucleus is ready to fall apart ('decay') at the slightest nudge, releasing unneeded energy and extra neutrons. In the diagram, the plutonium (B) is given that nudge by the outer casing of TNT (A), which explodes all around it.

Here's what happens; the process is called 'Nuclear Fission':


The plutonium is unstable, or radioactive. Its atoms are constantly 'falling apart', breaking up into smaller elements that are more stable. Every time one nucleus does this, it releases the extra energy it no longer needs to hold it together, as well as a few left-over neutrons. This energy, and the escaping neutrons, is what we describe as the radiation being emitted from the radioactive plutonium. This energy and flow of escaping neutrons can damage human cells, so radioactivity is dangerous.
Enough atoms in the chunk of plutonium are breaking down at any one time to make the chunk of plutonium warm up, but not enough to be considered an explosion.

What happens in the bomb, however, changes that! The force of the TNT explosion causes the plutonium to be squashed, or compressed in size, and become very dense. This is called its 'critical mass'; the plutonium is now so densely packed together that the neutrons escaping from the decaying nuclei of plutonium cannot escape from the plutonium without bumping into another plutonium atom!

When they hit another atom, they cause that nucleus to break down too, whether it was ready to or not. That second nucleus releases more energy, and more neutrons, which in turn go on to hit and break up further nuclei. The decaying nuclei cause more decaying nuclei, and so on, in a rapidly escalating chain reaction ... and all because the plutonium has been squeezed into such a dense state (by the TNT) that the escaping neutrons that normally would fly out of the material now can't, without hitting other nuclei!

Within a very tiny fraction of a second, all the nuclei in the chunk of plutonium have been hit by escaping neutrons, and have broken down. The extra energy in trillions of atomic nuclei is all released at once! This energy is considerable; the atomic bomb dropped on Hiroshima in WWII was an example of this process.



Here are the details of the fusion process:



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