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Nuclear Bombs ... How They Work
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.

Thankfully, more peaceful uses for this process have been found. Critical mass can also be achieved by just collecting together enough plutonium in one place; if it's thick enough, the neutrons can't escape without hitting another nucleus, and the chain reaction will start. By inserting special neutron-absorbing material in between portions of the plutonium, the rate at which the chain reaction proceeds can be controlled, resulting in a 'slow burn' instead of an explosion. This is the process that takes place inside a nuclear power plant. The heat generated by the nuclear fission is used to heat water into steam that turns a generator.


Nuclear Fusion
This is a much nastier bomb. Not only does it release much more energy, using a process called 'nuclear fusion', but it is triggered not by TNT, but by an atomic bomb!
The central core (B) is a mass made up of trillions of two kinds of atoms, which are both isotopes of hydrogen, called deuterium and tritium. (These are just hydrogen atoms with one or two extra neutrons in each nucleus). Small atomic bombs (A) scattered around the outside cause the deuterium and tritium to be squeezed into a very dense mass, which initiates a process called nuclear fusion, releasing great quantities of energy.
This process is difficult to achieve; it's been described as trying to squeeze an unopened can of Coke into a little ball without spilling any Coke!
But there's more! As the core explodes, it causes the bomb casing, (C), which is made from uranium, to undergo fission, creating even more energy. In other words, an atomic bomb sets off a fusion bomb, which also triggers another atomic bomb!

Here are the details of the fusion process:


The block of deuterium and tritium atoms are squeezed into a super-dense mass by the exploding atomic triggers. Nuclei of these two isotopes are squeezed together by the force of the explosion.



The force is so great that it causes the nuclei to combine. This process is called nuclear fusion.



A new nucleus is formed ... helium.




But this new nucleus requires less energy to keep it together, and there is one less neutron needed. This excess energy, and the neutron, escape as radiation.
The amount of escaping energy is huge; bombs of this type can release energy equivalent to the explosion of many millions of tons of TNT.
The escaping neutrons also cause the uranium in the bomb casing to undergo a fission chain reaction, the energy from which is added to the total output of the bomb. It also results in much radioactive material being expelled into the atmosphere, which can later descend as 'fallout', and kill living things. Bombs without the uranium casing just release a lot of energy and neutrons, killing in the immediate vicinity without leaving a lot of radioactive fallout. These 'cleaner' bombs are called neutron bombs.

We can hope that such devices of mass destruction will never again be used to kill people. Scientists now are trying to salvage some good from the effort that went into making these devices. Knowing how a hydrogen bomb works has led to an effort to recreate the fusion process on a small scale, using only a few atoms at a time. The immense temperature and pressure needed to cause nuclei to fuse together actually occurs naturally ... in the sun. Nuclear fusion, in a very similar process, is slowly converting the sun's hydrogen into helium, and releasing the energy as light and heat.
In order to reproduce this process on earth on a small scale, it is necessary to find a way to exert very high pressure on a small sample of deuterium and tritium. (ie: without using an atomic bomb and creating an explosion). This process, called 'controlled nuclear fusion', would allow massive amounts of energy to be released, with very little dangerous radiation, from a small amount of ... water! It would solve the earth's energy shortage forever!
Interested? Find out more on our Controlled Nuclear Fusion page!

You might also like to visit our page that explains E=mc2


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