What is Fission? - Definition, Reaction & Theory

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you'll learn what nuclear fission is, how it works, and what it has to do with Einstein's theory of relativity. You'll also be introduced to a fission equation and the dangers and uses of fission.

What is Fission?

If you weigh three apples individually and add up their weights, when you put them all on the scale together you should get the same number. But in particle physics, things don't work that way. As strange as it might seem, the whole really can be greater than the sum of its parts.

Nuclear fission is what happens when the nucleus of a large radioactive atom splits into two or more smaller nuclei. When this happens, a lot of energy is released, and this is how nuclear power stations work.

Fission Reactions

When an atom of uranium-236 undergoes fission, it will often split into an atom of krypton-92, an atom of barium-141 and three neutrons, as shown below:

Nuclear Fission Reaction
Nuclear Fission Reaction

To start the reaction above, you first have to take a sample of uranium-235 and fire a neutron at it. This neutron is absorbed, turning the uranium-235 atom into an energetic uranium-236 atom. This uranium-236 atom is extremely unstable, and very quickly the reaction above will occur to break it down into smaller, more stable atoms. In the process, a large amount of energy is released.

But where does all the energy come from?

Weighing Atoms & Nuclear Power

If you take a radioactive atom, like uranium-236, and weigh it, you'll find that it weighs 236.0456 atomic mass units. But if you total the products of the reaction described above: the Kryption-92 atom, the barium-141 atom and the three neutrons, all five together only weigh a total of 235.8666. That's a difference of 0.1790 atomic mass units. So where did the extra mass go?

Einstein's most famous equation taught us that mass and energy are really the same thing. The extra mass didn't disappear -- it turned into energy!

Einsteins Famous Equation

This energy is what is harnessed in nuclear power plants. The energy is released in the form of heat, which boils water. The steam produced from the water turns a turbine and generates electricity.

Dangers & Concerns of Nuclear Power

Radioactive materials warning
Radioactive Materials Warning

Nuclear power is controversial for a number of reasons. For one, a nuclear fission reaction has to be carefully controlled. The reaction described above produces three neutrons, and those neutrons can start other fission reactions nearby. This is known as a chain reaction, because the neutrons created by one reaction will start more reactions, and if the process isn't carefully moderated, the whole thing could cause a nuclear meltdown.

A nuclear meltdown occurs when the fuel elements inside a nuclear reactor get so hot that they melt. This can be extremely dangerous, because radioactive material can escape into the surrounding area, hurting humans and the natural world alike.

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