Fusion and Fission: How Can Both Produce Energy? Breakthrough Junior Challenge Video

This is my submission to the breakthrough Junior Challenge 2023, a competition where students from all across the globe create a video explaining a topic in math, physics, or the life sciences. The winner receives a $250,000 scholarship as well as a $50,000 cash prize to the teacher of his choice and a $100,000 science lab for his school.

Video transcript:

How can nuclear fusion and nuclear fission both produce energy when they are opposite reactions?

That’s a great question!

First, here’s a little clue: the fusion of small atoms and the separation of large atoms both increase the stability of the atoms involved. Keep that in mind.

Ok, so the key lies in the atomic mass defect, which goes like this: protons, neutrons, and electrons each have a specific mass. So when you combine a certain number of them to form an atom, you would expect the mass of the atom to be the sum of the masses of all the particles in the atom. But in fact, the atomic mass is slightly less than that sum. This is because atoms are more stable than the individual particles, meaning an atom requires less energy to exist than those particles. 

The difference in energy is called nuclear binding energy and is the amount of energy necessary to split an atom’s nucleus. As atomic mass increases, the binding energy (BE) increases, because the atom becomes more and more stable, meaning it requires less and less energy. Therefore, when you fuse two small nuclei, the new, large nucleus needs less energy than both of the small ones, and the excess is released.

Now, you’re probably thinking: if fusion produces energy, shouldn’t that mean fission only consumes energy?

Not exactly. See, BE only increases to a certain extent. Iron-56 is the most stable element and has the maximum BE. Elements larger than iron begin to decrease in BE, because the strong force—which holds the nucleus together—has less of a pull on the particles near the edge of the nucleus, making it unstable. So when a heavy, unstable element is fissioned into lighter, more stable ones, the BE is increased and energy is released.

Thus both reactions produce energy because they both increase the stability and therefore the binding energy of the atoms involved.

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