How can nuclear fusion produce more energy than nuclear fission?


I understand how breaking apart an atom can unleash stored energy…but how can combing two atoms seemingly create energy? Surly the merger of two atoms would take energy, not produce energy. Where is this “new” energy coming from…?

In: Physics

I think it has to do with part if the mass being converted to energy when the protons combine in the nucleus. It follows E=mc^2, so any mass conversion to energy is huge, and in order to compress the protons at that level, they automagically convert the mass.

I’m an electrical engineer, so some background but not enough for a solid answer. Hope someone can expand on it though!!


Energy is Mass times the speed of light squared.

Take a deuterium atom that weighs 2.014amu and a tritium atom that weighs 3.016amu and smash them together. In doing so, a neutron with 1.008amu is expelled and a helium, 4.002amu is made. Helium plus neutron is 5.01amu but the deuterium and tritium you started with weigh 5.03amu together. How do you end up with less than you started with?

This lost mass is turned into energy.

It depends which side of iron you’re on

Elements heavier than iron give off energy when they split, the heavier atoms like plutonium tend to give off more energy per splitting than lighter ones like Radon. Elements lighter than iron give off energy when they fuse but need energy to split them apart. Heavier forms of hydrogen give off the most energy.

Iron is super stable and every step towards it lets the atom get rid of some binding energy.

Probably one of the simplest ways to explain:


– 2 Hydrogen atoms fuse together.

– They then form one single atom of helium.

– Individually the 2 hydrogen atoms had MORE energy than the amount of energy contained in just a single helium atom.

– So that energy has to go somewhere.


So again: one single helium atom has LESS energy than 2 hydrogen atoms.

So if the 2 hydrogen atoms decide to get together, and combine themselves to make helium, they have to get rid of some of that energy, so they can present themselves as a “normal” well behaved helium atom, at the proper energy level of helium.

It’s all about keeping up with appearances!