ELI5 so nuclear reactors generate power by splitting atoms but what actually splits these atoms?

This is a bad and misleading way to talk about it. You are describing spontaneous decay or spontaneous fission. That is not what happens in a nuclear reactor.

What happens is that by absorbing a neutron, a fissile nucleus has a very high chance of getting pulled apart by its internal forces.

The verb I find most useful here is "stretching." If a fissile nucleus absorbs a neutron, it can become stretched along its axis. If it is stretched long enough, it will stop behaving as if it is a single nucleus, but instead start behaving like it is two nuclei that happen to be right next to each other. These are then rocketed apart by their electrostatic forces (like holding the two positive ends of two magnets together).

Another way to think about this is: an atomic nucleus — its core — has two main forces that we care about for this discussion. One is the electrostatic force, which, because the protons are positive, would be repulsive (again, like two positive ends of two magnets). The other is the strong force or nuclear force, which means that at very short distances, protons and neutrons are attracted to one another. The strong force is, as the name implies, very strong, so most of the time it wins out. But if you stretch out a very heavy nucleus, like uranium, you can make it so that about half of its positive charge is outside of the range of the strong force from the other half. And then the electrostatic force will kick in and blast it apart.

As with all things of this sort, this is a simplification. But it is a better simplification than "uranium just falls apart" (that is not what is going on here) and better than "neutrons just make it fall apart faster." That makes it sound like regular radioactive decay and that is not what is happening.

/r/explainlikeimfive Thread Parent