So i'm doing a bit of reading about the basics of nuclear fission these days, just to understand what's actually happening there.

Here are some interesting things that i think that i learned so far:³

• nuclear engineering is all about the neutron economy. If you had an easily-available cheap source of neutrons, we wouldn't need big nuclear power plants because energy could be easily generated by hitting Hydrogen-1 (protium) with neutrons to make H-2 (deuterium) out of it. According to this diagram, this releases around 2 MeV of energy per collision (resulting nucleus has 2 nucleons), which is a lot of energy.

• the problem is that we don't have a cheap source of neutrons, and that's about where the whole trouble with nuclear energy begins. we're smashing U-235 specifically because it's fissile and can sustain a chain reaction: It emits more neutrons per collision than it absorbs, so we get a net gain of neutrons out of it.

• the sun sources its neutrons from weak interaction between protons and electrons¹. Basically this fuses a proton and an electron together to make a neutron. This is the rate-limiting factor in the sun's fusion and determines the sun's lifetime before it burns through its fuel. The average time for a particle to undergo weak interaction is about 10¹⁰ years in the sun's interior which is about the sun's lifetime. If the sun had more neutrons, it would burn faster.²

[1]: well, sorta. this is kinda simplified and more precisely described in the proton-proton chain which is the same thing but with extra frills.

[2]: Also note that while the weak interaction is the rate-limiting factor of the fusion process, it barely releases any energy. Almost all of the energy is released due to the strong interaction which glues protons and neutrons together really tightly and that way releases a lot of energy.

[3]: if you know better, please do correct me.

Since you're reading about this topic, here are some free MIT courses on nuclear science.

I mainly have an issue with the image above your post. It’s mostly incorrect.

Left part is ok (except for the fact that the fissile atom is displayed as having the same size as the fissile products), but the problem is on the right, after fission has occurred: this image makes it seem like the fission products –or less-heavy atoms that are produced when the fissile atom splits– are also fissile. That is not the case. The neutrons from the fissile reaction have to interact with new fissile atoms, like U-235, to sustain a chain reaction. The fissile products are however radioactive, which means they can spontaneously decay releasing radiation and energy.

Next, in a nuclear reactor, you don’t want every fission to create three new fission events as displayed in the image, since that is how you get a bomb. You want about 1 neutron from each fission to interact with a new fissile atom and the other 2 to be absorbed, fly out of the reactor, or whatever.
In other words, you want the multiplication factor k to be around 1. That is how you safely sustain a nuclear chain reaction.

So this:

so we get a net gain of neutrons out of it.

Is not the goal in nuclear power. It is to sustain a controlled chain reaction that generates heat, which creates steam, which drives a turbine to create electricity.

Something that is a more minor issue, the fast neutrons created by the fission need to be slowed waaaay down to something called a thermal neutron to be able to cause fission, or rather dramatically increase the chance of fission occurring. So the neutrons can’t fly directly to a new atom and cause fission as displayed in the image.

And lastly, fission and fusion are two very separate processes that have very little to do with one another and are in fact each other’s opposites.

I like these kinds of posts :)

If anyone else finds it helpful to write out what they've learned, please do share it. Its a nice break from the subtle soapbox type posts

yeah i just noticed ... i didn't actually inspect the image so thoroughly before choosing it. it was mostly to have a fun little picture to go along with it, but you're right, it's wrongly labeled.