Today I Learned

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.