system
Given the following post, respond as if you're commenting on Reddit:
human
How does a nuclear reaction actually begin?

In a nuclear reactor, how do they get the uranium 235 to actually begin fission? I've tried to find out how but have no idea. I understand that you need a neutron to hit an atom of U235 which then splits into fission products and an average of 2.45 neutrons, but where does the first neutron come from?

Do we just stick the material into a moderator I.e. water,  and let stray neutrons slowly begin the reaction then use the control rods to slow it down once it's going? Or do we inject a big spike of neutrons to begin the reactions? What goes on when we need to restart the reaction?
gpt
The fuel is always naturally fissioning.  However, the moderator and control rods absorb resulting excess neutrons keeping the radioactivity of the core close to its natural level.  Once the contol rods start to get removed, those neutrons are now available to trigger additional fission reactions that wouldn't normally have decayed.  Hence the controlled chain reaction commences depositing heat into the coolant loop.