The Small Reactor is one of your first power sources available in the game. It is maxed at an incredibly early level and thus, it provides alot of power to your ship. Because of how much power it gives, it is usually the target of MANY people to completely disable your ship so defend it well.
There isn't much to do when you attack with the Small Reactor because well, it doesn't even attack in the first place. Just make sure that your crew are able to fix it if your AI cannot already do it by themselves. Also, System Hack and anything with EMP Damage has almost no effect on the Reactor's power giving.
The Small Reactor is usually the least armored in ships due to its large space consumption. This makes your ship's reactors high target for Penetrator Missile spam and boarders from the Teleport. So, make sure you have alot of repair crew that can easily access the Reactor, Armor is also highly recommended, but don't worry if you can't fit in any more armor.
Reactors provide power while it is still upgrading.
Reactors have no functional AI Response.
You cannot have less than one Reactor in your ship, the game simply prevents you from removing it if you do have less than one.
If you have no Reactors at maximum health, you will always have a 0% Escape Rate, even if you have a Bridge.