Small reactor

"Reactors generate energy on-demand and if no energy is used, no heat is generated. They have a base output but they lose some of that energy to heat before you can ever use it. They also always have an optimal running output. Any energy that is used beyond the optimal output experiences a double heat penalty, so if you plan to place a heavy load on your reactors it is a good idea to have extra radiators. Different reactors create and store different amounts of heat, so small reactors are easier to cool than large ones, but when they get to 100% heat capacity they explode, so every reactor has a cut-off point where it shuts down before overheating."