I was reading another post on the forum when I stumbled into the idea of a Fusion reactor. Here are my ideas after viewing it, including some of their ideas:
Fuel: Hydrogen and Lithium, close to the IRL counterparts, Deuterium and Li-8.
Cost to make: In the Large Printer, it takes 2 Nanocarbon alloy and either 2 Astronium or 2 Refined Astronium (Refined Astronium is another idea)
Results: Helium and 24A of power
To reach Ignition: 1 Hydrogen Gas Canister, 1 Lithium, and 16A of power
To keep it running, it uses one fifth of a hydrogen gas canister and one fifth of lithium over 2 minutes.
Fits on: XL Platform A
If it runs out of fuel, it will safely shut down, unless it hits an unstable temperature, it will try to cool itself and display red lights, make annoying sounds, and warn the player. If the temperature is way too high, beyond self-cooling, it will melt down, breaking machines. If you stand near a fusion reactor during a meltdown: you die.
IRL: Fusion Meltdowns are theoretically rare, unlike Fission meltdowns (Three Mile Island, Chernobyl, Fukushima).