Molten Salt Reactor - sswelm/KSP-Interstellar-Extended GitHub Wiki
Molten salt reactor ( wikipedia ) have the lowest temperature and thermal power of all the reactors, but its balanced out with long-lasting stores of UF4 or ThF4. Fission reactors can last a very long time without refueling. The actinide waste can also be reprocessed using a refinery or a science lab to vastly increase their lifespan. The fission reaction requires a minimum power level to sustain itself, and so these reactors cannot go below 25% power production unless they are shut off.
Fission reactors have two fuel modes: Uranium and Thorium. Thorium reactors produce more power than Uranium reactors and have a slightly higher core temperature (which is good!). However, Thorium reactors' power output declines as actinides build up, and so they need to be reprocessed regularly in order to maintain peak efficiency. The fuel a reactor uses can be set in the VAB, and can also be changed on-the-fly using a Kerbal on EVA. In order to swap fuels, you need to have enough of the new fuel to fill up your reactor and enough empty space for the old fuel to be stored into. The reactor must also be shut down and cooled completely.
Reactors can be shut down via an EVA. They will also automatically shut down if waste heat on your vessel exceeds 95%. Shut down reactors enter a cooling state, where they will produce 10% of the maximum thermal power, which gradually decays down to 0% over several days. Once they are cooled completely, they can be refueled, have their fuel type swapped, or restarted using the context menu with an EVA kerbal.
Reactors can also use thorium tetraflouride, using ThF4 gives increased power over UF4 but has drawbacks in that its output will decline over time meaning that it should not be used for unmanned long range craft. Its increased performance comes with increase maintenance in the form of fuel reprocessing using the ISRU.
