Fusion Reactor is used to produce energy and materials in late game.
The first reactor will take some work to create, but eventually you will find that you are surrounded by these reactors and have enough Helium Plasma (used for EU generation) to supply your entire base.
At the same time they make it easier for you to obtain some raw materials such as Indium, which is necessary for Superconductory.
Fusion Reactor functionality is quite simple. First of all, you put liquids into the input hatch (depending on the recipe) and then charge the reactor to max and once it is charged, run it with a soft mallet. A Reactor can have a maximum of 16 Energy Hatches. Reactor MK1 can store 10M EU per 1 energy hatch, so MK2 can store 20M EU and MK3 can store 30M EU.
Fusion Reactor doesn't work like other regular GregTech machines. 2x EU/t | 2x speed -> MK2 can make a recipe that MK1 can do in 1/2 of the time ( 2x faster ), but if it needs 2 energy hatches. and if you have for example 4 hatches it won't have any effect for speeding up.
| Reactor tiers | MK1 | MK2 | MK3 |
| Energy | Speed | 2x EU/t | 2x speed | 4x EU/t | 4x speed | 8x EU/t | 8x speed |
After you get the possibility of fusion energy, you have a very convenient and one can even say inexhaustible source of energy. The Fusion Reactor will produce Helium plasma and this can be used as fuel for the Plasma Generator / Large Plasma Generator (a dynamo hatch is required for energy generation [ Maximum dynamo hatch can be used UHV Dynamo hatch [Buffered] at 4A ] )
You can create Helium plasma in a Fusion Reactor -> Helium-3 + Deuterium ( both types of fluid can be mined on planets, so you don't have to create fuel via machines ). Then you can also use Deuterium + Tritium ( However, this is disadvantageous precisely because you still have to produce Tritium via machines, and from Deuterium. )