Available as soon as you've reached the Moon (late HV-early EV, or just get some titanium from aluminum gravel), nuclear reactors are a strong power source for EV and IV. The process of creating a reactor is quite complex and risks huge nuclear explosions. You are strongly advised to try all designs in a reactor planner before using them, and to encase your reactors in reinforced materials (2 layers minimum) or warded glass (thaumcraft) to prevent explosions from destroying the area. Test your reactor in creative first! Nuclear Control 2 is in the pack allowing various automation and information to be done.
There are in general two types of reactors, power producing and breeder (for the items/rods). For the power ones, you want the greatest efficiency (fewer rods, more power). For item breeders, you just want to consume as many rods as you can as fast as possible. For rod breeders, you want as many neighboring fuel cells as possible while generating the least amount of heat. Stacking also comes into play for large numbers, since they are not conveniently designed for that normally.
For items, you can look at this handy chart. Note that some items were excluded (for example getting thorium back from thorium rods). Note that the EU and Heat #s are for single rods (EU is /t, heat is /s as they only outputs each second), and that the EU should be multiplied by 10 for the actual amount per tick. See the planner for more details (make sure to grab 2.4.2+). Any rod that has the same heat values is equivalent in a planner design, and anything that produces less heat can also be substituted, unless it's a MOX-type design. Keep in mind that while quad thorium and dual uranium have the same heat levels, this is not true when placed next to other rods.
EU reactors produce raw energy directly with a 50% penalty. Use transformers to transform the power into the GT type.
Fluid reactors produce hot coolant. Use a Large heat exchanger to convert the Hot Coolant to HP Steam/Steam, and HP Steam Turbines/LP Steam Turbines to generate energy with very high efficiency. See the Fluid Reactors page for details.
Vacuum reactors produce EU, but rely on cooling the cells in a vacuum freezer to keep it from exploding. See the Vacuum Reactors page to find out how to use them, and also redstone automation info.
Wrench reactors are a meme, you can read about them here There are several fuel types:
Thorium is the cheapest and easiest to obtain, obtainable in large quantities from coal ore or thorium ore found in the nether. It lasts a long time but doesn't make much power. You need to make these to get Lutetium for Americium later on.
Uranium is obtained at Mars' moons from a couple ores. It doesn't last as long as thorium, but makes more power.
MOX is obtained by mixing plutonium (from depleted uranium rods, centrifuging uranium dust, or ores on later planets) with uranium. It returns enough plutonium to sustain itself. It outputs more power the hotter the reactor is.
Naquadah and Naquadria are obtained from mid-game ores, and are generally best used in naquadah generators.
Tiberium is made with tiberium. Get this from Ross128ba, the T5 planet.
The Core is made with naquadah and tiberium. Only use it with coolant cells!
Rod Breeder Reactors produce very little EU but instead transform a Breeder Rod into a different resource. Each reactor tick, all breeding targets get charged by the number of active rods they are next to. The best fuel rods for these reactors are Thorium because they generate the least amount of heat. Using a hotter fuel rod will produce more EU, but won't speed up the breeding process at all.
Item Breeder Reactors focus on consuming fuel rods as fast as possible to generate resources obtained from centrifuging depleted cells.
Glowstone breeder rods are used to make sunnarium.
Note: This information may not be accurate in the latest version. Always test designs in creative mode first before using them in your base!
Currently known bugs:
Prior to version 2.2.8, Glowstone breeder rods unintentionally interact with fuel rods, creating additional heat (and therefore requiring more cooling than they should).
Sample Power Reactors
The EU/t values given are for Thorium or Uranium fuel rods. As high density uranium produces the same amount of heat, it can directly replace uranium fuel at double the EU/t. When using the IC2 Reactor Planner, ensure Version: 1.7.10 and GTNH is set as the game version under the Advanced Tab, otherwise many components won't be selectable.
800 EU, 4 Quad Thorium
1 chamber, highly stackable, uranium version won't work