The inside of the reactor must be at LEAST 3x3x3 in size.If you choose not to, here is a quick rundown each reactor needs to meet certain build parameters These are listed below

To learn the basics of fission reactors you can follow this tutorial, by OnlyDraven. The fission reactor requires large amounts of tough alloy and uranium both to build and to power. Molten salt fission reactors require molten FLiBE salt solutions of nuclear fuel to generate power and cooled with molten eutectic NaK alloy, and will deplete the fuel and heat the coolant.The fusion reactor uses liquid or gaseous fuels such as hydrogen and will produce 4 byproducts, the reactor must also be powered in order for the electromagnets to work and for the fusion core to heat up.The solid-fuel fission reactor, or just the fission reactor, uses radioactive fuels such as LEU-235 and needs to be sufficiently cooled or else it will cause a meltdown, causing random blocks to be turned into molten corium.NuclearCraft introduces Solid-Fuel Fission reactor, Tokamak Fusion Reactor, and Molten Salt Fission Reactor, which are all multiblock structures for generating power. NuclearCraft is a mod created by tomdodd4598, with the help of others, which allows players to create RF via radioactive materials. NuclearCraft takes a semi-realistic view of reactor design and optimization so the mod is well-suited to players who enjoy complex design challenges. NuclearCraft offers three main methods of power generation: the Solid-Fuel Fission Reactor which directly harnesses the power of splitting atoms, the Tokamak Fusion Reactor which generates power by fusing atomic nuclei together under immense heat, and the new Molten Salt Fission Reactor, in which the heat of nuclear fission is passed to coolant to be transferred to water to produce steam in a Heat Exchanger, which is subsequently used by one or two stages of Steam Turbines to generate power.

Nuclearcraft fission reactor all fuels