A full size 17 cubed interior reactor
The Fission Reactor is a good way of making power using Nuclear Fuels. They are very versatile and can generate vast amounts of power. They can range from a core of 1x1x1 (roughly 3x3x3 exterior) to a whopping 17x17x17 core with a 19x19x19 exterior. To increase the size, input Reactor Upgrades into the Reactor Controller, one for every block in radius there is in the reactor interior from the walls. For example, that would mean a 3x3x3 core would need one Reactor Upgrade, and a 7x7x7 core would need three Reactor Upgrades. As the reactor increase in size, so does its power and heat, multiplying by the size of the reactor. (For example, a 3x3x3 interior reactor would have double the power as a 1x1x1, and a 13x13x13 interior reactor would have seven times a 1x1x1's output.) The speed at which the fuel burns also seems to go up slightly. [Needs Testing]
The exterior is constructed from reactor casings and in the interior there is one singular column of Fuel Cell Compartments. Around that column almost anything can be put, but it does little. The only things worth putting inside the reactor are Graphite Blocks and Reactor Coolers. Reactor coolers will remove 32 H/t per cooler in the Reactor and Graphite Blocks will generate [varies] RF/t and [varies] H/t per block while the reactor is on. Since there is no space for coolers in the smallest possible reactor (1x1x1 interior), the reactor cools itself for -8 H/t.