There are two types of nuclear reactor in Voltz: Fission and Fusion. Fission Reactors are easier to build and comparatively cheap, but fail catastrophically once they reach a certain temperature, irradiating the area around them. Furthermore, they apply the Radiation Poisoning to any entity near the reactor, forcing players to wear a Hazmat Suit while in the vicinity. Fusion Reactors are extremely expensive to build, but fail in a way that is easier to contain. They do not have a maximum temperature, and as such, are only prone to failure if damaged by an outside source, such as TNT or a Creeper. They are also also extremely expandable, and can end up providing many times what a fission reactor could, at a premium of space and materials. To sum up, if you are late game and looking to expand your power system, build a Fusion Reactor. If you are early game, however, look to a Fission Reactor for quick and dirty electricity.
Fission Reactors work by splitting atoms, to do this you must: