The Nuclear Reactor is a generator that produces EU by slowly breaking down refined Uranium Cells. As cells decay inside the reactor they produce heat. Heat may be removed by cooling methods. If cooling is insufficient, the reactor will gradually overheat and eventually explode, violently.

Copper cable is sufficient for basic reactors, advanced reactors will require gold or even HV cable.

Each cell will last 10,000 seconds (~2h 47min) inside the reactor, providing at least 10EU/t power (at least 2 million EU per cell). More advanced setups will generate more power, some setups reaching Extreme Voltage (2048 EU/t).

You can enlarge the space of your reactor by placing up to 6 additional Chamber Reactor Chambers directly adjacent to the reactor. 

It is generally advised that for anything but the most basic Nuclear Reactor setup, that the entire reactor be shielded  with Obsidian  or other blast-resistant materials to protect your base and the surrounding landscape from an accidental meltdown.