The Nuclear Reactor is a powerful tool in IC2, but extremeley dangerous if mishandled. It uses Uranium to produce power ranging from 1 Million EU in a cycle, to upwards of 400 million. Small designs will rarely top 100 million power, and large designs take extremely large and complex systems to maintain. However, the enormous amounts of power has its benefits, and there have been many successful Nuclear Technicians.
To effectivley use a Nuclear Reactor, you need some sort of Uranium Cell to power it, and some sort of Coolant Cells to cool the reactor, and avoid a critical meltdown of gargantuan and devestating proportions. There are other mods that are used in tandem with IC2 to help prevent meltdown, such as Nuclear Control.
If properly used, you too can reap the benefits of Uranium, and avoid a large gaping crater where your base was!