The concept mijnboogje expose is valid... BUT
1. Nuclear reactors in real life are a complex structure that include several BIG components, also including a turbine that is the real component that create energy in the reactor. In Minecraft his size is exactly 7 block, but we must assume that the concept at the base is the same, so even if is extreamly compress there must be a turbine "in the reactor" actually.
2. U burn the uranium, u gain heat, the turbine convert this heat into energy and there is heat excess that must be cooled like nuclear reactor does... but this excess is waste heat no more useful for produce energy because is no more in the distribution liquid but dissipated in all the reactor... if usable, we have a 100% heat-energy conversion directly in the nuclear reactor... translated into minecraft language, u can't use this heat into something that make EU.
3. beyond this, if u want to use geogen at all cost into reactors, u can make a new component, crafted from a geogen and something else, cooling like a coolant cell and generating 1-2 EU/t only if the reactor is over 2000 heat instead of his normal beheaviour... it seems to me more balanced...
4. even if minecraft mustn't be like reality, we have to make it a minium similar if we think industrialish... or we will start to think to create energy directly from dirt or divine blessing one day...